Is it too late to warn about frost? After all the rain, the recent and sudden cold weather was quite a surprise. Fortunately, these recent frosts were relatively mild. This sort of weather is probably just enough to start to satisfy plants that require chilling through winter without causing too much damage to too many sensitive plants. So far, only the most sensitive plants, like angels’ trumpet, canna and left-out houseplants show symptoms of frost damage. (Cannas should get cut to the ground at the end of winter anyway.)
Doing without all frost sensitive plants would be too limiting. Lemon, avocado, bougainvillea, fuchsia and Australian tree fern would be off limits. Such plants are worth growing, as long as we understand the potential for occasional frost damage. Those that are too big to protect may sometimes need to get pruned for removal of stems that get killed by frost. In milder climates, such damage will be very rare. In cooler spots, damage is more common, and may involve a few tougher plants, like jacaranda.
Smaller plants that are sensitive to frost, such as jade plant, angel wing begonia and the various pelargoniums, can be grown in containers so that they can be moved to sheltered spots before the weather gets too cold for them. The most sensitive sorts need to be moved under a porch roof or eave, or maybe into a garage. More resilient plants may be safe under overhanging trees or against a wall. South or west facing stucco walls radiate a slight bit of warmth at night.
Frost sensitive plants that get too big for containers should be planted in sheltered spots, like below eaves or larger trees. If a severe frost is predicted, young plants can be protected by burlap, paper, trash bags or any convenient sheeting suspended above by stakes. Foliage that touches the sheeting may get frozen, but foliage within should be fine.
Foliage and stems that get damaged by frost should not necessarily get pruned away immediately. Although unsightly, the dead foliage insulates damaged stems below from subsequent frost. Besides, premature pruning can stimulate new growth, which is more sensitive to subsequent frost.
This really is the best climate for gardening. Even though summers are mild, there are not many plants that want for more warmth. Even though winters are mild, it gets just cool enough for many plants that require winter chilling. Yet, there are a few plants, particularly plants from tropical climates, that can be damaged by frost. The best way to protect such plants from frost is to not grow them.
Of course, this oversimplified technique would prevent the cultivation of familiar plants like banana, angel wing begonia and angel’s trumpet. In cooler regions, it might involve bougainvillea, philodendron and some types of citrus. Most of us would prefer to take some degree of risk to grow marginal plants. Some may get damaged by frost and then recover. Others may be killed by frost.
Potted plants can be moved to sheltered spots when threatened by frost. Some can be brought into the home or garage temporarily. For some, the simple shelter of a porch or a dense evergreen tree might be sufficient. Bougainvillea and other plants that do not like to be potted might prefer to be planted under the eaves against a warm south-facing wall. A bit of warmth radiates from walls at night.
During the coldest nights, some of the most sensitive plants that can not be moved to shelter may need shelter brought to them. Burlap, plastic, paper or any sort of sheeting that can be temporarily suspended on stakes above the foliage should be sufficient. Leaves that touch the sheeting can potentially get frozen. The sheeting should be removed during the day so that it does not get too warm in the sunlight.
Foliage that does get damaged by frost should not be pruned away immediately. It may be unsightly, but if left for later, it helps to insulate inner foliage and stems from subsequent damage from later frost. Besides, immediate pruning stimulates new growth that will be even more sensitive to frost than was the foliage and stems that had already been damaged.
There is no doubt that frost will return next autumn. It does that every year. Right now, we are more concerned that is should not return prior to that. It is now safe to plant plants that are sensitive to frost. Even if the weather were to somehow get cold enough to necessitate protection of frost sensitive plants, it will not be severely cold, and it will not last long. It is best to start now than to delay.
It is also safe to prune away foliage and stems that were damaged by frost through winter. It was best to leave it in place through winter, both to provide a bit of insulation for undamaged stems below, and to not promote new growth. Pruning it away allows warming sunlight to the undamaged stems, and stimulates generation of new growth. A bit of new growth might already be apparent.
Many leafy perennials can be cut to the ground, or at least just above their rhizomes. The tall vertical canes of cannas can be cut back to the low horizontal rhizomes that creep along the ground. The canes are not really stems anyway, but are merely upright foliar shoots. Any shorter new shoots that are beginning to develop can remain, even if a few outer leaves happen to be damaged.
Zonal geraniums can likewise be pruned almost to the ground, leaving only stubs of lower stems, even if only upper and outer foliage was damaged by frost. Although they do not need to be cut back so severely, they respond to such pruning splendidly, with vigorous new stems and foliage. The fungal foliar disease known as ‘rust’ overwinters in old leaves that get removed in the process.
Lemons, limes and any other citrus that were damaged by frost only need to have their damaged stems removed as far back as viable growth, where new buds might already be visible. However, if more pruning is necessary, this would be a good time to do it. Major pruning should not be done later in summer because the sensitive bark of inner stems can be scalded by sudden exposure to too much sunlight. Small trees that are sensitive to frost become more resilient as the grow larger.
Timing is very important in gardening. Even when the weather in autumn still seems like summer, spring blooming bulbs must be planted on time. Bare root fruit trees will become available while they are dormant, and will need to be planted before they wake up. Roses need to be pruned before buds for new canes swell later in winter, even if their old canes are still awake and blooming.
Some procedures are influenced more by environmental factors than timing. Planting bare root fruit trees and pruning roses must be done sooner if the weather gets warm sooner than expected. Irrigation must be increased in summer, but only as warm and dry weather necessitates such increase. It gets decreased in winter, but only as weather gets cooler and rainier. Then there is frost.
Frost arrives differently every year, and is of course worse some years than others. It is only a minimal threat in some regions, but limits what can be grown in others. It happened to be sneaky this year, by hiding in between remarkably pleasant days. While the daytime weather is so mild and even unseasonally warm, a weather forecast may provide the only early warning of overnight frost.
By now, sensitive plants were either protected successfully, or possibly damaged by earlier frosts. Potted plants got moved to shelter under porch roofs, or maybe dense evergreen trees near the home. Those in the ground might have been tented with plastic or sheeting suspended above the foliage by sticks and maybe string. Old fashioned Christmas lights might have softened the chill.
Many perennials and annuals are just left out to succumb to the cold. Any remaining warm season vegetable plants are not worth protecting. They will not produce anything this late anyway. Canna and ginger can likewise be left out in the cold for their foliage to freeze and collapse to the ground. If protected, they can stay green through winter, but will replace their foliage in spring anyway.
Foliage of hosta and dahlia can be removed because the dormant parts of the plants are safe below the surface of the soil. Canna and ginger rhizomes on the surface of the soil might appreciate a bit of mulch if their foliage gets removed. Frost damaged stems of woody plants should not be pruned out too early. Premature pruning stimulates new growth that will be sensitive to later frosts.