Azaleas sometimes suffer frost injury in the fall before they're fully adjusted for winter. But this can largely be prevented, says USDA's Agriculture Research Service, by removing the mulch about three weeks before the first frost. Studies at Glenn Dale, Maryland, showed that mulch lowers the air temperature above it in fall and winter, when it insulates against heat loss from the soil.
Tests in October and November showed that air temperature two inches above the mulch was five degrees lower than at the same level above non mulched plots. in one case, a temperature of 28 degrees was noted a week before the first recorded frost. While there were frost particles on the hay mulch and on the mulched azaleas, no frost was apparent on bare ground or on unmulched azaleas.
The later the mulch remained, the greater the winter injury: Almost no dead wood was found on plants from which mulch was removed before the frost. A corresponding number of azaleas of the same variety that were mulched suffered severe injury.
Winter injury to azaleas consists largely of flower bud damage, leaf burning and splitting bark on stems. Usually the plants show these symptoms during the winter, but occasionally the effect will not be noticed until the next summer, when the plants collapse during a drought. Azaleas sheltered by buildings, shrubbery and trees may not suffer to the same degree.
If you have been experiencing winter injury, it may be a good idea to remove the mulch from your azaleas now and leave it off all winter.
But generally, after the azaleas have hardened for winter, return the mulch. Winter mulch has many advantages. It conserves moisture and encourages its penetration; it insulates the soil to prevent the ground from freezing deeply. Plants that benefit most from winter mulches are the broadleaf evergreens such as azaleas, rhododendrons, boxwood, Japanese and other hollies, camellias and Pieris japonica.
Materials for mulching include bark, wood chips, sawdust, straw, salt hay, chopped leaves and compost. They can be applied to a depth of two to three inches, but don't mound them up too much around the stems of the plants. Straw or chopped leaves may be applied a little thicker since both will settle and pack over the winter. GERMINATING GESHERIADS -- You probably have tried your hand with at least one gesneriad -- the African violet. This Saturday, 1 to 5, and Sunday, 10 to 5, you can see other members of this flowering family at the National Arboretum (24th and R Streets NE). The free show is sponsored by the American Gloxinia and Gesneriad Society. For details call 472-9100. BOXWOODS ON WHEELS -- The American Boxwood Society is sponsoring a tour of area mansions and gardens on September 29 and 30. Stops include Montpelier Mansion, an 18th-century Georgian home in Laurel; the Smithsonian Institution gardens; Gunston Hall, home of George Mason; Oxon Hill Manor, now in the process of being restored; and the garden of a family in Mclean. Meet the bus at either the Boyce Experimental Farm or the Old Colony Inn in Alexandria. Advance registration only, as bus space is limited. Write Thomas Ewert, Box 85, Boyce, Virginia 22620.