The following are highlights from a Fair Oaks Horticulture Center Workshop presentation by Master Gardeners Caroline Hathaway and Cathy Coulter. Also see the UC/ANR publication, "Frost Protection for Citrus and Other Subtropicals."
Before a frost:
1. Identify cold spots in landscape by monitoring with thermometers.
2. Identify plants at risk: citrus, succulents, tender perennials, tropical and subtropical plants.
3. Have supplies ready: sheets or frost cloths, lights, wraps for trunks, thermometers, stakes or framework to hold covers off foliage.
4. Prepare tender plants: avoid fertilizing and pruning after August to minimize tender new growth.
5. Rake away mulch to allow soil to warm up during the day and radiate heat at night into plant.
6. Monitor weather forecasts and note how low temperatures will be and for how long.
Some terms you might hear:
Local frost: clear, dry nights; but, it usually warms up during the day.
Hard freeze: temperature inversion or Arctic front, can last for days/weeks, very damaging.
When a frost is forecast:
1. Move potted plants to a warmer spot next to house or under patio cover, especially on south side.
2. Check that plants are well watered since dry plants are more susceptible to damage, and moist soil retains heat better than dry soil.
3. Cover plants before sunset to capture ground heat radiating upward at night, but remove covers daily if it is sunny and above freezing to allow soil to absorb heat.
4. Add heat by using outdoor lights: hang 100 watt drop lights or Holiday string lights to interior of plant. Use the old C7 or C9 large bulbs, not new LED lights which do not give off heat.
5. Wrap trunks of tender trees if hard freeze is expected, using towels, blankets, rags, or pipe insulation.
6. Harvest ripe citrus fruit. Generally both green and ripe fruit are damaged below 30 degrees, but there is some variation by species (refer to the chart in UC/ANR Publication 8100, "Frost Protection for Citrus and Other Subtropicals).
After a frost:
1. Identify damage: dark brown or black leaves and twigs.
2. Wait to prune out damage until after danger of frost is past, and new growth begins in spring.