The intelligence of market gardeners has been applied to means of forcing nature to produce in the middle of winter, in the middle of the frost, what is ordinarily produced only in the warm spring and summer weather; it is in this area that the science of the gardeners of Paris has revealed itself to be truly astonishing.
— Manuel pratique de la culture maraîchère de Paris, 1845
OUR GROWING SEASON in Quebec is short, and we therefore must take every means necessary to produce as much as possible in the little time we have. In this climate, our task as market gardeners is to find ways to control the growing conditions, in order to protect our crops from the cold and frost in both early spring and late fall. In this regard, I should clarify at the outset that when I talk of “season extension” in this chapter, I am referring to using unheated and minimally heated tunnel-like structures along with a whole set of simple and economical technologies and techniques that help growers force crops and defend them against harsh weather.
The idea of forcing crops to grow out of season is certainly nothing new, but doing so usually equates to heating greenhouses. In the last 50 years — during the era of cheap fuel — high-tech approaches to greenhouse production evolved tremendously, but so did the complexity and cost of these systems. In the northeast United States, and other regions where demand for local products is increasing, many small growers have set out to find simpler, less energy-intensive ways to get earlier harvests and extend their season into the winter. These exciting innovations are becoming better documented, and the body of knowledge about using hoophouses for cool-weather crops has grown each year. At the forefront of this trend is Eliot Coleman. His ideas and techniques have helped growers push the extent of their season by combining an understanding of plant biology and low-tech cover solutions. I have seen these methods at work with my own eyes on a number of cold climate farms and can assure skeptics that diversified production from October to May is possible. For those interested in year-round, cold climate production, there is a whole body of literature out there on the subject.*
In the 19th century, French market gardeners used cloches and cold frames to force vegetable growth during the cold season. Today we rely on floating row cover and polyethylene film to do the same thing.
On our farm, for time being, we have decided not to grow vegetables throughout the year. It is not the technical challenges that are holding us back; we just really look forward to taking some time off in the winter. Having said this, we still do force crops, especially in the spring. Over time we’ve come to the conclusion that people are more excited about vegetables in early summer, and our goal is to maximize harvest in June. To do so, we don’t hesitate to use propane gas to heat our tomato greenhouse, but other than that, the methods we rely upon are all passive. These techniques allow us to produce not just earlier vegetables but better vegetables—and more of them. The improved quality and yield are in my view the best reason to adopt season extension techniques.
Floating Row Cover and Low Tunnels
In my opinion, floating row cover is one of the all-time greatest technological innovations in the horticultural industry. It’s made of webs of unwoven polymer fiber, a material that lets air and water through while still acting as a physical barrier against wind and insects. When spread over crops, row cover increases the soil temperature by 2–3 degrees while also keeping in moisture, thereby protecting the plants against frost. Installing row cover over direct-seeded crops speeds up germination; installing it over transplanted crops protects the seedlings from harsh weather such as driving rain, strong wind, and hail. Essentially, row covers can create a microclimate anywhere in the garden, comparable to that of a hoophouse.
Row covers are available with thicknesses expressed in ounces per square yard. The thicker sheets have a higher thermal capacity but block more light. On our farm, we use 0.55 oz/ft2 (19 gr/m2) row covers in spring and fall. These are a suitable compromise between durability and light transmission (they obstruct about 15% of the rays). We also use heavier ones, which are about twice the thickness, over fall crops to act as a thermal screen on frosty nights.
In the spring, our entire production is under row cover. For direct-seeded crops, we spread the row cover directly on the ground, leaving a little slack to give the crops some room to grow. For transplanted crops, which are more fragile, we support the floating cover with hoops made of 9 gauge galvanized steel merchant wire. The wire is cut into five-foot lengths to create semi-circles that straddle our 30-inch beds with enough inside clearance for most crops. The hoops are pushed into the soil and spaced every 3 feet along the row.
For broccoli, summer squash, and other crops that grow tall, we use larger hoops made of ½” PVC plumbing pipe (PEX type) cut into eight-foot lengths. We space them every five feet or, if the same cover is shared by multiple adjacent beds, every 10 feet in a zigzag pattern. The hoops are driven into six-inch holes we make with a stake.
Finally, we use other hoops that we refer to as low tunnel hoops. These are especially useful when snow accumulation is still a possibility. They are made from 10-foot-long galvanized electrical conduit (EMT), bought at a local hardware store and bent into shape using a hoop bender. These low tunnel hoops are more expensive to make, but they are sturdy enough to handle heavy snow loads. In early spring or late fall, when snow accumulation is still a possibility, we cover our low tunnels with clear greenhouse film. The low tunnels then offer basically the same advantages as hoophouses but at a fraction of the price.
To hold down our row covers (both polymer fiber and plastic film), we bury the edges with dirt taken from the pathway, or if available, we use sandbags placed at the bottom of the hoops. To ensure that the bags will last a long time, it’s worth investing in the UV-treated bags. When installing row cover, we always make sure to stretch it tightly so that the fabric does not beat in high winds. Unfortunately, row cover is always at risk of being whipped up in the spring on windy sites like ours. The only solution we have found is to check regularly to make sure the weights are doing their job.
For three winters, we grew spinach, a very cold-hardy crop, in an unheated tunnel protected by floating row cover at night. Baby spinach could easily be called the queen of winter crops.
When we are done with the row covers, we stuff them into old grain sacks which we label according to the width and condition of the row cover. At the end of the season, one of the last chores we do is to try and patch up the ones with holes using UV-treated sheeting tape. Most of our row covers last approximately 3 years.
Floating row covers are effective, easy to install, and affordable. We attribute the success of many of our crops directly to these materials.
Caterpillar tunnels are inexpensive compared to permanent hoophouses. They provide shelter, warmth, and air flow to the growing environment.
Caterpillar tunnels are another great option for season extension. In short, a caterpillar tunnel is an inexpensive and simpler variation of a hoophouse, with the interesting feature that it is mobile. Because its structure is easy to set up and take down, it can be moved over established crops at different times in the season. We use ours to force early direct-seeded carrots and beets, and then move it over our Solanaceae crops, which always love the extra heat in the summer.
Caterpillar tunnels can be built using any number of techniques and materials. The more basic option requires nothing more than PVC pipes, rebar, and rope. We built ours with 1½” PVC piping, made up into 20-foot lengths using two pieces joined together firmly with a glued union. The hoops are spaced every 10 feet and anchored to the ground on 24-inch rebar (⅝” diameter) driven halfway into the soil. We tie a rope from hoop to hoop to form a purlin that runs the length of the tunnel. The rope is then firmly stretched to give rigidity to the structure and tied to stakes at both ends of the tunnel. The covering (we use old greenhouse plastic) is then pulled over the tunnel and held to the ground by sandbags. To keep the plastic from flying away on windy days, we stretch rope over it midway between each arch and tie the rope down to stakes in the ground, giving the tunnel its caterpillar-like appearance. Ventilation is controlled by simply rolling up the entire side of the tunnel, keeping it in place by hooks attached to every other arch.
In 2005, we spent about $400 in materials, not including the used greenhouse film, to make a caterpillar tunnel that covers an area of 10 feet by 100 feet. Providing cover for 6 beds (the tunnel covers 2 beds and is moved 3 times in the season) for an investment of $.33/ft2 is more than a bargain. The only downside of caterpillar tunnels is that they are low structures, providing little clearance to properly work in an upright position. Entering and leaving the caterpillar also involves a lot of bending.
A hoophouse (also referred to as a polytunnel, high tunnel, or tunnel) is a permanent structure made of semicircular steel hoops bolted down and covered with a plastic film. Unlike greenhouses, hoophouses are relatively low, easy to build, and usually not heated. The hoophouses I mention here are different from the big tunnels commonly used in large-scale vegetable operations: they are generally 16 or 20 feet wide and approximately 10 feet tall at their peak. A hoophouse can be built to any length and even extended by adding new sections—a useful feature for beginning growers who may expand operations in future years.
One of the main advantages of hoophouses is that they can be used all year long.* These shelters can be used for early spring and late fall crops, before and after heat-loving summer crops. If seeded at the right time, cold-hardy vegetables such as spinach and Asian greens may be harvested even in periods of frost. Hoophouses come in many models and are available from most greenhouse suppliers. Since new structures can be expensive—and the price of metal seems to go up every year—many small producers decide to make their own by bending steel pipes with a metal bender. In my opinion, however, the best way to reduce the cost of a hoophouse is to look for a used one. Although this often means dismantling someone else’s old structure and transporting it yourself, it’s often worth the trouble. Whichever way you decide to go about it, building a hoophouse is one of the best investments a market gardener can make. Sales of the crops grown in such a protected environment will pay for the structure within a few seasons, if not the first.
Since caterpillar tunnels are portable, we can reap all the benefits of keeping crops covered without compromising our crop rotation.
Car shelters are easy to find anywhere in Quebec. You can easily attach them together to make an affordable hoophouse.
We currently use hoophouses for growing trellised cucumbers and peppers in the summer. Prior to erecting our greenhouse, we grew our tomatoes in them as well.
Because hoophouses are permanent structures, they must be positioned carefully. You must ensure proper soil drainage beforehand so the beds are dry enough in the spring. Apart from leveling the ground, which requires heavy work in the garden, the best drainage solution is to install weeping tile around the structure. Our hoophouses have roll-ups and big doors at either end, providing the growing area with sufficient ventilation.
Season Extension Preferences of Certain Crops at Les Jardins De La Grelinette
* Eliot Coleman’s The Winter Harvest Handbook is a comprehensive manual on year-round vegetable production. Other American growers such as Steve Moore and Paul Arnold have also written extensively about their successful experiences.
* In very snowy areas, each hoop must be supported with a post.
