Spring Frost, Freezes Are Factors in Fruit Production

Spring frosts or freezes are major limiting factors in consistent production for commercial and home fruit growers throughout Jefferson County.
In many years, temperatures during the period leading up to and just past bloom drop to the point where bud damage may occur.

Several Factors play a role in the amount of damage actually received. The site, the type and variety of the fruit crop as well as the health of the plant can impact the amount of injury experienced during a cold event. A frost or freeze during the period leading up to bloom does not necessarily mean that the crop is lost. Most healthy fruit trees will set far more buds than will be needed for a full fruit crop. In fact, it is estimated that peach or apple trees can set a full crop on 10% or less of the fruit buds on the tree.

We experience two different types of cold events in spring. Radiation frosts are perhaps the most common. A radiation frost event is characterized by warm, calm, sunny days leading into calm, clear nights where temperatures may drop dramatically. Radiation frosts tend to be localized. Advective freezes are characterized by cold, windy days leading into cold, windy nights. An advective freeze generally occurs over a large area. Of these two scenarios, crop protection during a radiation frost event is more realistic than during an advective or wind freeze.
An inversion layer often is found with a radiation frost. The air temperature will actually increase as you go from ground level up to a certain height. The inversion layer acts as a blanket holding warmer air below it. If the inversion layer is not too high and the temperature differential from the ground level to the inversion is substantial, commercial growers can use wind machines or heaters to increase the temperatures throughout the plantings. Irrigation is also used to protect plants by continuously forming ice on plant parts, thus maintaining the temperature of the plant at 32˚F. There is no inversion layer with an advective freeze. Therefore, wind machines and heaters are ineffective. Windy conditions that accompany an advective freeze limit the usefulness of irrigation as a frost control technique.

Home fruit growers can also protect the blossoms on their fruit crops. Smaller fruit trees, bushes and vines may be covered with blankets or sheets. As heat is radiated from the ground back into the atmosphere, a cover over the plant will retard heat loss. While the cover does not have to go from the ground on one side over the top and to the ground on the other side, the more canopy that is within the cover, the more protection will be recognized. Plastic is not as effective. Plant damage will occur wherever the plastic touches the plant. Additional protection can be obtained by adding a small amount of heat under the canopy. If the plant is close enough to an electrical source, run an extension cord and suspend one or two lights in the lower part of the plant. The bulbs will give off enough heat to raise temperatures within the enclosed canopy. A small heat source is better than a large heat source as the latter may cause some plant damage.

Irrigation for frost protection works because water going from a liquid to a solid (ice), gives up heat. If new ice is continually being formed throughout the period that air temperatures are below freezing, the temperature of whatever the ice is forming on will be held at 32˚F, which is above the temperature at which damage will occur. Irrigation should start before temperatures drop to freezing and should continue until active melting begins. Interrupting the irrigation when temperatures are below freezing will result in more damage than if irrigation was never used. The weight of the ice can cause damage to plants. Training trees to develop a strong framework and pruning before irrigation for frost protection will lessen the potential for damage.

Washing the frost off a plant in the morning will not reduce problems due to cold temperatures. Damage occurs when ice crystals form in the tissues of buds and blooms as temperatures reach critical points during the cold event.

During the last week of March and first week of April, stone fruits and pears were near full bloom or just past this stage. Apples were at one half inch green to tight cluster or pink, depending on variety and location in the state.

As the buds on fruit crops progress from a fully dormant stage to full bloom, they lose their ability to tolerate cold temperatures without damage. The critical temperature is defined as the temperature necessary to cause a certain level of damage when buds or blossoms are exposed to it for 30 minutes or longer. The following table gives the 10% kill and the 90% kill critical temperatures for several fruit crops at various stages of bud development. Several factors such as plant health and previous cold weather events can actually influence the amount of damage received at these temperatures.

Frost or freeze damage to the blossom will be apparent within 4-6 hours after temperatures get above freezing. Cut a bud or blossom in half from top to bottom. The pistil will appear to be a dark, water soaked green color. Within a few days, it will it will turn brown then black and shrivel. Note that on a healthy bloom, the stigma and style will also turn brown to black and shrivel after pollination and fertilization of the bloom has occurred. However, the ovary will remain bright green and will begin to grow in an undamaged bloom.

For more information please contact Steven Huff at 397-2969.