You wouldn't think that the dead of winter would be a good time to sow seeds. But it is, for plants whose seeds need some kind of long-term treatment before they will sprout.
Such is the case for the tree peony seeds I recently planted. Well, "planted" may not be quite the right word. After soaking them in water for a few hours, I merely tossed them into a plastic sandwich bag with a handful of moist potting soil. The bag will sit on the kitchen counter for a couple of months, then go into the refrigerator for a couple more.
HOLDING BACK FOR A REASON
Such treatment is needed because tree peony seeds must lay down roots before any shoot growth takes place. To grow, the roots need some rain (or a good soaking) to leach out inhibitors, and some warmth.
The shoots, however, won't sprout until they've been exposed to a period of cool, moist conditions -- outdoors or in my refrigerator. Under natural conditions, all this might take two years. In my house, all systems should be go by spring.
Lily and viburnum seeds also respond to this treatment.
For tender young seedlings, a reluctance to sprout as soon as they hit moist soil often makes sense. In a climate characterized by cold winters and periodic drought, a wild tree peony seedling does well not to rear its head until it's sure that winter is over and it's got the support of an established root system.
DORMANCY REFLECTS NATURAL CONDITIONS
Other germination quirks reflect other natural environments. Some seeds have a double dormancy, one for the seedcoat and one for the embryo. Still others -- goldenseal, for example -- ripen with underdeveloped embryos. That warm then cold treatment also prepares seeds with either of these quirks for germination.
In places with consistent moisture throughout the year, it is winter cold that would snuff out any young sprout that began growing in the fall. So seeds of many of our native plants won't sprout until they feel that winter is over, a condition that could be mimicked by a couple of months in the refrigerator in a sandwich bag along with moist potting soil. While doing time in the refrigerator, it's not unusual for a whole batch of seeds to sprout in unison, as if a switch has been turned on, even before they're released into warmth.
Hormones in seeds are what bring them to life at the appropriate moment. Although the seeds appear to be lying lifeless in a bag on a refrigerator shelf, all sorts of things are going on hormonally. Levels, for instance, of a germination inhibitor called abscisic acid are decreasing, while levels of another hormone, gibberellic acid, are increasing. These hormones have been extracted from seeds or synthesized, and some seeds shed their normal reluctance to sprout with nothing more than a dip in an appropriate concentration of gibberellic acid. All is not so simple, though, because other hormones also are at work and other compounds, such as potassium nitrate, can, for unknown reasons, also promote germination.
Let's not blame dormancy only on hormones; some seeds stay asleep for purely mechanical reasons. The tough seedcoats of honeylocust, black locust and black cohosh are among those that cannot imbibe water as soon as their seeds hit the ground. And a seed that remains dry inside will not sprout.
In nature, these tough coats are eventually softened as soil microbes chew away at them, by cycles of freezing and thawing, by abrasion and by passage through animals. Microbes work best at warm temperatures, so a couple of months in a sandwich bag along with some moist potting soil could awaken these seeds just as they do those of tree peonies.
The potting soil, in this case, should contain some real soil or compost to supply living organisms to work on the seedcoats. Nicking seeds with a file or carefully nipping at them with a wire cutter also lets water penetrate.
The easiest pretreatment is that needed by many grasses and most annual flowers and vegetables. Seeds of these plants need nothing more than a period of dry storage of from one to six months before they'll germinate. Cold is not needed but does keep them fresh longer, so my vegetable and flower seeds are waiting out winter sitting in airtight plastic boxes in my garage.