Wide temperature swings can be hard on our plants at this time of year. Once buds begin to swell, a sudden drop in temperature, or a late frost, expected or unexpected, can damage any parts of a plant that are actively growing.
Early flowering of trees and shrubs, especially those that produce fruit, is a big concern. Not necessarily due a failure to pollinate, but because a cold blast can kill off the developing flower or fruit buds. Commercial growers have some methods to cope with late frosts, but there really isn’t much that a home gardener can do about cold weather other than using protective covers.
So how about a little science? What factors do determine when a plant will begin to grow and flower? Temperature is just one. Here’s where the science comes in. It’s important to remember that plants respond to a buildup of warmth called growing degree-days. A GGD is a heat-index that can be used to predict stages of plant, insect or animal development. Although plants develop in a stepwise manner that is strongly influenced by the temperature on any given day, they depend more strongly on an accumulation of warm temperatures over time. For instance, the flowers of a saucer magnolia will not begin to open until 750 GDDs are built up. Even with the week’s beautiful weather, Madison’s total stands at 7.
But that’s logic based on average numbers and has little to do with a worried gardener’s heart and mind fretting about favorite garden plants. What about those tulip bulbs on the south side of your house poking through the mulch? Or those crabapple buds that are showing a little bit if silver? The flower buds of apple crops for instance, and this includes all members of the Rose family, will experience only a 10% failure rate when flower clusters have formed and the temperature drops to 27 degrees. If all you see on your trees is a bit of silver at the bud tip, the temp has to drop 15 degrees before there is any measurable damage. And remember that we’re only talking about lost flowers. Your tree will be fine. If you’re curious and would like to look at some photos, Michigan State University has some nifty charts dealing with fruit crops. Follow this link. http://msue.anr.msu.edu/topic/apples/weather/growing_degree_days
There are times other than spring when cold temperatures can cause damage. Warm January and February days followed by chilly nights can cause injury to the thin bark on younger trees and shrubs. Windy icy weather can damage broad-leafed evergreens like rhododendron and boxwood, and any yews or arborvitae that was sheared in the late summer or fall. And roller coaster temperatures in December can heave newly planted perennials right out of the ground.