December 5, 2009
Temperatures Monday night and Tuesday morning are likely to drop into the mid-20's! Expect frost damage on subtropical plants. And watch the weather: lows in the upper teens and low 20's can cause serious damage to citrus trees and other subtropicals.
National Weather Service link for Sacramento.
Plant protection tips--
Cover plants: lightweight spun plastic fabrics usually sold as "floating row covers," or frost blankets can simply be draped over the plant, as they are light enough that there won't be damage from contacting the foliage. If you use any material that isn't clear, you need to remove it during daylight hours. Plants can't live without light! Wrapping the trunk with burlap can help prevent major damage in a severe freeze; probably not necessary this time.
Move plants: pull potted plants up against a south or east wall, under an overhang. Reflected or retained heat from warm walls or cement walks will provide additional protection. Protect from cold wind. Fences or walls will prevent additional stress from cold winter winds. If practical, pull the container into your garage for the next few days.
Water: make sure all plants, especially those in containers, are well watered. If dry soil freezes, it will pull moisture from the roots, causing them damage. If the soil is moist it can freeze without harming plant roots.
Spray with an antitranspirant? Though research results are mixed, products such as Cloud Cover or Wilt-Pruf applied just prior to cold weather may give the foliage 3 - 4 degrees protection against the cold as well as desiccating winds.
Provide a heat source: Christmas lights hung in citrus trees have proved very successful, even with temperatures in the teens. Landscape lighting and portable shop lights will work as well. Make sure the light source is plugged into a grounded extension cord approved for outdoor use.
Harvest fruit? Your call. This is a short-duration freeze, unlikely to do significant damage to the fruit of most varieties. Thin-skinned varieties of citrus could be damaged, especially those on the outer part of the tree (unprotected by foliage).
You may wish to harvest lemons (especially Meyer), and limes, as they are ripe now and will deteriorate quickly if damaged. Freeze the juice for later use. Mandarins that are outside the foliage might be damaged. Thicker-skinned types such as navel oranges will probably be adequately protected by covering the trees, but they are also ripe and could be harvested. Avoid harvesting Valencia oranges, grapefruits, and tangelos, as they don't ripen until later in the spring. Citrus fruit does not ripen further off the tree.
Cold as it is, this will not likely break the record for temperatures set in 1990, when we had 13 consecutive mornings below freezing, with low temperatures on Dec. 22 and 23 of 18 and 17 degrees. The freeze of January 2007 equaled the number of days, but not the absolute low temperatures, of the 1990 freeze.
Here are the temperature data for the 1990 and 1998 freezes.
For more articles about frost and freeze: The 2007 freeze
general frost protection guidelines,
Frost vs. Freeze,
and Winter Care of Citrus.
Some plants that may be severely damaged or killed:
Australian tree ferns
Bougainvillea (older plants likely to recover)
Mandevilla (evergreen types)
Serious damage, but likely to resprout:
Brugmansia (Angel's trumpet)
Bananas (mostly killed to the stem)
Citrus trees (limes and young trees are most vulnerable)
Hardenbergia (Lilac vine—expect damage to the flower buds; maybe no lovely purple flowers this winter!)
Jasmine (true jasmine; Star jasmine is fine).
Palms -- some, particularly Queen palm (outer fronds will dry up and look dead, but the growing point is protected inside the center of the tree).
Pandorea (Bower vine, considerable top damage)
Passifloras (cut 'em to the ground in late spring; they'll be back)
Potato vines (the purple ones look worse than white, but mine recovered in 1990)
Red trumpet vines (severely damaged, unlikely to flower this summer)
Any decision about replacing plants should wait until we've had several weeks of warm weather. Subtropicals can be surprisingly resilient, and may resprout as late as May. So don't be in a rush to pull them out.