Frost vs. Freeze
Written for the Davis Enterprise, December 1999
December 20, 1998
A cold air mass moved into Northern California just in time for Christmas. Temperatures plunged to nearly 20 degrees for five days straight, and citrus growers scrambled to save their oranges and lemons. Most of the crop was lost, with damage estimated in the millions of dollars. Meanwhile, home gardeners moved tender plants inside and covered landscape plants with plastic, sheets, and blankets. The cold weather lasted for a week, and nursery professionals and master gardeners were besieged with calls about which plants to protect and how to do it.
So, was it the coldest ever in Davis?
No! Previous freezes in December 1990 and 1975 were colder and lasted longer. The 1990 freeze was the coldest in 60 years, with temperatures dropping to 21 degrees on Dec. 21 and staying below freezing for 14 days, with the lowest temperature recorded in Davis at 17 degrees.
How likely is a freeze again this year?
Climate research models show this winter  to be affected by La Nina ocean conditions, similar to last year's. Those winters tend to be relatively dry and cold in Northern California, punctuated with occasional "pineapple express" storms that pull moist subtropical air in from Hawaii, leading to heavy rain. Freezes and floods are both possible in La Nina years.
Home gardeners don't need to worry about most of their plants during a normal frosty night here.
Some plants should already be inside...
or under an overhang in a south or east-facing exposure: Jade plants, Bougainvillea, Hibiscus, Mandevilla (pink-flowered forms). Any house plants you've kept outside during the summer should be in by now!
There are many plants that we call subtropicals that you don't need to worry about leaving outside...
Lantana, Fuchsia, Verbena, Solanum (Potato vine), statice, most Passifloras (Passion flower vines), and others will be damaged by frosts or freezing weather, but enough of the stem or root will survive that they can simply be cut back in the spring and will regrow vigorously. Many of these were severely damaged last December and in 1990, but most recovered. Most landscape plants need no winter protection even in a hard freeze. Roses, azaleas, Camellias and most flowering perennials will be fine.
What about Citrus trees?
Should we cover the trees? Pick the fruit? Water? Spray? Oranges, lemons, mandarins, and other Citrus fruit trees love our hot summer climate, but can sustain winter damage when we get a freeze.
Mary Helen Seeger is co-owner of Four Winds Growers, a wholesale Citrus nursery located in Fremont and Winters. She offers the following tips:
Potential cold damage is a combination of temperature(how cold) and time (for how long). Brief dips to the mid 20's will not damage most citrus. Prolonged temperatures in the teens caused much damage in 1990 and 1998. Limes and lemons are the most sensitive, needing some winter protection in colder locales. Other citrus are fairly hardy once established.
Most Citrus fruit will be damaged at about 26 degrees; lemons and limes at 30 degrees. Don't pick them unless you have to, because the fruit won't get any sweeter once it is off the tree. Lemons and limes can be juiced and frozen in ice cube trays. The fruit on the outside branches of the tree will be most vulnerable. Tender new growth will be killed, but this isn't very harmful to the tree.
Experts will usually tell you to cover the tree...
but to make sure that the covering material is not touching the foliage because there will be freeze damage where there is contact. Most people aren't willing to build a greenhouse around larger trees! But for a young tree, you can pound some stakes in the ground around the tree and drape or staple clear plastic over it.
Plant protection tips
Lightweight spun plastic fabrics...
usually sold as "floating row covers," 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 every day. Plants can't live without light! Wrapping the trunk with burlap can help prevent major damage in a severe freeze.
Microclimates can be sought in looking for planting sites.
Reflected or retained heat from warm walls or cement walks will provide additional protection. Fences or walls will prevent additional stress from cold winter winds.
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 citrus roots.
Use of an antitranspirant...
such as Cloud Cover or Wilt-Pruf once a month or just prior to cold weather will give the foliage four degrees protection against the cold as well as desiccating winds.
hung in citrus have proved very successful, even with temperatures in the teens (and your neighbors will be impressed by your elegance!). Landscape lighting at the base of the trunk is even more sophisticated. Portable shop lights will work as well. Tarps and plastic are not as effective.
"There is no need to panic with a few hours of temperatures in the high 20's such as occurs during a normal radiation frost," says Seeger. "The Christmas lights and anti-transpirants do wonders in the low twenties, and help prevent severe damage in the rare occasions when temperatures are in the teens."
Seeger notes that experts used to recommend against fertilizing Citrus trees after late summer to "prevent" damage to tender new growth. Four Winds Growers now recommends fertilizing Citrus year-around. While there may be some damage to new growth, they have found that healthy, well-fed trees resist and recover from frost damage better.
Frost versus Freeze:
What's the difference between a frost and a freeze? Frost occurs on a clear, still night, as heat radiates from surfaces (your car window, a field, etc.) to the sky. The temperature drops below 32 degrees, and water vapor freezes on those surfaces. The temperature usually won't go much below freezing, because energy is released as the water freezes. Most plants that aren't tropical can go 2 - 3 degrees below freezing with no problem. Heat will be trapped by any overhang, so tender plants will usually not freeze if they are up against the house. Clouds or fog will trap the heat, usually preventing a frost.
A freeze is when cold, Arctic air moves into the region. Local news stations give us warning as these cold fronts bear down from the Gulf of Alaska. The air is much colder than freezing, there is wind, and the humidity is very low. Much of the damage we see on plants in a freeze is from desiccation--severe drying of the foliage. An overhang provides less protection; sheltered plants may need to come indoors.
© 2008 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
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