BATON ROUGE - Louisiana citrus farmers and others with citrus trees are preparing for damaging cold conditions later this week -- but not all crops face the same level of vulnerability.

The LSU AgCenter is sharing a chart that rates the hardiness of those fruit trees.

Kumquat trees are best suited to handle the sub-freezing conditions, with satsumas also a hardy variety. Lemons and limes are most susceptible to the cold, as a general rule.

There are three main factors involved in determining freeze susceptibility of citrus trees:

1) The type and age of the citrus. In order from most cold hardy to least cold hardy: satsuma, kumquat, orange, grapefruit, lemon and lime. A citrus tree also increases in hardiness as it gets older. Trees with larger, denser canopies deal with the cold better as they trap more heat.

2) Threshold temperatures are approximately 20 degrees for satsumas and kumquats, and about 26 degrees for all other citrus.

3) The duration of the sub-freezing temperature is very significant. If the temperature is below freezing for 24 to 36 hours, damage can usually be expected.

Those who want to protect their crops can take the following steps:

• Clean cultivation under the canopy of a tree, mechanically or by herbicides, prior to winter is recommended. 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. Or, the coverings may be applied during freezes and removed during mild weather. Although tree tops may still be lost during freezes, a tree can recover if its trunk and root system are intact. Banking or wraps should be removed in the spring.
  
  
• 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. This must be done well in advance of the freeze. 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. Outer leaves may be lost to a freeze, but complete loss of inner leaves is averted by a thick canopy.
  
  
• 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 per year of tree age may be used. Stop increasing the rate when you reach 15 pounds. If using 13-13-13, the rate is one and one-half pounds of fertilizer per year of tree age. Stop increasing the rate when you reach 10 pounds. Spread the fertilizer around the edge of the branches in the area of the feeder roots. 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.
  
  
• Oil sprays used to control insects and mites decrease cold tolerance and should not be used later than August 15.
  
  
• 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. Before covering, the tree could be generously draped and wrapped with small, outdoor incandescent Christmas lights to provide additional warmth and increase the level of protection. Incandescent Christmas lights will not damage the tree even if they come into contact with it.
  
  
• The frame and cover can stay in place indefinitely but will need to be vented. Air temperatures within should not be allowed to go above 85 degrees to 90 degrees F. Venting should be provided on sunny warm days to prevent overheating and to maintain a supply of fresh air.

The WBRZ Storm Station weather team says a hard freeze will arrive late in the week, with warnings expected as Christmas nears.

FOLLOW WEATHER DEVELOPMENTS HERE

Thursday afternoon, a strong but mostly dry cold front surges through the Capital Area. Showers will be isolated due to the lack of moisture expected in the atmosphere. Temperatures will rapidly drop behind this line and by Friday morning, temperatures will be well below freezing.