For citrus and subtropical trees such as avocado, loquat, and guava, cease pruning and fertilizing trees after early summer, to allow new foliage enough time to mature before the onset of cold weather.
Carefully monitor soil moisture and keep your trees adequately irrigated, making them more able to withstand frost than those weakened by inadequate irrigation. If it has not rained and soil is on the dry side, deep irrigate resulting in warmer soil, which radiates heat to plantings. Keep ground around trees clean and free of much, weeds and ground cover, which allows the moist soil to radiate more heat to protect the trees from frost. Wrap trunks of tender trees, using towels, blankets, rags or pipe insulation.
For nights when severe frost is forecast, a temporary shelter can be constructed from 2' x 4's, PVC pipe, or stakes covered with a tarp or plastic, taking care to not allow cover to rest on leaves. The cover should be removed during the day to allow sunlight and ambient warmth to penetrate into the canopy. Replace cover as needed if frosty nights persist. It is not a good practice to cover trees indefinitely during winter 'just in case' of a frost.
If a severe frost is predicted, due to either extreme low temperature or for a sustained length of time, a 100-watt lamp (designed for outdoor use) may be placed in the interior of the tree, to emit enough heat to reduce frost damage. Alternately, string incandescent holiday lights (not LED, which give off no heat) throughout the tree to emit enough heat to reduce damage.
For more information, see UC ANR Publication 8100 Frost Protection for Citrus and Other Subtropicals.