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When to use antidesiccants

Q: Are antidesiccants worth the investment? If so, when is the best time of year to apply them?

Claudia Larrow, Burlington, VT


Antidesiccant sprays can help reduce water loss and protect plants from drought.
Antidesiccant sprays can help reduce water loss and protect plants from drought. Photo/Illustration: Allison Starcher

A: Lee Reich, contributing editor and author of Uncommon Fruits for Every Garden, responds: Antidesiccants, also called antitranspirants, are products that slow the loss of water from plant leaves. The best reason to apply such a product is to stop or reduce the amount of water being lost through leaves at times when roots are unable to take up sufficient water. Without the help of an anti­desiccant, leaves can scorch or a plant could even die.

Evergreens might find themselves in such a predicament in winter, when bright sun, wind, and air temperatures above freezing coax water from leaves yet the soil remains frozen.

Even during the growing season, a plant might not be able to keep up with water lost through its leaves and could benefit from protection. Bare-rooted or balled-and-burlapped trees, for instance, lose roots during transplanting so they have trouble replacing water, even in moist soil. In a dry summer, all plants struggle for water. Plants that are susceptible to drought and cannot be watered are good candidates for antidesiccants.

Antidesiccants have been known to help reduce the incidence of certain diseases, such as downy mildew on zinnias and black spot and powdery mildew on roses. The first lines of defense in fending off plant diseases, however, should be plant selection, site modification, and pruning. Antidesiccants can also help preserve the foliage on Christmas trees and on evergreen branches cut for vases.

Some caution is needed when using antidesiccants. Because leaves lose water through the same pores that take in carbon dioxide for photosynthesis, antidesiccants can be toxic to plants. In addition, these products are gener­ally more hazardous to evergreens than to deciduous plants. For example, you should never apply antidesiccants to blue spruces or other plants that have a bluish waxy coating on their leaves. That waxy coating is the plant’s own natural antidesiccant; spraying an antidesiccant washes away that wax and the blueness.

You can minimize the toxic effects from antidesiccants by reading the label carefully and noting the warnings for specific plants and the directions for dilution and timing. For winter protection, spray the leaves with an antidesiccant in late fall and then again toward the end of winter. Spray only when temperatures are above freezing, and wash out the sprayer with warm, soapy water immediately after each application.

It’s best to avoid plant-moisture problems in the first place by siting your plants appropriately, watering the soil as needed, and mulching to conserve soil moisture and to reduce and delay soil freezing so that roots remain active.

From Fine Gardening 99, pp. 21