Drought is said to be the arch enemy of the dry-farmer, but few agree upon its meaning. For the purposes of this volume, drought may be defined as a condition under which crops fail to mature because of an insufficient supply of water. Providence has generally been charged with causing droughts, but under the above definition, man is usually the cause. Occasionally, relatively dry years occur, but they are seldom dry enough to cause crop failures if proper methods of farming have been practiced. There are four chief causes of drought: (1) Improper or careless preparation of the soil; (2) failure to store the natural precipitation in the soil; (3) failure to apply proper cultural methods for keeping the moisture in the soil until needed by plants, and (4) sowing too much seed for the available soil-moisture. [Emphasis mine]
—JOHN A. WIDTSOE, Dry-Farming, 1911
CASCADIAN GARDENS seem to require irrigation. Rain rarely falls between June and late September. When it does rain in summer it usually doesn’t amount to much. Most gardeners irrigate veggies with lawn sprinklers, but these put out water so fast they almost inevitably spread too much. Gardeners watering with hose and nozzle tend to spread too little. Either way, too much or too little, diminishes the result. And not just a little.
Most gardeners have seen plants get gnarly, or stunted in dryish soil and, when moisture stress is extreme, wilt and die. They have also seen plant growth speed up after irrigating. So gardeners water prolifically without realizing that overwatering leaches soil fertility, slows growth, and lowers productivity.
This chapter explains how the plant density you establish strongly influences how much water the garden will need, how often irrigation must be done, and how to construct an irrigation system that spreads water evenly and controllably. You may design a garden that needs watering once a week. Or one that may yield a bit more but requires watering almost every day in summer. Or if you’re fortunate enough to have deep moisture-retentive soil, you may design a garden that during the heat of summer only needs irrigation every two to three weeks, allowing you to take worry-free summer vacations. On that kind of soil you also can dry garden, an extreme style that yields a good deal less per square foot but rarely needs watering, if ever. It’s your choice.
When it rains hard, the soil’s pore spaces are temporarily filled with water. Soil air contains a high concentration of carbon dioxide produced by the microecology and plant roots. When the moisture inflow stops, surplus water drains and fresh oxygen-rich air gets pulled back into the soil pores. The remaining moisture adheres to soil particles. To see adhesion at work, dip a small stone into a glass of water and then remove it. The stone is wet on the outside. A few drops may fall off, and then the rock, still moist, stops dripping. If this rock were a layer of soil being irrigated, when it stopped dripping we’d term the quantity of moisture still adhering to its surfaces its capacity to hold moisture.
As soil dries down, the films of water adhering to its particles get thinner; the thinner they get, the stronger they stick and the more effort it takes plants to extract that moisture. Plants cannot grow fast when they must expend a lot of energy getting moisture. Even a short period of moisture stress turns lettuce bitter. If a moisture-stressed cauliflower plant manages to form a head at all, it will probably be harsh tasting. Water-short zucchini become dry, fibrous, and sometimes bitter, while moisture-stressed winter squash vines don’t set as many fruit and they’ll be smaller fruit. Snap beans become thin and tough, radishes hot and woody.
When a plant can’t extract soil moisture fast enough to keep up with the amount being lost in strong sunshine, it wilts—a self-protective action that reduces its rate of moisture loss. As the day cools down, or the sun goes behind clouds, or night falls, the plant straightens back up and looks okay the next morning. Actually, temporary wilting is a huge stress that slows growth for days after. Extreme moisture stress makes plants wilt and promptly die.
Root crops like parsley and carrots store water. By drawing upon this reserve they can transpire more moisture than the roots can take in, without wilting. However, enduring a moisture deficit makes the carrot or radish tough to chew and bitter tasting, while the parsley stops making new leaves. No wonder gardeners quickly learn to fear dry soil.
If the garden is important to your family’s health and economy, then I think it is crucial to irrigate systematically. But investing in convenient equipment is not necessary; it is possible to get an excellent result with a cheap lawn sprinkler or even a hose and nozzle, as long as you know what you’re trying to achieve, discover what your equipment actually does, and in the case of hose and nozzle, are willing to invest your time.
The least costly way to irrigate is with hose and nozzle. You wetting down the garden works quite well if you enjoy the task. But the garden gets watered too briefly if you don’t relish the occasion. If the garden is hand-watered every single day but too briefly, moisture doesn’t penetrate deeply enough before you move on. Under a damp surface layer the bed may have been sucked bone dry. The vegetables may never wilt, but they’ll become severely stunted.
To avoid this possibility, John Jeavons recommends repeatedly wafting the spray back and forth across several square yards of bed (area A) until the entire surface sparkles and becomes shiny wet. During the few moments the surface is sparkling, the surface layer contains more water than it can hold against the force of gravity. Spreading even more water faster than the pore spaces drain would make water run off the bed and possibly wash topsoil down on the paths. Better to move on to the next few square yards (area B), continue on B until that section gets sparkly, then return to A until the shine reappears, and then go back to B until the shine reappears, repeating this pattern until the sparkling shine lasts long enough. At first the shine only lasts a second or so, but as the soil gets saturated ever deeper, the shine lasts longer. How long you want the shine to persist depends on your soil type. In the edition of Jeavons’s book I read, it says “long enough” can vary from a second or so on sand to 10 seconds on a clay soil.
That’s quite a difference! How to choose? Jeavons suggests initially calibrating your soil’s moisture-assimilating ability by trying different shiny times combined with digging some test holes to see how deeply the soil has become saturated. Without this check, gross overwatering or underwatering could result. The desired shiny time only has to be determined once.
Clayey soil holds a great deal of moisture but assimilates it slowly. So clay can show a sparkly surface for many seconds while being quite dry a few inches deeper. On the other hand, coarse sands usually take in water so rapidly that it can be difficult to get a shine to last more than an instant. However, sandy soil is the type most prone to leaching. Garden writers (including me) tend to generalize broadly from limited experience, so I’m not surprised Jeavons did his research on clay soil.
In my first few years of backyard gardening I painstakingly hand-watered exactly as Jeavons suggested and it worked, but I had a small business to manage. So I switched to a sprinkler system that achieved the same result without consuming my precious time.
| Soil | Moisture Remaining |
| Permanent wilting point | 20 to 33 percent |
| Temporary wilting point | 50 percent |
| Minimum moisture for intensive vegetable beds | 70 percent |
| Field capacity | 100 percent |
Available Moisture (Inches of Water per Foot of Soil)*
| Soil Type | Total Holding Capacity | Available Moisture |
| Sandy | 1.25 inches per foot | 1.0 inch per foot |
| Medium (loam) | 2.5 inches per foot | 2.0 inches per foot |
| Clayey | 3.75 inches per foot | 2.7 inches per foot |
*When the soil has delivered all of its available moisture, it has dried to the permanent wilting point. Obviously, sandy soil has far less ability to supply moisture than clay soil.
Daily Moisture Loss in Summer, Willamette Valley (Inches per day)*
| Average Day | 0.2 |
| Hot Day | 0.25 |
| Very Hot, Breezy Day with Low Humidity | 0.3 |
*This table assumes the sun is shining and the soil is growing a dense leaf canopy.
Amount of Water Needed to Bring 1 Foot of Soil from 70 Percent to Capacity
| Soil Type | Irrigation in inches |
| Sandy | ¼ (0.25) |
| Medium (loam) | ½ (0.50) |
| Clayey | ¾ (0.75) |
Many vegetables grow very poorly when soil falls below 60 percent of its peak moisture-holding capacity. So be guided by this principle: once the top foot of soil has dried to where it holds about 70 percent of its total capacity to hold moisture, it should be brought back to capacity again.
Sandy soil covered by a dense leaf canopy can lose enough moisture in two sunny summer days to drop 2 feet of topsoil from capacity to 70 percent of capacity. This garden benefits from having yesterday’s moisture loss replaced at the beginning of every hot sunny day. A clayey garden loses the same amount of moisture as a sandy garden does each day, but clay could be watered every three or four days and you’d spread three or four times as much water with each irrigation.
When plants are small, their root systems feed in the surface foot. In that case, spread the quantity of water recommended in the table “Amount of Water Needed to Bring 1 Foot of Soil from 70 Percent to Capacity.” When the plants are two months old (after one month, if the vegetable makes a taproot), their root systems reach down 2 feet or more. And their tops probably have formed a canopy that transpires a lot of moisture.
Soil moisture should be measured 4 to 6 inches below the surface. Firmly squeeze a handful of soil from that depth into a ball—this is the classic “ready to till” test. If the ball feels wet or gooey, or sticks together solidly, the soil moisture is above 70 percent, unless you have sand that won’t form a ball no matter how hard you squeeze. If the soil ball sticks together firmly but breaks apart easily, moisture is 65 to 70 percent. If the soil feels damp but won’t form a ball when squeezed hard in the fist, then the soil is below 65 percent moisture.
Cascadian soils receive more moisture than crops transpire from midautumn through midspring. Some of this excess runs off the surface. Some passes through the soil and enters the water table. “Leaching” is another word for this circumstance. The soil dries down during March and April, although clay soils, especially bare clay soils, may not reach 70 percent of capacity until May.
Daily moisture loss varies with the season and with the amount of vegetation the soil supports. The scientific name for this loss is “evapotranspiration,” meaning a combination of evaporation from the soil’s surface and transpiration from leaves. If a bed is covered by a dense leaf canopy during June, about an inch of water is lost through evapotranspiration each week. During the intense light and heat of July and most of August, average water loss will be around 1½ inches per week. On days that bring a scorching east wind I call a “Umatilla,” loss can exceed ⅓ inch each day. By September, losses average around 1 inch per week.
I determine how much to irrigate by assuming the soil is losing as much water every day as the chart says and spread 10 percent more than that amount.
Lawn sprinklers spread water thick and fast. This seems convenient. But how uniformly do they spread it? And how long does yours have to run in order to spread a half inch of water? Before you irrigate a veggie garden with a lawn sprinkler, please test its application rate and how uniformly it spreads water. Position a few irrigation-rate measuring gauges—cylinders like empty tin cans or straight-sided drinking glasses—in different parts of the sprinkler’s pattern and operate the sprinkler for exactly 30 minutes when the wind is not blowing. Locate one gauge close to the sprinkler, put another a few feet inside the far limit of its throw, and put a couple more in between. After 30 minutes, measure the depth of water in each container, average those amounts, and then multiply the average by two to derive the sprinkler’s average application rate per hour.
In my experience lawn sprinklers spread water at over 2 inches per hour. Perforated hoses and spot sprinklers designed to cover small areas usually put out an even higher rate. How much leaching do you suppose happens in a sandy garden from running this sort of sprinkler for 1 hour? The tin can test also reveals how uniformly the sprinkler spreads water. Usually, the ones closer to the sprinkler holding a lot more water than those on the fringes.
Agricultural (and institutional) sprinklers spread water more uniformly. They initially cost more, but durability makes them far less expensive in the long haul. I recommend them. Sometimes the local home-improvement store or big-box garden center department sells all-brass impact sprinklers resembling those used by farmers, but usually what’s offered to consumers is designed for lawns, not vegetables. I suggest you buy commercial-grade crop sprinklers from a supplier that understands why there are optional nozzle sizes and a range of sprinkler head sizes.
THE BOTTOM LINE
Sandy gardens in Oregon should receive half an inch every two days in summer. In periods of extreme heat they should be irrigated every morning. Washington gardens should get about the same amount, but every three days. And Washington gardens almost never have to cope with periods of extremely hot dry winds.
Clayey soils growing large-size vegetables can easily accept 1 to 1½ inches of water. In Oregon this much every five or six days in summer is enough except in very hot weather.
Frequent light irrigation may be needed on all soils when sprouting seed, when nursing recently transplanted seedlings during hot weather, when nursing small seedlings until their roots penetrate far enough to find stable moisture, and for species with unusually high moisture requirements, such as radishes and celery. These extra needs are best supplied with hose and nozzle.
But before buying any sprinkler, discover what your water pressure is because this factor may determine which, if any, sort of sprinkler is possible. Municipal water usually is well over 35 pounds per square inch (psi) at the street, so a pressure reducer can hold household pressure at a steady 35 psi. Homestead well pumps can be adjusted to produce between 30 and 50 psi. You can roughly test water pressure without a formal gauge with a hose nozzle that makes a strong smooth jet. A nozzle like that should throw water 40 to 50 feet if you have over 30 psi.
An ideal agricultural sprinkler for vegetable crops emits between 1 and 3 gallons per minute. My ¼-acre garden is covered by 40 1-gallon-per-minute sprinklers arranged in five lines holding 8 sprinklers each. I have enough water to operate 16 sprinklers at once. To get an idea of the number of gallons per minute your outlet can supply, measure how many seconds it takes to fill a 5-gallon bucket through a full length of garden hose with a nozzle at the end putting out its strongest jet. The nozzle mimics the back pressure from an array of smaller sprinkler nozzles. Before building a system that needs the entire amount, first see what happens if someone takes a shower while the bucket is filling.
Agricultural sprinklers come in a range of sizes. Large crop sprinklers use nozzles that can throw water over 50 feet; some really big sprinklers with nozzle openings the size of a 10-gauge shotgun barrel spray a mixture of water and dairy manure over a circle hundreds of feet in radius. Sprinklers most suitable for vegetable crops use nozzles between ⅛ inch and 1/16 inch. At its designed pressure a 1/16-inch nozzle can be expected to throw 25 feet; a ⅛-inch nozzle throws around 35 feet. Small-bore nozzles like this are better in the home garden because (1) they put out fine droplets that cause less soil compaction and (2) a shorter throw radius helps keep the water off adjoining buildings and neighbors’ yards. The impact of large droplets breaks up soil crumbs and floats silt and clay particles to the surface, where they dry and form a smooth crust, much the same as screeding concrete lifts the fine sands to form a smooth skin. Crust formation is the last thing you want to cause; it blocks germinating seeds and stops air exchange.
Application rates between ¼ and ½ inch per hour are ideal for most food gardens. But low-application-rate sprinklers have limitations. A combination of sun plus wind plus high temperatures can evaporate much of a fine stream of water before it hits the ground. This means small-bore sprinklers should be run early in the morning on light soils before the sun gets strong and the wind comes up, or else all night on clay. A clay soil could be watered from bedtime to breakfast without leaching the root zone if the application rate is below ⅕ inch per hour. Watering at night is reputed to cause disease. Actually, watering all night prevents disease by continuously washing bacteria and fungus spores off the plants before they can germinate. This principle is well understood by nurseries that propagate healthy plants from cuttings by frequently misting them. What can harm plants is to turn off the water just before or soon after dark. This makes plants damp all night—ideal conditions for the multiplication of disease organisms.
Spreading water uniformly using only one sprinkler in one fixed position is nearly impossible because to achieve uniformity, the sprinkler must deposit nearly 10 times as much near the far edge of its pattern as in the center. Every point in between must get a different amount. Please carefully read the caption below the illustration on the next page.
The oscillating lawn sprinkler seems to overcome this problem by watering in rectangles. But this design spreads less uniformly than most circular sprinklers. I suspect the reason is that the Achieving Uniform Water Application from a Single Sprinkler cam that pivots the spray arm pauses at the turnaround points, putting too much water at the ends of its rectangular pattern and too little above the sprinkler itself.
Achieving Uniform Water Application from a Single Sprinkler
The formula for the area of a circle is: A = πr2. Imagine water being spread by a sprinkler with a 25-foot-throw radius. The innermost 5 feet of the sprinkler pattern occupy 78.5 square feet (3.14 by 5 by 5). The area in the outer 5 feet of the pattern is 706.5 square feet, calculated this way: the area of the full circle (3.14 by 25 by 25) minus the area in the inner 20 feet of the pattern (3.14 by 20 by 20). Thus, the nozzle must deposit nearly 10 times as much water on the outermost 5 feet of the pattern to end up with the same thickness of coverage as it spreads on the innermost 5 feet.
The impact sprinkler can’t spread water uniformly because when the rocker arm momentarily interrupts the nozzle jet (its bouncing rotates the sprinkler), it creates heavy, slow-moving droplets that fall close to the sprinkler. Most consumer-market impact sprinklers come with a diffuser paddle or adjustable needle-tipped screw to shorten the water throw by diffusing the spray. But diffusing the stream makes it throw even less water to the fringes. The more diffusion, the worse this effect becomes. Agricultural-quality impulse sprinklers do not have diffusing devices; they come with precision nozzles that, if operated within the designed pressure range, achieve the best-possible compromise, putting only about twice as much water near the center of their coverage as on the outer half.
Farmers obtain fairly uniform distribution by positioning impact sprinklers in overlapping patterns. Any multiple sprinkler pattern still leaves a fringe area where fewer overlaps occur. On the farm, throwing some water beyond the margins of the field is of little consequence; in the backyard, it may be essential to keep all spray within your own yard or off your own buildings.
Fringe areas can be used for dry gardening. Or you can position a tall growing crop like climbing beans, asparagus, or sunflowers along the garden’s edge to intercept the overspray.
Sometimes sprinklers are arranged in a square pattern, sometimes in a hexagonal pattern. The hex arrangement distributes water slightly more uniformly, but the square pattern works slightly better when the garden is close to buildings or where the sprinklers could throw water on the neighbor’s property.
For the home garden the most useful, highly durable, and least costly ag-quality impulse sprinklers I know of (and personally use) are Israeli-made Naan 501s with 1/16 or 5/64 nozzles (1.7 to 2.0 millimeters). Naans should be widely available through irrigation and farm suppliers but are not at present. A good online source is www.growerssolution.com, offering both 501s and 502s (see below).
A sprinkler designed to throw water the maximum-possible distance has its nozzle angled about 12 degrees above horizontal. This “high-angle” design covers the largest-possible area with the fewest sprinkler heads while drawing the least number of gallons per minute. However, high-angle water streams are strongly affected by wind. High-angle sprinklers with ⅛-inch nozzles create application rates so low they allow watering a clay soil all night and half the next morning. Low-angle sprinklers throw a stream that is only a few degrees above horizontal. These are best for windier situations and for daytime use. However, their throw radius is shorter, so more low-angle sprinklers are required.
Naan 501 series sprinklers have low-angle nozzles. They are durable! My current set has been exposed to year-round sun and weather for 15 years now, and every one of them is still operating. Naan sprinklers are rotated by a brilliantly simple design that does not require close tolerances. There are only a few moving parts and no springs; Naans are quick to disassemble and clean if the nozzle becomes blocked. To disassemble a Naan, wiggle and lift the top cover to detach it from the two posts that hold it, remove the spinning wheel, pry up the nozzle using a flathead screwdriver or the back of a small peeling knife, remove the blockage, and then reassemble.
Sprinkler heads require a support stand and water supply that usually cost more than the sprinklers themselves. Conveniently, the Naan 501-U plus stand assembly includes a yard-long galvanized steel rod that supports the sprinkler head above most vegetable crops and a corresponding length of feeder pipe with a 7-millimeter quick-disconnect barbed fitting at the end—ready to plug and play. Naan also makes the 502 series, small-bore high-angle sprinklers that use the same stand/support assembly. If I were market gardening a half acre or more, I’d use the 502.
Using several correctly spaced sprinklers creates overlapping water patterns, permitting uniform coverage. The optimal spacing between sprinklers is about 60 to 70 percent of the sprinkler’s throw radius. Left: sprinklers arranged in triangular patterns. Right: sprinklers arranged in square patterns.
Holes in supply lines for barbed connectors should be made with a proper punch so that the barb doesn’t leak. Barbed connectors set with a nail almost inevitably leak.
Some gardeners avoid spreading water outside the garden by using part-circle impact sprinklers. Keep in mind that reducing an impact sprinkler’s coverage to a half circle doubles its rate of application; cutting it to a quarter circle quadruples the rate of application. Impact sprinklers spread water even less uniformly when running in part-circle mode because while the head is reversing, the rocker arm’s action deposits even more water close to the sprinkler. Part-circle sprinklers come with other downsides. Actuating the reversing mechanism requires considerable force; this means they must use a large-bore nozzle that makes higher application rates and bigger droplets.
A simple way to restrict an ordinary full-circle impact sprinkler is to attach a shield made from a cutout tin can or a small plastic bucket to a garden stake pounded in directly behind it. This method does not increase the application rate (except immediately below the sprinkler), nor does it lessen uniformity of distribution.
Turbine-powered sprinklers are an enormous improvement over the impact design. The nozzle is rotated by a propeller spun by the water passing through. Multiple water streams emerge from a slowly rotating disk. One or two of these jets throw an undiffused stream that covers the outmost parts of the circle; others spread water close in. Some nozzle openings greatly diffuse the spray; others make a rather clean stream. The original was made by Toro. Now other manufacturers are in the business. I think the best design is the MP Rotator made by Nelson. It is not expensive, allows handy adjustment of the throw angle (distance), and can restrict the amount of arc covered without increasing the precipitation rate. (www.nelsonirrigation.com).
Agricultural sprinklers are designed to operate within a specific range of water pressures. Farmers understand that if pressure is too low to diffuse the spray properly, far too much water is thrown to the outer few feet of the pattern. The impulse arm’s action still causes much water to be laid down near the sprinkler. Too little water goes into the midzone. The resulting distribution pattern is termed “doughnuting.”
Consider the opposite. Operated at excessively high pressure, the water jet becomes too turbulent and breaks up too much—“sprays too much,” as a farmer would say—actually reducing the throw distance while greatly increasing the amount laid down close to the sprinkler, making the fringes too dry.
Nozzles can be designed to diffuse properly at pressures ranging from 10 psi (for mini-sprinklers) to 100 psi most designs require from 30 to 60 psi. Homesteaders having their own well and pump can, within limits, adjust their water pressure. Naan 501s, for example, operate effectively from 20 to 50 psi. Under 20 psi they don’t diffuse enough; over 50 psi they diffuse too much and operate so violently you might eventually wear them out.
High-angle sprinklers should be spaced no further apart than 65 percent of their throw radius. This creates enough overlap to equalize distribution and allows for (light) wind blowing the spray. Low-angle sprinklers are usually spaced at 75 percent of their throw radius.
The cost of supporting the sprinklers and of bringing water to them can much exceed the cost of the sprinklers themselves. Inexpensive sprinkler supports that can supply low gallonage heads can be made by gluing a plastic micro-sprinkler spike into the end of a piece of ¾-inch white plastic pipe that has its bottom end cut off at a sharp angle so that it can more easily be pushed into soft soil. The white plastic pipe carries no water.
If a permanent sprinkler system that covers the entire garden is beyond your interest or budget, uniform, precise irrigation can still be accomplished with a single impulse sprinkler on a homemade stand tall enough to position the sprinkler above most crops, supplied by an ordinary hose. It is run for the same amount of time in each of the positions you’d have put a dozen sprinklers. I watered my trials ground this way for the first few years. I made the stand with one sack of ready-mix concrete, a 5-gallon white plastic bucket, a few feet of ½-inch galvanized pipe, and some fittings.
Drip systems are the last method I’d ever choose for vegetables. For six years I had to supply my household and irrigate a ¾-acre trials ground and a 5,000-square-foot kitchen garden from a 3-gallon-per-minute well. Three gallons per minute can’t supply even one sprinkler nozzle large enough to be effective when the sun is strong or the wind is blowing. So as soon as I could afford to, I switched to drip tapes, lightweight plastic hoses with a pinhole every foot. I’ll soon tell you more about that disappointing well and the lessons it taught me.
I came to know way too much about drip, and here are the downsides. Drip tapes are expensive even when purchased in 2,500-foot-long spools. They are short lived, but I did not care what a trials ground cost in terms of money, time, or effort—it was producing information that made everyone’s garden grow better. Drip tubes are easily cut when hoeing. The emitter holes must face upward or else they rapidly become plugged with soil, but the tape moves with changes in temperature. To keep the water on narrow rows of young seedlings, the tubes have to be pinned firmly to the earth every few feet; and even so, it is not possible to germinate seeds or make sure every newly transplanted seedling gets watered when the drip line lengthens and shortens, twists and lifts as the temperature changes. Even though the water supply first went through a filter, every emitter hole on every line still had to be checked each and every time a line was turned on. Because the runs were over 100 feet long and went down a slope, I had to use “biwall” drip tubes. This design equalizes the pressure from end to end because there is an inner, larger pipe with internal openings into 10-foot-long sections where the emitter holes are. With biwall pipe, when an entire 10-foot section stopped emitting, it could not be fixed.
Drip tapes, individual drip emitters, or soaker hoses do not suit sand because the water goes straight down without wicking out horizontally, leaving areas of totally dry soil. If the soil contains a fair amount of clay, water wicks out horizontally as much as 2 feet to either side of the drip line. Drip lines might be useful for permanent plantings such as raspberries, but given a choice, I’d choose sprinklers.
Microirrigation is a hybrid between drippers and crop sprinklers, using low-pressure black plastic pipes, quick-connect fittings, and cheap plastic spike stands holding miniature ultrashort-radius sprinkler heads with emission rates of a few gallons per hour, not per minute. Microirrigation provides an inexpensive alternative for establishing high-density orchards and vineyards, and for narrow ornamental beds around houses. I have used them in tunnel cloches. Microirrigation parts often are bubble packed in garden centers, but you’ll find a broader range at urban irrigation suppliers. If you’re considering microirrigation, don’t assume the rate of application is low. Each sprinkler may not emit much water, but the nozzle doesn’t throw far. And use a very effective particle filter! The nozzles are extremely fine.
Irrigation makes a huge difference. Some kinds of vegetables absolutely require irrigation. Irrigation hugely increases yield and usually increases quality. Irrigation always makes the outcome more certain.
But in case the need arises, I reckon you might like to know how to grow a garden without much or any irrigation. Understanding dry gardening also clarifies plant spacing. Even if you have abundant water, the information in this section can save you work and trouble.
In early May of 1978, when I was 36, I had just settled on a 5-acre Oregon Coast Range homestead. We’d come there by way of Los Angeles, where, like the biblical Jacob, I had spent the previous seven years earning the wherewithal. In early May the Lorane valley is covered with green grass punctuated by liquid sunshine and rainbows. New to Cascadia, I thought summer would be like that too.
Homesteading. I intended to grow as much of my own food as possible, raise a legal crop to pay the bills, live simply so as to need little cash. Because I had horticultural ambitions, my offer to purchase was subject to drilling a well delivering at least 15 gallons per minute (gpm). With that much water I could irrigate one acre. The fourth (and what I vowed would be the last since I was paying the bill) drilling attempt brought forth slightly more than 15 gpm, and I bought the place. The wells were drilled in April.
My 5 acres had been part of an exhausted pasture that no longer produced enough grass to profitably mow and bale, an east-facing hillside that before growing hay and fattening calves had lost all of its topsoil from a half century producing plowed crops. I rocked the driveway, put in a septic system and a power pole, and hauled in a new single-wide mobile home—not the dream house we’d wanted, but it was instant shelter that I could pay for without debt. I immediately built a tool/firewood shed with space for a workbench. We were free and clear with enough left in the bank to live frugally for 2 years if nothing went wrong.
To establish a food garden, I spread a few trailer loads of sawdusty horse manure and a few sacks of lime and cottonseed meal on the silty clay subsoil, and hired a neighbor who did custom tilling; then I erected a deer fence around the roughly 5,000-square-foot area and assembled an excessively permanent irrigation system using 10 small brass impulse sprinklers that emitted 1 gallon per minute each. All 10 sprinklers were turned on and off with one big gate valve. I used galvanized pipes for supply lines and risers, because I knew I was going to be there for the rest of my life. Wasn’t I a clever young man! Unfortunately, every time I’ve known that something would be so for the rest of my life, I’ve been painfully wrong.
Summer arrived. Those lush green pastures browned off. The days got quite warm. My garden needed irrigating for the first time. So I opened up that big valve and stood outside the gate admiring 10 little impact sprinklers bouncing out crisscrossing streams, psit, psit, psit. But after a few minutes the pump shut down. Not yet knowing much about Oregon Coast Range wells, I spent hours repeatedly repriming and restarting the pump only to have it run for three minutes and then quit again. It wasn’t a pump issue. The pump wisely turned itself off because taking 10 gallons per minute lowered the static level below the intake point. But I could run seven sprinklers.
Over the next month the well’s output steadily decreased. I accordingly reduced the number of sprinkler heads I operated at one time. I was deeply worried. By mid-July the well could only sustain three sprinklers. Fortunately, a bit over 3 gallons per minute was the well’s lowest production level. By running three sprinklers all night I could cover the entire garden in three nights.
Two years later that well was also called upon to supply Territorial Seed Company’s ½-acre trials ground. Initially, I watered the trials with only one high-angle 2½ gpm impact sprinkler. I remember starting up that sprinkler at about 8:00 p.m., waking up by alarm clock at 2:00 a.m., stumbling down the cold dewy path barefoot, moving the sprinkler to its next regular position, going back to bed, and then shutting the system down at 8:00 a.m. so that the household could have morning showers. This went on four to five nights a week from June through August; on the remaining summer nights the water went to the kitchen garden.
For two years the trials ground depended upon my talent for getting back to sleep with cold feet. No wonder when the business could afford drip tape, I started using it. With drip I could water the trials in the daytime.
Once I had to replace the pump during high summer. After only a few days without irrigation, my intensive kitchen garden began to complain. The more extensively spaced trials ground grew okay through seven days without water. I was forced to face how dependent my garden was on technology. The experience started me wondering how the original settlers managed. Asking that question led me to discover dry gardening.
Before the intensive system, garden books from east of the Rockies suggested arranging vegetables single file in rows 3 to 4 feet apart. Irrigation was desirable but not absolutely required east of the 98th meridian, a north–south line running through Dallas, Texas. It rained often enough in summer to keep things green, although some years there were rainless periods lasting weeks. Fortunately, drought does not necessarily start after two weeks without rain. John Widtsoe, the agricultural scientist whose wise words begin this chapter, defines drought as beginning when the crop is damaged. The onset of drought has little to do with how much time elapses between rains; it has everything to do with how the grower manages moisture already in the soil.
While investigating, I turned up an old guy growing unirrigated carrots on alluvial sandy loam on the Rogue Valley floor, the hottest, driest part of Cascadia. He sowed carrots in spring while the soil remained moist enough to germinate the seed. He soon thinned the row to 1 foot apart; the rows were 4 feet apart. Despite getting no rain and no irrigation all summer, his carrots grew to enormous sizes and the overall yield per area occupied by the crop was not as low as you might think.
I found hints in a book by Gary Nabhan called The Desert Smells Like Rain about Native Americans growing remarkable Arizona desert gardens using moisture left in the soil by a brief period of seasonal rainfall. I knew of native South Americans in the Andean highlands who grew food crops in their cool climate with only 12 inches of annual rainfall. And I discovered John Widtsoe’s book, Dry-Farming (see Additional Reading, this page).
My first dry-gardened vegetable happened by accident. I had sold Territorial Seeds; we resettled on 16 acres of Class I silty clay loam near Elkton, Oregon. I had a long list of plant breeding projects in mind, one of them being to develop an open-pollinated late Savoy cabbage out of a commercial hybrid because this kind of cabbage was a family staple food item, while the only remaining open-pollinated variety, Chieftain Savoy, had degenerated into a useless mess. (Incidentally, Chieftain has been restored by Tim Peters and is available again from Adaptive Seeds.)
I knew that irrigating seed crops while they are drying down lowers germination and vigor. So in late winter I dug up six fully headed out hybrid Savoy cabbages and transplanted them beyond the sprinkler system’s throw. Big Savoys are usually arranged 24 by 24 to 24 by 30 inches, but for seed making I spaced them 48 by 48 inches because blooming brassicas make huge sprays of flower stalks. I did not intend to water these plants at all because cabbage seed forms during late spring while the soil is naturally moist. The seeds mature as the soil naturally dries down.
Seeds formed as expected. Except that one plant did something slightly unusual for a refined brassica—it started growing a new head among its seedstalks. Amazed, I watched this unwatered cabbage enlarge steadily through the hottest and driest summer I had yet experienced in Oregon. I realized I was being shown something, so I gave the plant absolutely no water, although I did hoe out a few weeds around it after I harvested the seedstalks. At maturity the head weighed seven pounds and was as sweet and tender as any other of its type when given all the water it could have asked for.
Because Nabhan pointed out the extreme plant spacing used by desert gardeners and I knew about dry farming carrots in southern Oregon, that huge unwatered cabbage spoke! Next spring I dry gardened a pair of 100-foot-long rows that were 5 feet apart center to center. I tried an assortment of vegetables I hoped could cope. I chose to dry-garden like a playful purist, to use absolutely no water at all, not even to germinate seeds. So I sowed everything before the soil dried out.
I tried kale, late-maturing Savoy cabbage, purple sprouting broccoli, carrots, beets, parsnips, parsley, endive, shelling beans, potatoes, French sorrel, and a couple of corn seeds. I also transplanted one compact bush (determinate) and one sprawling (indeterminate) tomato plant. (Each tomato seedling got a cup of water when I set them out.) Most of these vegetables grew surprisingly well. The plot produced extraordinarily good-tasting tomatoes until the end of summer. Kale, Savoy cabbages, and parsley fed us the following winter. The purple sprouting broccoli looked gnarly at summer’s end, but it grew lushly as soon as the rains returned and produced abundantly the next spring.
Almost everything was pleasantly edible. The potatoes yielded less than I’d been used to and had a thicker than usual skin that stored better but also had a richer flavor. I found out later that potatoes grown dry often have more protein compared to those given all the irrigation they can seem to use. Unirrigated tomatoes were numerous, richer tasting, and smaller than usual with thicker skins. The enormous carrots were a bit chewy but tasted fine. The rutabagas grew huge but became inedible by summer’s end. I could have eaten them in July.
The following year, I grew a pair of similar gardens. My insurance garden was, as always, thoroughly irrigated and the usual size so that no matter what came of my dry-gardening experiments, we would still have plenty. Another garden of equal size was grown entirely without watering.
By midsummer, some kinds of unirrigated vegetables looked fine, and others seemed severely moisture stressed. I recalled Widtsoe reporting on an 1882 dry-farming experiment where it took 1,100 pounds of water to grow 1 pound of dry plant matter on poor soil, but only 575 pounds of water to produce that same amount of plant matter on fertile land. Wondering if the real cause of what appeared to be severe moisture stress might actually be severe nutrient deficiencies, I tried foliar feeding full-spectrum liquid chemical fertilizer. Within days I could see it helped a lot.
I reasoned that I had fertilized the topsoil, but the topsoil had become too dry to feed the plants. The still-moist subsoil on this never-before-gardened area was infertile; it might take another few years of building topsoil fertility before many plant nutrients leached down that far. So I improvised a subsoil FertiGator. Next to some of the plants (and not others) I placed a 5-gallon white plastic pail with one ¼-inch-diameter hole drilled in its side just above the bottom. The bucket then was filled with liquid fertilizer that slowly emptied out, leaving a small wet spot on the surface. Most of the moisture had gone into the subsoil. I gave the lucky plants another such drink three weeks later.
Unfertigated winter squash vines looked moisture stressed all summer. Each unfertigated hill yielded about 15 pounds of very average-tasting food. When given just two 5-gallon fertigations, the vines extended twice as far and yielded about 60 pounds per hill. And the squash tasted good.
Some plants got fertigated with an equal mixture of liquid seaweed and liquid fish emulsion. Others were fed soluble chemicals. Both approaches worked, but the chemicals worked better. At that time I thought the reason why was that chemical fertilizers offer far more phosphorus to the plants. Now I understand that fertilizers like Miracle-Gro and Peters are closer to being in balance regarding all 11 major plant nutrients than any honestly labeled combination of fish and kelp could possibly be.
Next year I grew only one garden with irrigated, intermediate, and dry areas. Water-loving species like lettuce, Chinese cabbage, and celery were assigned to an adjoining pair of fully irrigated raised beds 4 feet wide by 100 feet long. These two beds occupied about 15 percent of the garden. The rest was in long rows four feet apart. The two long rows closest to the sprinklers got enough overspray to mean something. The remaining area was either given no water at all, was fertigated, or in a few cases, was foliar fed. Everything worked! Many species grew surprisingly well. At the end of that summer I wrote a little labor of love about these experiments called Water-Wise Vegetables.
Dry gardening is possible because the plants draw upon last winter’s rainfall stored in the subsoil. Leached Cascadian subsoils are not fertile enough to grow vegetables, but many of them hold a lot of moisture and some allow root penetration. Even if that subsoil is so compact or clayey that roots cannot survive in it, the moisture it contains still rises up slowly and replaces some of the water lost from the topsoil. Fertigation increases subsoil fertility and supplements that moisture. Fertigation enormously increases yield while using very little water.
If you doubt that vegetable crops can produce without watering, please go blackberry picking in August. Go where exhausted fields are fattening feeder calves on protein- and mineral-deficient grass. Notice that blackberries grow much better in some places than in others. Deep, open, moisture-retentive soil can be located immediately; that’s where blackberries grow huge and lush. You’ll find blackberry vines 7 feet tall covered with big, sweet berries; you’ll find a lot more patches only 4 feet tall with smaller berries that may taste okay; you’ll find places where stunted canes yield nothing worth picking. These differences are mostly an indication of how much soil moisture is in storage.
Now ask yourself how the forest survives rainless hot summers.
Imagine keeping a Cascadian garden entirely bare all summer by repeatedly hoeing it from March 1 through the end of August. On March 1 we scientifically measure the amount of water being held in a slab of subsoil starting 1 foot below the surface and going down to 2 feet. It’ll be at capacity, holding all the moisture it can. Now suppose it proceeds to be a typically hot, entirely rainless summer. On September 1 again we measure the amount of water in that foot-thick subsoil layer. Most people would reason there would be little water found in the soil no matter how deeply we dug.
But that is not at all what happens! The hot sun does dry out the surface, but if we dug down 6 inches, we would encounter slightly damp soil. Go deeper and there will be a lot of moisture. When topsoil loses moisture, it is slowly replaced by the uplift of subsoil moisture. However, the drier the soil gets, the slower subsoil moisture moves upward. If the top few inches get dry enough, moisture uplift stops almost completely. Frequent weed cultivation makes the surface inch get so dry and loose, it acts like mulch.
I suggest making a quick study of the root system drawings in Weaver’s Root Development of Vegetable Crops (see Additional Reading, this page). This free downloadable book makes it instantly apparent that the underground parts of most kinds of vegetables can be more extensive than the top. You’ll realize from Weaver’s drawings why spacing corn plants on a 4-by-4 foot grid provides each plant with sole access to all the soil its roots can reach. You’ll discover which vegetable species make weak root systems. And perhaps you’ll see why subsoil moisture can sustain widely separated vegetables through long, hot rainless months.
A field’s moisture-supplying ability depends on depth of soil, on the subsoil’s clay content, and on how much root penetration that subsoil permits. Theoretical calculations won’t help you much; to find out if your site can support dry gardening, you’ll have to run a performance test. I can assure you that there’s a lot of water in storage if your soil is more than 3 feet deep and the subsoil contains at least 25 percent clay. If it is less than 3 feet to bedrock, or if it is sand, you can still use dry-gardening techniques to reduce the frequency of irrigation.
In the best of circumstances the upward movement of subsoil moisture will not fully support high-density crops. I suppose that’s why intensive gardening and industrial vegetable production completely ignore the possibility. However, when given sufficient uncontested growing room, plants can forage enough moisture to support themselves through long rainless periods, even for months. If you can irrigate a dry garden, once every three or four weeks spread enough water to recharge the subsoil 3 or even 4 feet deep. Then the soil supplies the plants better for a long time.
Modern vegetable varieties have been bred to make big yields from high-density irrigated plantings. Genuine heirlooms and some commercial varieties in use before 1970 direct more of their energy budget toward root formation. These varieties usually grow for more time before maturity and should be provided with more room to do that. Chapter 9 provides what little I know about specific varieties.
The amount of water a crop transpires is determined by the nature and density of the leaf cover, amplified by wind and sun. In these respects, the crop functions like an automobile radiator. With radiators, the larger the metal surfaces, the colder the air, and the higher the wind speed passing over the cooling fins, the better the radiator works. In the garden, the more leaf surfaces; the faster, warmer, and drier the wind and the brighter the sunlight, the more water is transpired. Where there are no plants growing and a dust mulch has been created, little subsoil water will be lost. If a thick leaf canopy develops in summer, the rate of water loss will approximate what I recommend be added through normal summertime irrigation.
On sandy soil a crop with a full leaf canopy can experience moisture stress the day after being irrigated if it has been hot, sunny, and breezy and the humidity has been low. On a clay soil that same crop could comfortably grow through four or more days of such heat. But if those plants were given more growing space, they might grow entirely unstressed for an entire week without irrigation. And if that crop was separated enough that no leaf canopy developed, such that a considerable amount of bare, dry, dust-mulched earth were showing, this apparent waste of growing space would result in an even slower rate of soil-moisture depletion. This is dry gardening in a nutshell. And this is how to garden without having to irrigate so frequently in the same nutshell.
VEGETABLES THAT MUST BE IRRIGATED
Bulbing onions (for August/September harvest)
Cauliflower (except overwintering varieties)
Celeriac
Celery
Chinese cabbage
Lettuce (for summer harvest)
Winter radishes (spring radishes grow on rainfall)
Scallions (for summer harvest)
Spinach (spring-sown varieties do okay without irrigation)
Absolute dry gardening requires very deep, open, moisture-retentive soil, and most people don’t have that. But anyone who stands atop more than 2 feet of soil can irrigate less frequently by lowering plant density.
Suppose you have a 100-square-foot bed. You could raise quite a bit of lettuce on that bed by irrigating frequently. Or bravely put only four indeterminate tomato seedlings in that space. I assure you, given adequate soil fertility and a bit of fertigation, four tomato seedlings will thickly cover that space by mid-August. A tomato plant given 33 square feet of competition-free growing room might survive the summer without fertigation while still making a useful yield. But if you could provide each vine with 5 gallons of fertigation every two to three weeks from mid-July through August, you would harvest five times as much.
I suggest digging a 3-foot-deep hole in your garden. Evaluate what you find against the table in the first part of this chapter, “Available Moisture (Inches of Water per Foot of Soil).” If you discover a plow pan, it should be broken up by deeply digging the entire garden. Otherwise, your plants will be forced to form roots in the topsoil only. If there is airless clay beneath a shovel’s depth of topsoil, I can’t advise you for sure. This might prove to be a great site for dry gardening. Then again, it might not. To find out for sure, conduct a trial.
