Coping with the cold

When we get a serious cold snap, like the current one, we can bundle up or throw more logs on the fire. Birds can fly to warmer places and animals can seek shelter. But the trees, shrubs and vines we have planted in our gardens just have to sit there and take it.

The low temperatures we've been feeling lately are not a problem for most of the local forest and woodland native plants, but they can be devastating for some of the broadleaf evergreens we choose to grow in our landscapes.

The freeze of late December 1990 was a wake-up call to some regarding selection of plants with reliable cold-hardinesss. Some of the less hardy species were killed or severely damaged in that event, when day-time temperatures remained below freezing for several days. After that, they were avoided for a while, but lately I've noticed some of them creeping back into landscape designs.

Bloodred trumpet vine (Distictis buccinatoria), a broadleaf evergreen vine with spectacular red flowers, is a good example. Though it is listed as adapted to zones where "20-year" lows may drop into the teens Fahrenheit, they were severely damaged or killed around the Bay Area in the freeze of '90. Planting them here is risky.

Gardeners and landscape designers seem to be willing to take risks, sometimes careless, sometimes calculated, for the sake of the potential benefits or the sheer joy of seeing the flowers or fruits of one's labor.

Citrus is another good example of broadleaf evergreens with a risk factor in our climate. Look what's just happened to the California citrus crop. About 70 percent of it has been destroyed because of prolonged sub-freezing temperatures.

Lemons and oranges are integral to Napa Valley gardens, yet they are subject to moderate to severe damage in a freeze event. So, what do we do? We grow them anyway. But we either select varieties that are relatively resistant to the expected low temperatures in our zone, grow them in containers that can be moved in from the cold, implement cold weather protection measures, or just let nature take its course.

Freeze damage can have a variety of effects on tender plants. It may range from light foliar damage and "tip-burn" to the death of small branches, larger branches or the whole trunk

The results of freeze damage may be subtle. For example, my collection of freeze-damage notes from 1990 shows no problems with olive trees. But the Olive News (UC Davis, August 1999) mentions an interesting effect. A freeze in '98 defoliated the branch tips of many 'Manzanillo' olive trees in Glenn and Tehama counties. There were few signs of damage to the stems, but they soon became infected with olive knot, a bacterial disease that may enter the plant through small wounds and leaf scars. The author of the article, Farm Advisor Bill Kueger said "In retrospect, additional copper sprays, when defoliation became apparent, may have reduced olive knot infection and limited the damage."

The other day a client commented that he was glad the wind blew the frost away, so he didn't have to worry about it. The comment brought out the need to distinguish between "frost" and "freeze." Frost may be like the frosting on a cake, pretty much on the surface. It forms when humidity is relatively high and the surface of the object is freezing cold. But there can be severe freeze damage in the absence of frost.

It's the freezing of deep tissue in stems that causes serious damage to susceptible trees and woody plants. The reason the freeze of 1990 was so damaging was the combination of freezing cold temperatures and the fact that it lasted for several days. Even so, it had no severe effects on most of our common hardy shrubs and trees and the broadleaf evergreen coast live oaks in our woodlands.

In our current cold spell, the afternoon temperatures have been well above freezing, so the effects in our gardens should not be severe. Still, some of our riskier tender plantings might be killed if protection measures are not implemented.

If broadleaf evergreens have already been damaged or defoliated by cold weather it may be hard to tell what is dead and what is alive. The best course is to wait for spring and sprouts and then prune to remove dead and severely damaged stems.

In the mean time here are a few protective guidelines:

* Drought-stressed plants tend to be more susceptible to freeze damage. Be sure your plants are well irrigated.

* Christmas lights strung through the canopies of citrus have proven to be effective at frost protection. Remember to turn them on when temperatures are freezing.

* Anti-transpirant sprays (Cloud Cover, Wilt Pruf) can give tender plants a few degrees worth of frost protection.

* Clear poly sheeting or canvas painting tarps and bamboo poles can be used to build protection tents over frost-tender plants. They are more effective when open underneath, so heat from the ground can rise into the canopy under the tent.

Bill Pramuk is an ASCA registered consulting arborist. Please send questions to bpramuk@pacbell.net