Winter Dormancy and Chilling for Plants

It’s been a weird winter so far in a lot of places. Here in Florida, wearing shorts and flip-flops while Christmas shopping is normal and expected – doing the same in, say, Boston is NOT. And if you had dandelions blooming on your front lawn instead of grinning snowmen throughout December, you weren’t alone. But in most places, temperatures have finally dropped to something a little more usual, and while you might be shivering, your garden is actually thankful. Some of your favorite plants need winter dormancy and chilling in order to bloom when spring arrives.

Dormancy is basically the time when a plant stops active growth. It’s a way for plants to defend themselves against harsh conditions like snow and ice, but the process of dormancy begins long before winter starts. In the late summer, as days begin to grow shorter, plants begin preparing for winter dormancy. They slow down their growth, sometimes setting buds for the following spring, and shed their leaves or die back to the root. By the time cold weather arrives, the plant has shut down and is ready to wait out the rough weather ahead.

As it turns out, though, that rough weather is absolutely necessary. Many plants require a period of chilling to break the dormant stage. In other words, they need a certain number of hours of temperatures between 32 and 45 degrees F. Once the plant has experienced its necessary amount of chilling time, it will be ready to resume growth and spring flowering (known as vernalization) when the weather warms up again for good, rather than becoming active too soon due to an unexpected warm snap in January. For instance, most apple varieties require 1,000 chill hours on average, so warm days in December before they’ve had this chilling won’t usually cause them to start flowering.

Not all plants need dormancy and chilling, but for those that do, it explains their inability to grow well in warmer climates. Here in Central Florida, I miss flowering lilacs (Syringa vulgaris) desperately each spring, but it would do me no good to try to grow them. Lilacs require a chilling period each winter in order to flower in the spring, and Central Florida just doesn’t get cold enough. Any lilac planted here would most likely put on growth for a year or two, but it wouldn’t flower, and eventually would simply falter and die.

So if the weather has finally turned cold in your part of the country after the unusually warm November and December, it’s actually something to be thankful for. When warm temperatures arrive again this spring, proper winter dormancy and chilling ensures you’ll have lilac flowers, apple blossoms, and tulip blooms galore!