“I have great faith in a seed,” wrote Henry David Thoreau. “Convince me that you have a seed there, and I am prepared to expect wonders.” Faith in a seed—is there any truer embodiment of nature’s brilliance? Within this miraculous package is a miniature leaf and root, an embryo of a plant that we may never have seen before.
The seed is a perfect time capsule: it can remain viable for days, years, or even centuries, snug in a water-resistant coat. Common wisdom has long held that when the light, temperature, and moisture are just right for germination, dormancy ends and the embryo stirs. But dormancy is not really the right word for this state. Unlike established perennial plants that have elaborate mechanisms to allow them to cope with seasonal extremes such as leaf loss and underground hibernation as a root or tuber, seeds are not taking a siesta from active growth; their lives as green plants have not yet begun.
A seed may seem to be “sleeping” while it waits to germinate. But unlike dormant hardy plants, the seed’s rest does not end when the daylight hours increase in length or the snow melts. Water plays a large role in germination —quick-sprouting zinnias, marigolds, and foxglove all germinate soon after they have absorbed moisture in the indoor propagation area or in warm weather outdoors. However, these same seeds don’t germinate while they are still held in seed heads —even if they are drenched by the hose or summer rains. Nature has built into each seed germination inhibitors that prevent premature sprouting —encrypted codes that only specific conditions can decipher.
Pepper seeds rarely sprout inside their moist and sun-warmed vessels, although you may have cut a fruit open to find a dislodged seed sprouting as it free-floats inside the fleshy chamber. Dr. Norman C. Deno, professor emeritus of chemistry at Pennsylvania State University, has devoted years to studying methods of counteracting the chemical devices that suppress germination. He hypothesizes that the seeds inside of a pepper may receive chemical inhibitors through the fine thread that holds them in place, because pepper seeds often germinate as soon as they detach. His studies have also shown that a lack of oxygen may be an inhibitor, perhaps explaining why seeds enclosed in other fruits do not germinate until after dispersal.
In nature, chemical and physical inhibitors are automatically destroyed as part of the life cycle. Natural “conditioners” can range from simple changes in moisture to complex chemical shifts relating to weather, or even symbiotic relationships with animals, fungi, or plants. Dr. Deno suggests that instead of thinking in terms of trying to break seed dormancy, we consider “conditioning” seeds instead. The propagator’s task becomes more than just adding water or heat; she or he must eradicate these ingenious mechanisms just as nature does. Giving thoughtful consideration to a particular plant’s native habit and environment, we can deduce the necessary procedure for conditioning its seeds and sowing: indoors or out.
For example, many seeds from cold climates have germination inhibitors to ensure that the seeds don’t sprout until spring, when the weather settles and mild temperatures are guaranteed. However, storing dried seeds in envelopes in the refrigerator should not be confused with an artificial winter. Often cold-climate seeds need a period of damp cold followed by a prolonged period of warm and moist conditions before they will germinate. Other seeds have nearly impenetrable seed coats and may not germinate for years until their outer defenses have broken and water can reach the embryo. To initiate germination for growing these seeds indoors, the propagator must damage or scar the seed coats so the endo-carp and embryo can absorb moisture.
Seeds that are easily cultivated from plants that grow well year-round in your area might best be sown outdoors so that they can be conditioned naturally. However, precious seeds and those that pose specific challenges for germination might benefit from the intervention of the gardener-cum-alchemist.
Dry sweet pea seeds are soaked for 24 hours to condition them prior to sowing.
Ripe crabapple seeds must not germinate inside their fruits in winter, and the ice that covers these is part of the conditioning process. Freezing breaks the cells of the fruit, beginning its disintegration. Cold also destroys the inhibitors, so when the seeds find warm soil, moisture, and sunlight, they will germinate and grow.
Seeds have chemical and physical inhibitors that prevent them from sprouting prematurely. Chemicals may be supplied to seeds, such as the pepper’s, through the filament that attaches them to the fruit wall. When the seeds are liberated, germination will be possible.
Although seeds may appear to be inert, they, like every other organism, have a life span. Commercially packaged seeds of vegetables or flowering annuals are labeled with their packing date and should be used in the year packed. Most vegetable seeds can remain viable for from two to fifteen years, depending on the type and conditions of storage.
If you have annual and vegetable seeds left over from just last year and have stored them in their packets, folded shut and taped closed, in the jar in the refrigerator, you can risk sowing them at the appropriate time. But if they were not stored in optimal conditions or have been stored for two years or more, a viability test could be worthwhile.
Convention suggests that you use ten of each kind of seed to determine the viability percentage. Space the seeds evenly in the center of a paper towel and fold over the sides so that the seeds are completely covered. Dampen the towel with a spray bottle, and slip it into a plastic sandwich bag with a zipper lock. Place the package in a warm spot —about 75 degrees F (24 degrees C). The flat top of a water heater, away from the vent, maintains this temperature quite well.
The seeds of most annuals will sprout in three to ten days, but other plants can take longer. Check the package to determine the standard germination time. Halfway through the number of days supposedly required, hold the bag up to a light. If the seeds have life, there will be a recognizable change in their shape. Remove the towel from the bag and carefully unwrap and examine its contents.
If eight out of ten seeds have swollen and distorted, owing to the expanding radicles, 80 percent germination can be expected from this batch of seeds. If only one or two seeds react, consider replacing the entire lot with the closest variety you can find.
I perform the paper-towel test close enough to the normal sowing date so that I will not have to discard the test batch if the seeds are good. I pot the seeds in medium and grow them along with my other late-winter sown plants. If any roots have grown through the paper towel, I cut them out with the surrounding moist paper to minimize root disturbance.
If you are testing annual flowers and fruits with large seeds, you can pot these into biodegradable containers, such as peat pots or recycled-newspaper ones, so they can be planted directly into the garden with the least disturbance. If you use peat pots, be sure to tear as much of the pot away as possible. Peat shrinks, breaking roots, and if the top edge of the pot becomes exposed to dry soil or air, moisture will be wicked away from the developing seedlings.
The paper-towel germination scheme can have other applications, such as for pre-germinating certain species and for two-step germinators —described here.
The result of a viability test of last year’s pumpkin seeds is seen after about a week.
Newspaper can be recycled to make degradable pots with a special form.
The sprouted test seeds do not have to be discarded; large ones destined for a vegetable or annuals garden can be planted in peat pots that will disintegrate.
If fewer than half of the seeds react, consider replacing the entire lot with the same or a similar variety.
Commercial seed sellers happen to provide one method of conditioning, simply by cleaning and drying certain seeds. Drying may be nature’s principal means of destroying chemical inhibitors. Ripe seeds on a sunflower, for instance, do not germinate while they are attached to the seed head because the inhibitors are active. These and other warm-season annuals may require a certain amount of desiccation before exposure to moisture can initiate germination. Generalization is nearly impossible, but it may be that seeds that are exposed to the air and sun while ripening, such as those in the daisy, mustard, and grass families, need a period of drying, or curing, as a precursor to sprouting.
There may be some seeds that need only a short period to dry sufficiently, which occurs in their capsules while they are still attached to the plant. Bob Stewart of Arrowhead Alpines Nursery in Michigan has extensive experience sowing seeds and notes that many ornamental species, however, require an extended time in dry storage. Campanula and Penstemon are two examples where “old seed germinates much better than fresh seed.” This aging could be useful in the case of store-bought perennial seeds or ones received from plant-society exchanges. But as a general practice prior to conditioning or sowing, the dried seeds from perennials with moist fruit should be soaked for at least 24 hours —with the water changed once or twice or the seeds rinsed in a sieve —for a bit of refreshing.
In the last chapter, bending the rules in some cases produced excellent results, such as the fresh sowing of certain deciduous woody plant seeds and others collected just before their fruits were fully ripe. The seeds of warm-season annuals such as marigolds, nasturtiums, and zinnias you harvest from the garden, as well as those of fruits such as tomatoes, cucumbers, and zucchini, or courgettes, need to be first cleaned and air-dried before they are placed in envelopes and stored until the proper day for sowing. Then, these annuals often sprout within days. Chances are good that by the time you are ready to sow store-bought seeds of easy-to-grow annuals, they will be “old” enough.
The inhibitors in many seeds are destroyed during drying and aging. Ripening seeds of cosmos, black-eyed Susan, and purple coneflower (shown), for example, do not sprout in their flower heads, even if they are drenched by rain. The drying process delays the ability to germinate—the approaching winter postpones the opportunity. The seeds will sprout outdoors when spring rains and warm days arrive, or if harvested when those conditions are simulated earlier indoors.
The mechanism for delaying germination is often a moisture-resistant, rock-hard seed coat. The castor bean is an 8-foot-tall ornamental annual (mature leaf shown in framed photo) with mottled seeds (in dish) that have a lacquerlike surface. The coat will have to be breached before seeds can imbibe moisture—the first step toward germination and seedling production. Many kinds of seed might be peeled by hand, such as a lemon’s, but take care with the castor bean: all of its parts are poisonous.
Hard seed coats protect the tender embryos within, but they also prevent germination by excluding moisture. Without intervention, seeds such as those of the perennial pea vine (Lathyrus latifolius) may not germinate for a year, decades, or ever. The ornamental castor bean plant (Ricinus communis and varieties) has a beautiful seed —exquisitely mottled and patterned like snakeskin in tan and maroon. (It should be noted that all parts of this plant, the source of castor oil, are poisonous.) When thoroughly dry, the seeds have a glass-smooth coat as hard as armor.
These seeds will sprout when there is a breach in the seed coat. Nicking or filing, a process called scarification, will allow moisture to penetrate to the dormant embryo, triggering germination. To scarify a good-size seed, you can hold it in your fingers and drag it across a sheet of medium sandpaper, or use a metal file to scrape into the coat, creating hundreds of minuscule entryways on the hard surface. Use caution when doing this, filing only a side or a blunt end, so the embryo is not damaged.
Performing this task seed by seed is not practical when one is working with large quantities, so some nurseries tumble the seeds in bins with sand. Horticulturists at public gardens often rub seeds between two blocks of wood covered with sandpaper to penetrate the coats. At home, you can put small seeds in a jar with about an inch or two of sand, cover, and shake vigorously. Although it is preferable to separate the seeds from the sand, if the seeds are too small to sift or pick out, you can sow pinches of seeds and sand together.
Getting moisture into the seed is the goal of scarification, but water itself can be used to effect this process in some cases. Morning glory seeds are notoriously hard; you can nick or file them by cutting their points off with a single-edged razor blade or dragging them across sandpaper, but they will also soften with a soaking in warm water overnight (mimicking the warm spring rains of tropical climes perhaps). The coats will tear and the seeds will be swollen by the next morning. Sow the morning glory seeds as soon as you remove them from the bath, for, just as with every seed, they must not be allowed to dry out after the germination process has begun or they will perish.
Chemical scarification is used to replicate the caustic effects on seeds of an animal’s digestive juices. Birds and other animals are attracted to certain fruits, which they ingest and break down. The seeds are deposited in a new location, their seed coats damaged enough to allow the seeds to absorb moisture. In laboratories, plant scientists experiment with acids. Sulfuric acid, for example, is used to etch the coats of certain species of roses, witch hazel, and linden.
Professional propagators also use hormones such as gibberellin (in the form of gibberellic acid) to stimulate germination in other types of seed. Gibberellic (GB) acid destroys abscisic acid, the primary chemical germination inhibitor. GB is available to the home gardener from specialized sources, but the techniques involved in using it require a great deal of precision, so you should consult references before beginning any of these procedures (see the Bibliography).
The hard coats of other seeds, such as legumes (members of the pea family) and pines, can be cracked with nearly boiling water. In one experiment with the seeds of the honey locust tree, water heated to 170 degrees F (76.5 degrees C) was poured over the seeds and let stand overnight. These seeds were planted in one pot, while untreated ones were sowed in another. The pots were given the same culture from that point on. None of the untreated seeds germinated, while all of the seeds immersed in hot water pushed through the medium after a week —a higher success rate and months sooner than germination in a natural environment, where the repeated freezing and thawing pattern of winter breaks the seed coat.
Extreme heat is used in nature as well to weaken seed coats. When fires swept Yellowstone National Park several years ago, some seeds not only survived, but sprouted and thrived; they needed the exposure to intense heat to break open dried fruits or cones, or to crack hard seed coats. With the fire, the resins that sealed the cones of Pinus radiata melted, the cones opened, and seeds fell out onto cleared land where they had the chance to grow.
Seeds of certain plants lie in the duff of the forest floor or the sandy soil of scrubby areas waiting for fire and smoke to destroy their germination inhibitors. A few suppliers of specialty seeds, such as Rhodocoma capensis from Australia and Erica glauca from South Africa, include bits of paper impregnated with the same compounds found in brushfire smoke. Before the seeds are sown, they should be soaked along with the paper to absorb chemical reminders of their environmental heritage.
Morning glory seeds are used for a demonstration of nicking using a single-edged razor blade to cut a chip off the seed’s pointed end. The seeds can also be filed by dragging the point across sandpaper or, when there are many seeds, by mixing them with sand in a jar and shaking vigorously. Morning glory seeds can also be soaked in warm water for 24 hours, which will soften their coats and plump them for immediate sowing.
Most legumes—members of the pea family—have hard coats. Very hot water (160 to 170 degrees F [71 to 76.5 degrees C]) can be poured over some of these seeds, such as those of the honey locust tree (Gleditsia triacanthos). The water may simulate the effects of winter freezing and thawing to crack the seed coat. In an experiment, twenty-four ripe honey locust seeds were harvested from their pods in autumn. Eight were sown outdoors in a bed near the place where the fruits were found. On April 12, eight seeds were sown in pots and placed under fluorescent lights, while the third set were treated in hot water, left to soak for 12 hours, and then sown.
Within ten days, the treated seeds began to push through the medium.
In ten more days the eager sprouts had become strong seedlings. In late June, six of the outdoor seeds sprouted. The untreated indoor seeds never germinated and were discarded three months later.
Even though the majority of seeds are covered when they are sown, whether indoors or outside, most seeds do not require darkness. Covering the seeds, however, definitely helps to keep them moist, and covering them is a good practice for all but the specific seeds that require “surface-sowing” and light.
Light, however, is another one of nature’s ways for destroying inhibitors. Fern spores and many tiny seeds that float lightly to the ground, or the seeds of epiphytic plants, which spend their entire lives above the soil in the crotches and branches of trees (bromeliads and orchid cacti, for example), need light to germinate. This type of conditioning is accomplished by sowing seeds on a moist medium or sprinkling them over a top-dressing of grit and not applying any additional covering.
Heat for conditioning commonly calls for moisture and warmth —perhaps simulating a natural state for hardy-plant seeds that ripen in summer. Moist-warm stratification takes place around 70 degrees F (21 degrees C) —far below the temperatures of a desert summer. Certain references say that exposure to light is a prerequisite for cacti germination, but in nature, these seeds need shelter from searing sun in cool, protected spots, for instance, beneath Opuntia pads, where juveniles over fifteen years old may be found.
Many of the agents that repress germination in hardy plants’ seeds are naturally eliminated through the seasonal stages of chilling and warming. These seeds will not germinate until they have experienced the cold of winter and the spring thaw signals that safe temperatures are close at hand.
In horticulture, the term stratification is used to indicate that moisture and controlled temperatures are required before germination. Many references use the term when recommending a “pretreatment” with cold. But it is worth noting that warmth with moisture may also be part of the stratification process.
One way to sow and germinate seeds of hardy plants for gardens is to let nature provide the necessary temperature fluctuations outdoors, as described in the following chapter. But in certain cases, providing controlled environments with specific temperatures could have advantages over the vagaries of weather. For example, when there are precious few seeds of a plant, risks are limited by mimicking nature’s forces. Artificial temperature controls could also shorten the period needed from sowing, to germination, to having garden-ready plants. When seeds that ripened through the summer are stored and sent from commercial sources, or a society seed exchange, in February or March, six months of what could have been the natural conditioning process are gone.
The first thing to do with seeds that originated in the moist fruits of perennial plants and that arrive dry in envelopes and packets is to soak them for at least one day, and up to three —changing the water once or twice a day, or rinsing them in a sieve. This reconstitution is one step toward catching up on time lost. More time in the life of a plant may be reclaimed, however, through accelerated indoor treatments. Experiments have shown that the artificial winter can be shortened in the refrigerator. Some seeds will have gained the benefits of cold after as little as three weeks.
Recent research has also demonstrated that freezing —which may occur outdoors and is recommended by several sources —is rarely necessary. Seeds that are subjected to cold may not be damaged by temperatures below the freezing point, but the conditioning process is put on hold at these low temperatures and resumes only when the seeds are not quite so cold. This is one reason that controlled stratification in the refrigerator can be a truncated version of the outdoor process.
By now it’s clear that the most important piece of technical equipment for performing the operations of stratification, the refrigerator, is already possessed by amateur gardeners. Moisture and a constant temperature of 40 to 45 degrees F (4.5 to 7 degrees C) works best. Seeds brought out of the refrigerator to a warm environment may sprout right away, offering an opportunity to get a huge head start on the growing season. The outdoor-sown seed might just be appearing by the time the cold-treated seedlings indoors are large enough to transplant in the garden.
Another benefit of using the refrigerator for stratification, followed by germination and development in protective custody, is that the process can be closely monitored and yields will be high. For a plant such as an oak tree, no acorns will be stolen from the refrigerator shelf, as they might be in the garden.
To moist-cold stratify clean seeds indoors, place them in a wad of whole sphagnum moss that has been dipped in warm water and wrung out as much as possible. Small seeds, which would be difficult to pick out from the moss at sowing time, can be mixed into a little clean, barely moistened vermiculite to sow with the seeds at the appropriate time. Or the seeds may be sown (here) prior to chilling. The moss, vermiculite, or pots with seeds next go into plastic bags labeled with their name, dates of moist-cold conditioning, and when they should move to the next step of germination. Seal the bags tightly and place them in a prominent section of the refrigerator.
Here’s an example: I saw a tree with wine red leaves growing in a private garden on the “Point” in Newport, Rhode Island. I recognized the shape of the leaves as being those of the peach (Prunus persica)— and sure enough, the branches bore small, fuzzy, maroon peaches. So began a fruitless search to find a source for this ornamental tree. I discovered a reference to ‘Royal Red Leaf’ (‘Follis Rubus’), an old variety once used as an understock since it would be easy to notice and remove purple suckers that sprouted below the graft.
In 1996, I met the peach again in a Pennsylvania garden, and this time I did not hesitate to ask if I could gather fruits from the ground. After I cleaned the “stones,” I sowed half of them in a pot of sowing mix in a sunny window, while the other half went into the refrigerator for sixty days to then be potted and placed next to the first. All of the treated seeds came up. None of the untreated seeds sprouted.
Cold is one of the most common factors in conditioning. If seeds of hardy herbaceous perennials, shrubs, and trees sprouted as soon as they were ripe in autumn, the seedlings would be killed by frost. However, as noted, there are chemical inhibitors present in the seeds of hardy plants that delay germination. Exposure to cold disables the inhibitors. The process of subjecting seeds to periods of moisture and cold (or moisture and warmth, or both) is called “stratification.” As with most conditioning, the payoff is more seedlings—and sooner.
In propagation, we often have to mimic natural circumstances, and in the case of the jack-in-the-pulpit to “trick” seeds into reacting as if they have passed through a winter. The cleaned seeds are placed on moist sphagnum moss.
This is folded over and slipped into a labeled plastic bag, then put in the refrigerator at 40 degrees F (4.5 degrees C) for six weeks to two months.
The conditioned seeds sprout soon after they are sown.
These red-leaved seedlings were the results of a long search for an old peach variety used for grafting—a red sucker would be easy to spot and remove—but a nursery source could not be located. Fourteen peaches were collected from the ground beneath a rare specimen, soaked overnight, and washed of pulp. Seven “stones” were sown and placed in a south-facing window. The rest were cold-stratified for sixty days prior to sowing. The cold-treated seeds germinated quickly; the others did not, except for one—which appeared after two more winters next to the compost pile.
Some plants have what is often called a “double dormancy.” Dr. Norman Deno refers to plants that need more than one period of conditioning as “two-step germinators.” Seeds that need only one conditioning begin the germination process when the radicle, or immature root, emerges from beneath the seed. Germination is complete after cotyledons, or seed leaves, slip up through the sowing medium, spread open, and turn green as photosynthesis begins. With two-step germinators, there is a resting period between the emergence of the radicle and the cotyledons; in nature, it occurs during the first or second winter.
Species peonies are exquisite plants —but unknown to most gardeners, and rarely available as plants from nurseries or catalogs. The only way to grow these may be from seed. About half of all lilies are considered epigeal, and produce leaves from seed quickly, such as Lilium formosanum. The other half are hypogeal—two-step germinators —and produce a leaf only after a period of cold. The process can be shortened using the refrigerator.
Species peonies take two steps toward germination. During warm-moist stratification, an immature root emerges; then there must be cold followed by warmth to continue germination. In nature, it could be two years before a leaf appears, but with warm and cold stratification, leaves may show in six months.
By then, one of the original trees, planted behind tree stumps in a shrub border, had reached 7 feet, and fuzzy, maroon peaches were greatly anticipated.
