Frost Protection for Sensitive Plants

Before a frost

• Identify cold spots in landscape by monitoring with thermometers.

• Identify plants at risk: citrus, succulents, tender perennials, tropical and subtropical plants.

• Have supplies ready: sheets, blankets or frost cloths, lights, wraps for trunks, thermometers, stakes or framework to hold covers off foliage. Frost cloths come in different weights that can provide 4° to 8° of protection. Because the frost cloth allows some light and air to penetrate, it can stay on plants for a few days at a time. Frost cloth can lie directly on plant foliage.

PVC hoops to hold frost cloth

Frost cloth protects young tree

Lights on under frost cloth

• Prepare tender plants: avoid fertilizing and pruning after August to minimize tender new growth.

• Rake away mulch to allow soil to warm up during the day and radiate heat into the plant at night.

• Monitor weather forecasts and note how low temperatures will be and for how long.
– Local frost: clear, dry nights, usually warms during day
– Hard freeze: temperature inversion or Arctic front, can last for days or weeks, are very damaging

For information about the difference between chilling injury and frost and freeze injury click here.

When a frost is forecast

• Move potted plants to a warmer spot next to the house or under a patio cover, especially on the south side.

• Check that plants are well watered because dry plants are more susceptible to damage, and moist soil retains heat better than dry soil.

• Cover plants before sunset to capture ground heat radiating upward at night. Remove sheets, blankets and other covers daily if it is sunny and above freezing to allow soil to absorb heat.

• 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. Old style holiday lights that give off heat can provide up to 3° of protection. Use lights, extension cords, and multi-outlets or power strips that are rated for outdoor use and grounded (3-prong). Avoid connecting together more than three light strings in a line.

• Wrap trunks of tender trees if a hard freeze is expected, using towels, blankets, rags, or pipe insulation.

• Harvest ripe citrus fruit. Generally both green and ripe fruit are damaged below 30°, but there is some variation by species (refer to chart in UC ANR Publication 8100, Frost Protection for Citrus and Other Subtropicals).

At the Fair Oaks Horticulture Center we use these guidelines for protecting our citrus trees when temperatures are predicted to drop below:

32°: turn on the lights
29°: also cover young trees (1 to 4 years old) and older sensitive varieties such as limes and lemons
27°: also cover remaining trees
26°: add a layer of plastic sheeting over the frost cloth if a prolonged arctic freeze is expected

Frost cover on year-old citrus trees

Frost cover on two-year-old citrus

After a frost

Be kind to birds—break up ice in bird baths.

• Identify damage: dark brown or black leaves and twigs.

• Wait to prune out damage until after danger of frost has passed, and new growth begins in spring.

Written by Master Gardeners Caroline Hathaway and Cathy Coulter, November 2008