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Baby, it's cold outside--and do our plants know that all too well

By Tony Tomeo

Contrary to popular belief, a few plants are rather discriminating about architecture. Bougainvillea, jade plant, philodendron, tropical hibiscus and many other tropical plants prefer homes built after the Victorian period but prior to about 1975. Their favorites are single-story Eichler and California ranch houses, particularly with original windows, and lacking insulation.

This time of year, other architecture leaves a bit to be desired by the standards of frost-sensitive plants. Modern homes are too well insulated to share their interior warmth with the garden. Victorian homes lose plenty of warmth but are a bit too high above grade to warm low perennials, and their eaves are too narrow and high above the ground to hold in much radiant warmth.

Regardless of architecture, though, frost protection becomes a concern during the coolest winter weather. Certain architecture merely affords more options. The most practical option is still to avoid growing plants that are likely to be damaged by frost. Since this is somewhat too exclusive for many garden enthusiasts, the next best option is to grow frost-sensitive plants in pots that can be moved to sheltered areas when the weather gets uncomfortably cool.

However, it is not practical to grow all frost-sensitive plants in pots. Many simply grow too large or are not adaptable to root confinement. Those that will be happy in partial shade can be strategically located and planted in the ground where they will be at least partially sheltered by larger trees or wide eaves. This is when energy-inefficiency and large eaves become advantageous.

Plants such as bougainvillea, hibiscus and lemon trees that need full sun exposure during the summer may unfortunately need other protection from frost during winter. This is most commonly done by covering the plants with plastic or fabric at night. Because foliage that touches the covering is likely to freeze to it and be ruined, the covering should be suspended by simple wooden frames, stakes or wire strung between stakes or nearby trees.

Coverings should be removed during the day to prevent unseasonably warm conditions underneath. Black plastic accumulates warmth outside, then radiates the warmth inside. Transparent plastic allows sunlight through to warm the interior, but does not allow the heat to dissipate outside--the "greenhouse effect." This warmth can stimulate new growth that is even more sensitive to frost.

Of course, large lemon trees and other frost-sensitive plants that cannot be covered will sometimes be damaged by frost. If they are too big to be covered, they are likely big enough to not be too bothered by the damage, and should replace it with new foliage in spring. Damaged limbs may need to be pruned later.

When plants happen to get damaged by frost, their damaged foliage should not be pruned away immediately. Pruning stimulates premature emergence of very sensitive new growth. Besides, the damaged foliage helps to insulate foliage within from additional damage. My geraniums (pelargoniums) are too abundant to be covered, so I allow them to succumb to frost completely exposed. They look bad for a while, but get cut down in late winter, and vigorously restore themselves during spring.

Tree of the Week:
Red gum

Since the historically severe freeze of Christmas Eve 1990, the weather has been cold enough to damage the otherwise resilient red gum, Eucalyptus camaldulensis, only a few times, including last winter. Fortunately, frost damage is typically very minor. In 1990, many trees, including those in Los Osos, were damaged enough to be disfigured. Some were even killed.

Although red gum is the second most common eucalyptus in California, it is quite uncommon in landscapes. Both the red gum and the most common blue gum live mostly in naturalized groves or wind breaks. They were planted extensively in the early 1900s for lumber or wood pulp, but later abandoned. Red gum is usually a bit smaller and shapelier than blue gum, but does not hesitate to get more than 100 feet tall and almost as broad. It is therefore rarely planted near homes. However, homes are sometimes planted near established red gum trees.

Horticulturist Tony Tomeo can be reached at lghorticulture@aol.com or 408.358.2574.