Keeping cool: Sleeping plants fend off cold the best

March 12, 2019 GMT

Each warm spell makes the next cold spell feel that much more frigid to me and — more importantly — to my trees and shrubs. These plants would be much better off if the weather would stay cold until it was ready to turn warm in earnest.

This time of year, cold is the only thing keeping most trees and shrubs in their winter sleep.


Such was not the case back in autumn. Short days and cooling temperatures put these plants into a kind of sleep from which they could not awaken until they felt that winter had passed. Plants mark time by the amount of cold they experience, each kind of plant requiring different amounts of cold before it can awaken.


Even with warm temperatures, for example, apple trees will not start growing again until experiencing an accumulated total of about 1,000 hours of temperatures between 30 and 45 degrees Fahrenheit. (It is the amount of cold, not frigid temperatures, that triggers the hormonal changes that jolt plants awake.)

Once that “cold bank” has been filled, warmth can awaken these plants, which is what I fear happening with early warm spells. New growth, being succulent and lush, is especially susceptible to cold. Frost biting at new stems weakens a plant, could even kill it, depending on the vigor of the plant and the timing and degree of cold.

Early flowers are part of that awakening growth. If nothing more, cold threatens to turn colorful petals to brown mush. But dead flowers also cannot go on to become fruits, leading to further disappointment if the hope was for something luscious or again decorative.


We gardeners can make our plants less susceptible to the ill effects of early warm spells.

One way is to grow plants adapted to your region. Even within a species, individual plants vary in their winter cold requirement. A red maple native to Georgia, for example, is more likely to awaken prematurely in New York than would a red maple that is native to New York. New York red maples having “early wakening” genes would have died off or been overgrown by those with genes that kept them asleep longer.

Most of what we plant, of course, is not native or necessarily perfectly adapted to where we garden; we can help by trying to keep such plants asleep longer. Look around your yard and note how frozen snow lingers longest on the north side of your home or on north-facing slopes. Plants on north-facing slopes or on the north sides of buildings similarly feel less warmth, so begin growth later.


Even a tree or shrub that needs sun can thrive on north-facing slopes or near north facing walls. These locations become sunnier as the season advances and the sun rises higher in the sky and wraps further around the horizon.

I’ve even heard of gardeners keeping prized plants asleep by spraying their branches with diluted, white latex paint. The white color reflects the sun’s heat.

Water resists temperature changes, so proximity to large bodies of water or to air that’s travelled over large bodies of water keeps things cooler in late winter and early spring. That’s why gardeners near large lakes, in coastal regions and in western Europe have fewer problems with overly eager plants in the spring.

Before you start packing up plants for a move to coastal Ireland, however, keep in mind that water masses also subdue summer heat, which makes ripening heat-lovers like peppers and cucumbers more of a challenge.