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Cold Hardy and Winter Protection of Citrus in North Florida

January 04, 2010 1:59 PM
Collin Adcock, Washington Co. Horticulture Extension Agent

When it comes to growing citrus in Northwest Florida, the homeowner is at an extreme disadvantage. Every year in North Florida there is a high probability that a freeze will occur sometime in December well into January. Even though the home citrus grower cannot control the outside temperatures, there are many factors that can be changed that will influence citrus survival in freezing conditions.

One of the first things that need to be considered is selecting the proper variety of Citrus for North Florida. The most cold tolerant form of citrus is actually trifoliate orange. However, trifoliate orange is not edible. The most cold hardy edible varieties of citrus are kumquats and Satsuma. Due to the fact that trifoliate orange is the most cold hard citrus it is commonly used as a rootstock for the already cold tolerant kumquats and Satsuma. This produces an ultimate cold hardy citrus.

The next step that needs to be considered is location of the citrus trees. Cold air always drains to low elevations. Higher elevations that posses good air movement and drainage are ideal sites for citrus plantings. Another thing to consider in proper citrus placement is location. Citrus will suffer less cold damage if placed on the south side of a structure, such as a house or building. Air and wind from the North will be force up and over the structure, thus protecting the citrus tree from these factors.

A good clean and packed surface will absorb and retain heat during the day and give it out over night. Damp soils also tend to absorb more heat than dry soils. Furthermore, watering 2-3 days before a freeze can help prevent some frost damage. Overall plant health can help prevent the severity of frost damage. Follow proper fertilizer recommendations and cultural practices to maintain plant vigor.

After the proper tree is planted in the correct location, there are a few additional things that may be done to further help protect citrus from freezing conditions. There are many methods for protecting existing citrus trees. These include use of heaters, wind machines, fog generators, tree wraps, soil banking, overhead irrigation, and micro irrigation. Heaters, wind machines, and fog generators are usually geared toward large scale commercial citrus groves and may be very impractical for home growers.

Tree wraps, soil banking, overhead irrigation, and micro irrigation can be used by commercial growers as well as home growers. A tree wrap is where the trunk of the tree is wrapped from the ground past the graft union to just below the canopy. Tree wraps work by delaying heat loss from the trunk. This method is mostly used only during mild to moderate freezes. Materials commonly used in tree wraps are fiberglass, polyurethane foam, polystyrene foam, and polyethylene foam.

Another method of protecting citrus from freeze damage is by soil banking. This process is done to ensure that the graft union is protected by mounding soil around the trunk. During the day, the soil retains heat and during the night it lets heat off. It protects by conduction and insulation. Trees should be banked the day before a hard freeze and then removed shortly after the threat of all freezes is over. In North Florida this usually occurs during the month of February. Even if the top of the tree dies, the tree will regenerate from the grafted area in the spring. Blankets and quilts may be used in place of mounding soil.

Lastly, utilization of irrigation can protect from cold damage. Irrigation may also be used in combination with tree wraps. There are two types of irrigation methods used in cold protection of citrus. They are overhead and microsprinkler irrigation. Overhead irrigation is the use of a sprinkler which sprays water over the tops of citrus. Micro Irrigation is the use of low- volume misting sprinklers installed at ground level on short risers. These systems should put out 10 to 50 gallons/hour. Irrigation works by using the heat of fusion. Heat of fusion is the heat given off when water freezes to ice. This process protects the trees by transferring the created heat to the citrus tree. This heat is only transferred on the inside of the ice. Parts of the tree exposed to the exterior of the ice are subject to cold damage. Once Irrigation has begun during freezing temperatures, it must be continued until temperatures have risen.