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Protecting against frost: what you can do to help tender plants.

A YEAR AGO THIS month, the freezing "Siberian express" left many Western gardens decimated--and gardeners wondering how to protect borderline-hardy plants when temperatures dip well below normal for prolonged periods.

"Plastic wrap, roof, and cloud cover all failed," wrote Sunset reader Mary Trapani, from Pleasant Hill, California. Unlike common radiation frosts--caused by loss of heat from surfaces to the sky--last year's freeze resulted from a weather system that pushed south from the Arctic to replace existing warmer air. Because this cold air was driver, it increased evaporation, which also removed heat.

Temperatures fell too low for most frost-protection techniques to work. Only those solutions which both provided and trapped heat, such as the box pictured above, protected vulnerable plants like jade.

If you live in a mild-winter area and you indulge in growing frost-tender plants such as boungainvillea, bird of paradise, and Natal plum, freezing temperatures can seriously damage or kill plants unless you take preventive action. Here's what you can do to protect your plants from occasional frosts.

Precondition plants. As winter approaches, slow down and harden plant growth by withholding nitrogen fertilizer, gradually watering less, and avoiding heavy pruning. Let plants form seed or fruit, to force slower growth.

Keep soil moist. Since dry winter winds and cold temperatures cause faster evaporation from soil and leaf surfaces, plants can wilt unless soil is sufficiently moist. Plants weakened by wilting are more susceptible to damage from freezing temperatures. Also, moist, bare soil absorbs more heat than dry or light-colored soil.

Mulch. Apply a 6-inch layer of loose material such as leaves, redwood sawdust, or straw around the base of shrubs to help insulate their crowns and roots from cold. (To avoid stem rot, move mulch away from stems if temperatures stay mild and rains are heavy.)

Trap heat. Overhead protection helps prevent heat loss. Existing heat from soil, paving, or nearby structures can be obtained to protect plants. Boxes, bags, blankets, or tarps can provide temporary cover; their effectiveness depends on the amount of stored heat, the soil, and condition and type of plant. During last December's freeze, Charmaine Kaiser of Los Osos, California, successfully used an aging lawn umbrella of heavy opaque plastic, secured in a stand, to shelter a young lemon tree.

For best results, cover the plant completely; to avoid leaf burn, keep thin materials (such as plastic) from touching the foliage.

Protect from sun. If plants are frozen, shade from morning sun with shadecloth, sheets, or burlap. Otherwise, damage from quick thawing may result.

Spray with water. When frost is expected and the air is extremely dry, set an oscillating sprinkler under plants and trees sturdy enough to support the weight of ice; keep it running all night (a routine practice of citrus growers). As ice forms, the heat released protects plants. (Do this as a last resort, and only if the water situation allows.)

Set up heaters and lights. Supplying additional heat protects plants from frost, especially when used with a cover or windscreen. (Hanging a light bulb from an orange tree's branches and covering with a tarp pulled one tree in Alameda County, California, through last winter.) How much heat is needed will depend on how hardy the plant is and low the mercury drops.
COPYRIGHT 1991 Sunset Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Publication:Sunset
Date:Dec 1, 1991
Words:542
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