Archive for Thursday, February 2, 2006

Protect plants for when winter chill returns

February 2, 2006


Warm days and cool nights - so this is what winter is like in the South. If so, I like it. Unfortunately, plants may not agree as the mild temperatures can come to an abrupt end at any time. And when they do, what will happen to trees, shrubs and bulbs that think it's spring? Hopefully not much. Winter cold and a budding plant do not have to lead to plant death. Here is what you need to know and what you can do about plants that already have started to grow when the winter chill returns:

The plant life cycle is an interesting and complex chain of events. As fall approaches, a chemical process takes place in the plant that results in dormancy. The plant will only awaken after it has experienced a predetermined number of "chill hours." A "chilling" hour is one in which the temperature remains between 32 and 45 degrees. Plants differ in the number of chill hours required to complete dormancy. Those adapted to colder climates usually require more than those adapted to warmer zones. Even plants within the same species can differ dramatically in the number of hours required for bud break. For example, apple varieties range from a low of 250 (or less) chilling hours to a high of 1,700. This mechanism is the plant's way of making sure that it does not begin spring growth too early.

Even though this winter has been one of the milder ones on record, as far as plants are concerned, we already have met the chilling requirements for many of them in our area. For these plants, dormancy is over, and continued warmth will lead to bud swell and growth. However, winter is far from over, and buds that have swollen lose virtually all of their winter hardiness and can be damaged by severe cold.

Spring flowering bulbs are not much different. Their natural growth cycle is to grow roots in the fall, go dormant, then begin vegetative growth when soil temperatures warm again in spring. Often times, soil temperatures are less variable than air temperatures and warm faster in the spring than do air temperatures. The result is that bulbs begin to grow even though the nights fall below freezing. In many cases, this already has begun to happen.

So what do you do if you have a plant that has swollen buds or spring bulbs that have green shoots sticking out of the ground? Not much. Bud development and vegetative growth are completely dependent on weather. However, watering during dry weather may help. Readily available soil moisture will aid in keeping the plant healthy so that it will be better able to recover from cold damage if some occurs. As for bulbs, do not be tempted to cover the green shoots with mulch.

Even though the tissue looks succulent and tender, it is rather cold-hardy. In addition, the actual flower bud is still protected and is not likely to freeze. By piling mulch or other forms of organic material on top, you will only be trapping the soil's warmth, causing the plants to grow even more. By leaving the plants uncovered, they should stop the growth and wait for warmer days.


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