Protecting citrus from frost

Louisiana normally has its coldest temperatures in January but freezes can occur as early as November and last well into early March.
Protection for citrus trees mainly becomes a concern during hard freezes in which the temperature remains below freezing. It is difficult to determine the threshold in which precautions should be taken to protect citrus trees from the dangers of frost. The biggest difficulty in this determination is the differences in the various types of citrus and their levels of cold tolerance
Satsumas, for example, do not need protection until the temperature approaches 20 degrees Fahrenheit. Lemons, limes, and oranges generally need to be protected when the temperature dips below 26 F. However, these trees may be killed or damaged at these temperatures if they are not sufficiently hardened with enough pre-conditioning cold temperatures below 50 F.
All ripe fruit should be harvested from trees prior to a significant freeze event. Temperatures cold enough to damage the tree will also ruin the fruit. It takes temperatures in the mid- to low 20s five to 10 hours to freeze the fruit.
A citrus tree increases in hardiness as it gets older. Trees with larger, denser canopies deal with the cold better as they trap more heat.
Seven steps are suggested to reduce freeze damage:
•Clean cultivation under the canopy of a tree, mechanically or by herbicides, prior to winter. Grass, weeds, and straw mulches prevent heat from entering the soil during the day – therefore, less heat energy is stored in the soil under the tree for release at night.
•For trees too large to cover, banking the lower trunks of trees with soil or using tree wraps of bubble wrap, foam rubber or Styrofoam will help prevent cold damage to the trunk. This must be done before the first killing freeze and can be left on through the winter. Trunks should be treated with a copper fungicide before wrapping or banking to prevent foot rot
•If the weather has been dry, several days in advance of a cold front the soil beneath citrus trees can be irrigated. Good soil moisture acts as a cold buffer, and trees that are drought stressed may experience more cold damage. If this is done at the time the front arrives, evaporation may occur and result in colder temperatures near the tree.
•If pruning is needed, it should be done in spring to allow tree growth to mature before winter. Do not prune in the late summer or fall. Cuts should be made at branch crotches leaving no stubs. Prune to maintain a full, dense canopy. Trees need good leaf canopies to cut wind speed through the canopy and reduce the rate of cooling. Leaves radiate heat to each other.
•Fertilizer should be applied to citrus trees in late January or early February. A complete fertilizer such as 8-8-8 at the rate of two pounds or 13-13-13, the rate is one and one-half pounds of fertilizer per year of tree age. Apply a subsequent application of nitrogen when good soil moisture exists in June. Late summer or fall applications of fertilizer should be avoided as they can reduce the hardiness.
•To protect a single smaller tree, construct a simple frame over trees and encase the tree with one or two layers of translucent plastic. This is generally most practical for smaller trees. In southeast Louisiana, such an extreme practice would be needed only on a few severely cold nights.

DeLane Ross is an assistant county agent for St. Martin Parish.