Just when it seemed I would have to write about coping with hot weather in the middle of winter, along comes record-breaking cold.
Suddenly the topic shifts to preserving plants that have been damaged by last weekend’s freeze and protecting sensitive species from subsequent freezes should they occur this winter.
The most obvious signs of cold damage are burnt foliage and mushy wood. The foliage and bark of cold-sensitive tropical plants, such as ficus trees, may appear to be water-soaked following a freeze, and for good reason: The freezing and thawing of plant cells causes them to burst.
Citrus fruit, much of which ripens during winter and is still on the tree at this time of year, may not look damaged on the outside, but its lack of firmness will give away the softened and inedible condition of its pulp.
There is really nothing you can do to salvage cold-damaged plants other than to wait and see if they recover. I remember one severe freeze that killed an 8-foot-tall umbrella tree (Schefflera actinophylla) back to the ground. A single bud near the trunk remained viable, and it managed to regrow into another 8-foot tree.
Under no circumstances should the dead portions be pruned back until consistent warm weather returns in the spring. Pruning now would expose still-healthy plant tissue to the cold and also stimulate new, tender growth even more susceptible to injury than the already-damaged older growth.
To protect plants from future freezes, you can either wrap them in old blankets or burlap, or build simple protective mini-greenhouse structures covered in plastic. If you wish to keep your mini-greenhouses in place all the time, you should use clear plastic film. But if you just want to put them over your plants at night, the plastic can be either clear or black. During the day, the ground is warmed from the radiant heat of the sun. At night, this heat radiates back up from the ground into the sky. As long as you have plastic to block the escape of this warm air and keep it around the plant, you should be all right.
Floating row cover, a tightly spun fabric that protects from sun, cold, and insects, which nevertheless allows plants to breathe, is another option for frost protection.
Tropical trees such as citrus, avocado and mango are most susceptible to cold damage when they are young. There are a number of ways to protect these trees from frost damage. Aside from enclosing them in plastic cages or cardboard boxes, you could put a bank of earth around them, covering their trunk to the bud union — the discernible collar that develops as a result of grafting scion to rootstock — four to eight inches above the base of the tree. Aside from insulating the trunk, the mound of earth radiates heat it absorbs during the day to the upper portion of the tree at night. Make sure you remove earth as soon as weather starts to warm or your trunk could develop a fungus disease.
Tree trunk wraps made of fiberglass, polyurethane, polystyrene foam and closed-cell polyethylene foam are also available.
Another way of protecting a young tree is with a mini-sprinkler — a low-volume sprinkler mounted on a drip irrigation line. Situate a 90-degree or quarter-circle mini-sprinkler on the side of the tree from which the prevailing wind blows. As water turns to ice, heat is released. If you keep the mini-sprinkler on all night, a 32-degree coating of watery ice develops on the trunk and foliage, serving as a protective covering since the danger zone for most tropicals is only reached when the temperature drops several degrees below freezing. Using a tree trunk wrap together with a mini-sprinkler provides an added measure of protection.
If there is fruit on your trees, spraying the fruit with water at nightfall may protect it from freezing.
TIP OF THE WEEK: If freezing weather is forecast, check the evening sky. A clear sky is more of a threat than an overcast one. Clouds trap heat and prevent it from dissipating completely. If you do not have mini-sprinklers or drip irrigation that can be left on all night, you can minimize frost damage by giving your garden a good soaking as night approaches. Tropical plants in containers should be moved under trees or patio overhangs or placed next to walls or fences.