Azaleas are sometimes seriously damaged by an early frost that occurs before they have hardened (adapted) for winter. Those that are mulched are particularly vulnerable.
Scientists at USDA's Agricultural Research Service recommend that the mulch be removed from around the azaleas three weeks before the first fall frost if winter injury has occurred to them in the past.
Tests by these scientists have shown the air temperature two inches above the mulch to be five degrees lower than that at the same level above unmulched areas, enough to cause frost formation.
A portion of the sun's energy is absorbed by the earth during the day, and at night the earth's warmed surface gives up the accumulated heat to the cold, thin, dry upper atmosphere.
As this radiation cooling occurs, the air immediately above the radiating surface is warmed slightly, often enough to prevent frost. The mulch lowers the air temperature in the fall and winter by insulating against heat loss from the soil.
In one case, a temperature of 28 degrees F. was noted a week before the first recorded frost. Frost particles were observed on the hay mulch and on the mulched azaleas when no frost was apparent on bare ground or on unmulched plants.
Winter injury was found to be directly related to the date in autumn that the plants remained mulched. The later the mulch remained, the greater the winter injury.
Almost no dead wood was found on a number of plants from which the mulch was removed before frost, but a corresponding number of azaleas of the same variety that were mulched suffered severe injury.
Azaleas sheltered by buildings, shrubbery and trees may not suffer to the same degree.
Winter injury to azaleas consists largely of flower bud damage, leaf burning and splitting of the bark of stems.
If you have not been having trouble, your azaleas may not benefit greatly from mulch removal. A winter mulch has many advantages: It conserves and encourages the penetration of moisture and insulates the soil to prevent the ground from freezing deeply.
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