[Image: citrus]Citrus trees, such as satsumas, oranges and grapefruit, are popular for home fruit production. Citrus trees offer gardeners beauty and fragrant flowers as well as a tasty fruit. Their dark, glossy evergreen foliage is attractive year round.
These versatile plants can be used effectively as specimens, espaliers and in containers. Their relatively small size – about 10 to 15 feet tall and about 10 feet wide – makes them useful in a wide variety of situations.
Trees often become available to purchase in the fall and early winter. Since south Louisiana gardeners never know if a severe freeze will occur or not during winter, it is chancy planting trees in the fall or winter. It is generally a good idea to wait until late February or early March to plant trees into the landscape. Place container-grown trees in a sunny location and water regularly through the winter. Move them into a protected location anytime temperatures are predicted to go below 30 degrees. This allows you to keep your tree alive through winter even if temperatures in the teens occur.
If you do decide to plant a citrus tree in fall or winter, you must be prepared to provide protection should temperatures go below the mid to upper 20s. Gardeners who already have citrus trees growing in the landscape should think about when and how protection will be provided to the trees.
Factors that Determine Cold Damage
Older, well-established citrus trees are more likely to survive severe cold. Even if badly damaged, older trees have trunks that are more massive and are able to live through freezes that would kill younger trees. Different types of citrus trees vary in their resistance to freeze injury. The following list is arranged from most freeze-tolerant to least freeze-tolerant: satsumas, kumquats, sour oranges, sweet oranges, grapefruits, lemons and limes.
How cold it gets and how long it stays below freezing are important. Satsumas and kumquats do not need protection until the temperature drops below 25 degrees and generally survive without protection if temperatures do not go below 20 degrees. Other types of citrus are more likely to be badly damaged by temperatures in the low 20s. Temperatures in the teens will severely damage or kill trees that are not protected. In addition, the longer temperatures stay below freezing, the more likely extensive damage or death will occur. If temperatures rise above freezing after just a few hours, damage is usually slight.
The weather prior to the freeze can make a big difference. Citrus trees gradually exposed to lower temperatures go through a process called hardening off, and there is a decrease in the freezing point of the trees’ tissue. Trees that are semi-dormant because they’ve been hardened off by earlier temperatures in the 40s and mid to upper 30s are less likely to be damaged.
Cultural practices that do not encourage late growth are important to the citrus trees’ ability to harden off as temperatures drop. Avoid pruning and fertilizer applications after July.
Protecting Trees
You can help reduce cold damage and ensure recovery to citrus by maintaining a healthy tree with a thick canopy of leaves. Weak trees that are in too much shade, those with insect or disease damage or those with nutrient deficiencies are the ones most severely damaged and are the slowest to recover after freezes. In addition, keep the tree properly watered when freezing temperatures are predicted.
Plant citrus trees in protected areas of your landscape whenever possible. Southern exposures enclosed by hedges, fences or buildings on the north side offer protection from the north wind. When possible, plant citrus trees close to the house (but at least 5 or more feet away).
Protect trees planted in the ground that are not too large by constructing a simple frame over each tree and encasing them in one or two layers of plastic. The frame should hold the plastic off of the foliage. An incandescent 100-watt light bulb or strings of small Christmas lights inside can raise the temperature in the enclosure a few degrees. Use outdoor extension cords and make safe connections. Do not allow the 100-watt light bulb to touch the foliage of the tree or the plastic covering. If the next day is sunny and mild, vent or remove the plastic to prevent excessive heat from building up under the plastic.
In south Louisiana, this sort of protection is needed only on the few severely cold nights that occur here in December, January and February, but it can make a big difference. Even one night of 15-degree temperatures can destroy years of growth without protection.
Care After Freeze Damage
Do not prune or cutback freeze injured citrus trees until May. It is difficult to tell where regrowth will occur until the tree resprouts. Even then, new growth may collapse and die during summer. If the only new growth occurs from ground level or below the graft union (the graft union is often noticeable as a knob or swelling about foot or so above the ground), you should replace the tree. Citrus trees are grafted onto a rootstock that helps them grow better and makes them a little hardier, but the rootstock will not produce acceptable fruit. If new growth occurs from the above the graft union, it can be trained into a new tree.