The Big Chill: What cold is good for

A period of cold is essential to many plants. We call it winter. — Claricethebaker/Creative Commons

Bitter temperatures, biting winds and the odd surprise snowfall should be enough to remind you that, wishful thinking aside, winter will have its way with us.

As you button up your coat and pull on your gloves, you might imagine that there’s nothing good about the season of frost and cold. From a gardener’s point of view, you would be wrong.

Just as we need sleep to recharge body and soul, plants of our climate are programmed to slip into a period of rest we call dormancy. If you want continuous growth, my chilly friends, you’ll have to move to a lower latitude — and there would be trade-offs.

In tropical and subtropical locations, the weeds never die, the bugs never cease and the gardener gets no reprieve. Moreover, you’d be doing without a host of beloved plants that need a prolonged period of cold to trigger flowering, daffodils and lilacs to name two.

New research has identified the crucial gene that delays flowering until after a period of cold. The gene — and here’s proof that scientists have a sense of humor — is known as FRIGIDA (FRI, for short) and the process is known as vernalization, “vernal” meaning spring. Perhaps the most familiar example of vernalization occurs among the spring bulbs.

Crocus, fritillaries, snowdrops, hyacinths, bulbous irises, daffodils and tulips need sustained cold for 12 to 17 weeks before flowers will form. This is why you don’t see these plants growing in the deep South, unless gardeners have refrigerated the bulbs for the required amount of time. Of the familiar bulbs, only the South African amaryllis and the Chinese paperwhites don’t require chilling.

Flowering trees and shrubs also need the Big Chill. Bring a leafless branch of dogwood or forsythia into the house in autumn, right after leaf fall, and you get . . . nothing. But wait until late winter or early spring, after cold temperatures have broken the dormancy of the buds, and you’ve got blossoms aplenty.

Apples need cold of 40 degrees or less for 250 to 1,700 hours, depending on the variety; blueberries need 150 to 1,200 hours; and raspberries 800 to 1,700. Do you really want to be without these fruits? They don’t grow in the land of papaya and banana, where perpetual warmth is the rule.

Many seeds (including maple) need cold to germinate. — Philipbouchard/CC

Seed dormancy is another issue. It would do trees, shrubs and perennials no good at all if their seeds germinated as soon as they matured and fell to earth. Tiny seedlings, without much in the way of root systems or stored foods, would almost certainly perish in their first winter.

Many garden plants, and most species native to the northeast, get around this problem neatly by producing seeds that will not germinate until they are exposed to a prolonged period of cold, commonly known as winter.

Gardeners can overcome this inhibition by providing artificially both moisture and cold, mixing seeds with damp peat and then refrigerating them — a process called stratification. The same thing also can be accomplished by sowing seeds in the ground or in a cold frame in autumn to await their germination the following spring.

You might think that extreme winter cold might help knock down populations of insects, but these wily, cold-blooded creatures are all too good at surviving winter in bark crevices, leaf litter or underground, where temperatures remain much warmer than on the surface. Some good news: Dog owners swear that fleas and ticks are reduced by real cold, and boll weevils can be killed off by sustained freezes (a big whoop for northern, non-cotton-picking gardeners).

It’s actually severely hot and dry weather that does the bugs more harm. You can pray for another blistering summer — but until I get central air conditioning, I’d rather you didn’t.

There are few experiences that fail to yield some fresh insights, and a blast of hard cold is no exception. This is an excellent time to learn more about the lay of your land, to mentally map the cold spots where frost damage is likely and the warmer micro-climates that offer plants a little more protection.

You might think that the coldest places in any yard are the ones most exposed — sites at the top of a hill, open to the wind and weather. Actually, the opposite is true. Cold air, being denser than warm, flows invisibly across the terrain and seeks out the low-lying areas of the garden. Cold, still air can be trapped by fences, hedges or thick evergreen shrubs creating “frost pockets.”

In your yard, it's the low spot where the landscape dips down to that stone retaining wall or band of yews that will be the coldest. Any barrier that prevents cold air from flowing lower still will accentuate the affect. Although it's counter-intuitive, a breeze helps move the coldest air along — which is why some dedicated growers trying to prolong the growing season will station a fan in the garden to blow off a killing frost.

Conversely, anything that remains warmer than surrounding air can provide a measure of protection. Plants tucked up against your house will enjoy the protection of that expensive heat leaking out from even the tightest modern construction.

Just two reminders in the nature of a public service announcement: It’s time to make sure all the storm windows are pulled down tight and to shut off outdoor water taps lest they freeze. Batten the hatches, my friends. Old Man Winter is on the way.