I'm not alone when I ask: "What happened?" I'm referring, of course, to the minus 15-degree temperatures of a few nights ago. Although most gardeners were wrapped up toasty warm in their beds, many landscape plants were outside suffering in the brutal cold. Unfortunately, the damage will not appear until spring, long after the ground has thawed and plants have come back to life. Here is what to expect in the months ahead as a result of the brief encounter with the deep freeze.
Point blank: Low temperatures can directly damage plants. Those most tender are the fruit buds and canes of older varieties of thornless blackberries. They are generally damaged at temperatures near 5 degrees. However, many of the newer varieties, such as Arapaho and Navajo, have increased winter hardiness and should tolerate temperatures to 10 degrees below zero. Some thorny blackberries may be damaged at temperatures near 5 degrees below zero. Others will tolerate significantly lower temperatures. Also affected are the flower buds and woody stem tissue of peaches and nectarines. They are likely to be damaged when the temperature drops to 10 degrees below zero.
Luckily, the fruit buds of most apple cultivars will survive temperatures as low as 20 to 25 degrees below. Red Delicious is among the most tender of apple cultivars and may be damaged at 15 degrees below zero. The condition of the plants can influence the extent of low-temperature injury to fruit buds and wood. For example, a plant that had severe disease and insect injury to the leaves may have a low level of nutrition in the plant. This plant will be injured more than a healthy plant.
But what about wind chills - are they harmful to plants? For warm-blooded animals, wind chills can have a profound effect on their ability to keep warm. However, plants do not respond to wind chill indexes like warm-blooded animals. The difference is that plants do not need to maintain a temperature above their surroundings. For example, a wind chill of 40 degrees below zero at a temperature of zero degrees will not cause any more cold injury to plant tissue than a wind chill index of 20 degrees below zero at zero degrees. This is not to say that wind does not harm plants in winter. Wind alone is damaging because it can dry plant tissues. Plant tissues require moisture to survive, and a high wind velocity can cause moisture loss. This desiccation may be great enough to injure or even kill tissue, particularly the smaller size wood found in peach twigs, apple spurs or blackberry canes. There is no scientific evidence to show that an increasing wind chill index will directly increase plant damage due to cold injury.