The short answer to this question is it depends on how cold, where you live and what types of plants you are trying to grow. In the Dallas area, our winters tend to be mild with cool temperatures interspersed with warm temperatures and a few freezes.
When is Cold Good?
Vernalization is a physiological process in some plants where the flowers or seeds must go through a prolonged period of cold in order to blossom or germinate in the spring. The amount of cold required by a plant is measured in “chill hours.” For many perennial plants, such as fruit trees, a period of cold is needed to break dormancy, prior to flowering. Peaches, for example, typically require 700 to 1,000 “chill hours” (below 45°F and above 32°F) before they break their rest period and begin growth. Nuts trees and berry bushes also have varying chilling requirements. That pretty much sums it up for cold = good.
When Cold Bad: Frosts and Freezes
To understand frost, you need to understand a little bit about dew point. Dew point is a water-to-air saturation temperature. When the temperature falls to the same temperature as the dew point, dew forms because that is temperature at which the air can no longer hold all its moisture. When the dew point is below freezing, frost forms instead of dew. Frost typically forms when the temperature drops to near or just below freezing and there is no wind. Frost can form when the temperature is above freezing, but frost is a sign that the plant tissues have dropped below freezing. When you see frost there has been a freeze at the point of the plant surface. The absolute low as well as the length of time frost conditions remain can affect how much damage is done to plants. Real damage occurs when the water in a plant’s cells freeze, damaging the cell wall.
When a plant is frost-damaged, growth can take on a translucent appearance or become limp, then turn black or brown and dry up. Frost problems can be worse where plants face the morning sun because they defrost more quickly, which ruptures their cell walls.
A freeze is a more extended period of below freezing temperatures and may or may not include wind. In the Dallas area, extended periods of freezing temperatures are rare. When they do occur, soil becomes frozen. Many people think that trees and shrubs “go dormant” in the winter. This is true, but the internal functions of plants do not stop, they just slow way down. As long as the soil is not frozen, trees, shrubs, and even some perennials will still be growing roots. When soil becomes frozen roots are unable to take up water and plants can die from lack of moisture.
Plants are most vulnerable to freezing temperatures in the spring when periods of warm weather increase and suddenly there are several nights with temperatures well below freezing. A hard freeze can damage fruit and buds. Buds may freeze and drop off or fully opened flowers can turn brown or fall to the ground.