Farmer Fred's January Gardening Tips

Protecting Plants from a Frost or Freeze

by Fred Hoffman

Lifetime Master Gardener Fred Hoffman is host of "The Garden Show" on NewsTalk 1530 KFBK and "Get Growing" on Talk 650 KSTE on Sundays in Sacramento.

We are now officially entering the shivering season for much of California. December through mid-February is the most critical time for protecting frost-susceptible plants.

This is especially true for citrus tree owners, who are anxiously keeping an eye out on the upcoming weather forecasts.

Several days before an expected frost (temperatures dipping down to 32 degrees) is the time to gather the necessary implements to protect your citrus trees, including giving the ground beneath them a good soaking (moist soil is better than dry soil at moderating the temperature beneath the tree).

Most gardeners first thoughts about protecting their citrus trees during a frost or freeze is, "protect the fruit!"

Lemons, limes and citrons are most sensitive to frost; damage to the fruit can occur at 32 degrees. Sweet oranges, grapefruit, tangerines and calamondins are hardy to 28 degrees. Kumquats and Owari Satsuma Mandarins are the most frost-tolerant, braving temperatures into the mid-twenties.

But what about the overall health of the citrus tree? How low can temperatures go during a freeze event before the tree is toast?

22 degrees is terminal for citrus tree cambium cells.  The cambium layer is the growing part of the tree, the cells that are producing new wood and healing wounds. It is located just beneath the bark.

And that's for a citrus tree in tip-top shape: a plant in good health, with moist soil around it during a freeze.  An extended period of time in the 20's, however, could signal serious problems.

28 degrees for four hours probably won't kill a Meyer Lemon tree, provided the rootstock cambium doesn't freeze.  It will defoliate and lose twigs.  At 24 degrees, however, permanent damage can occur.

That's when the frost blanket, frost bonnet and lights will provide up to four or five degrees of protection. And don't forget: moist soil helps send warmer air from the ground up into the tree at night. Some growers will run specialized overhead sprinklers when the forecast call for temperatures dipping into the 20's. Backyard gardeners, however, run the risk of those sprinklers freezing up as well as branch breakage on limbs covered in ice.

Citrus trees most at risk to fatal damage from a frost or freeze are the young trees. It is vital that they be covered completely when a heavy frost or freeze is predicted, and provide protection for the trunk, bud union and rootstock area. That can include trunk wraps, newspapers, old carpeting. 

In his book, "Citrus", author and grower Lance Walheim also suggests applying a copper-based fungicide to the trunk and then mounding or banking soil against the trunk and lower limbs. Just don't leave it on too long; fungal rots can develop (that copper can only work for a limited time). He advises leaving that soil next to the trunk from Thanksgiving until February, or March, in colder areas.

What about your other plants and possessions? Here are some pre-frost pointers:

• Identify cold spots in landscape by monitoring with a thermometer that registers high and low temperatures.

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

• Have supplies ready: sheets or frost cloths, lights, wraps for trunks, thermometers, stakes or framework to hold covers off foliage.

• Prepare tender plants: avoid fertilizing and pruning from late summer through February to prevent damage to new growth. 

• Plant insurance: In September and October, take cuttings from frost sensitive perennials; keep cuttings in a sunny, indoor area.

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

• Monitor weather forecasts and note how low temperatures will be and for how long. 

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 with a row cover 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. 

When a Freeze or Hard Freeze is Forecast (temperatures remain at or below 28 degrees for several hours)

1. Wrap any exposed plastic water pipes; cover outdoor faucets, as well. Turn off the water supply to outdoor irrigation faucets, if possible. Allow those faucets to drain.

2. Disconnect garden hoses and lay them out straight...away from driveways!

3. Adjust your pool, spa or pond filtration timers so that they are running when the chance of freezing temperatures is greatest, between two and nine a.m. Moving water is less susceptible to freezing.

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.

3. Make sure the backyard birdbath isn't frozen over in the morning. Daily fresh water for dogs and cats is also a good morning habit.