What season are we in? It seems to vary day to day. We yo-yo in and out of winter and spring, with a day or two of summer thrown in. If you’re a plant, you must be very confused. Should you start growing, or remain in hibernation. Will you begin sprouting, only to be “nipped in the bud” by the next deep freeze?
What effect will this yo-yo cycle have on trees, shrubs and perennials in our yards? When weather warms in mid-winter or late winter, do plants think it’s spring, and start to grow, only to be killed when temperatures plunge? Luckily, not necessarily.
Trees, shrubs and perennials are smart. They don’t rely on calendars and they aren’t easily fooled by false late winter warmups. Instead, they have “brains” that tells them when to stop growing for the season, and when to restart. Trees, shrubs and perennials have the good sense to go dormant in the fall, which puts them in safe-mode, making them winter-hardy to survive cold weather. Plants sense the shortening days of autumn, and begin winding down to enter their safe-zone dormancy.
Plants have evolved strategies to prevent being fooled into thinking it’s spring, when it’s not. And it’s a doubly protective system. It’s easy to understand that cold temperatures keep plants dormant. That’s one factor in dormancy. But plant have a more mysterious backup system of internal dormancy.
Trees, shrubs and perennials require a certain quantity of what has been termed “chill hours.” Until the required number of chill hours have accumulated, plants won’t start re-growing. That’s why even if we get 3 weeks of 70 degree weather in early January, our trees and shrubs will not begin growing. Their “brains” recognize that the required number of chilling hours haven’t been met.
In our region, according to maps that track chilling hours, we receive about 1000 to 1200 chilling hours during the average dormant season. It’s a complex science, and we don’t have all the answers yet. The most effective chilling temperatures are between 32 and 45 degrees. Very cold temperatures don’t count as much, because, well, they’re too extremely cold.
It varies by plant type, but most trees and shrubs in our area require about 1000 chill hours before they can start growing again in spring. When the required number of chills hours has accumulated, plants are ready to grow. Plants are somehow able to keep track of the number of chilling hours they’ve received.
When plant’s chilling needs have been met, they’re ready to grow. At that point, they’re at the starting gate, and all that’s needed is warm temperatures. So, where are we at today? How many chilling hours have we had, and are trees and shrubs ready to go? There’s a national chill hour calculator, and as of March 8, 2017, Fargo has received approximately 1000 chill hours, depending on which model you use. Because most of our region’s common trees and shrubs require 1000 chill hours, their needs have likely been met.
Because their chill hours have been met, trees and shrubs are just waiting for air and soil temps to become favorable. Because our average last spring frost occurs in May, there is always danger of injury if plants start growing too early. Early spring extended warm periods followed by extended cold in late April or May can be harmful.
So what’s happening this spring? Plants are ready to grow, and some have already begun. We probably won’t know whether this year’s yo-yo winter has injured plants until after-the-fact. By early June we should have a complete picture.