Mulching trees, shrubs, flower and vegetable gardens has long been considered very beneficial. A good mulch can hold water and control weeds, be coarse and porous, nontoxic and attractive. Francis R. Gouin, University of Maryland professor of ornamental horticulture, has found, however, that too deep a mulch can be harmful and worse than no mulch at all. Also, years ago, researchers at USDA Agricultural Research Center, Beltsville, discovered that mulches should be removed from evergreen azaleas about three weeks before the first fall frost to avoid damage to the plants. "Piling mulch around the stems of trees and shrubs appears to encourage the formation of stem cankers, restrict the growth of tree trunks and suffocate the roots," Gouin says. "During the past three years I have been asked to investigate a number of complaints believed to be related to natural- gas-line leaks. In almost every instance the cause of death could easily be traced to over-mulching. "Layers of mulch five to 10 inches thick had accumulated as a result of applying mulch every year for many years. "In one case, a mulch 14 inches thick was found at the base of a large Bloodgood Japanese maple that had been mulched yearly for eight to 10 years. Not only was the base of the stem heavily infested with cankers, but the stem portion beneath the mulch was several inches smaller in diameter than the stem above the base. We have known for many years that crown cankers on spruce trees can be caused by excessive mulching. I can assure you that spruce is not the only plant to suffer from this disease. "Apply mulches one to two inches thick, incorporate existing mulches before applying a new mulch and do not pile mulches around the stems of plants." The Beltsville researchers found that azaleas sheltered by buildings, shrubbery and trees are less likely to be damaged by leaving the mulch around them. This shelter gives them a measure of protection from frost. They learned that the mulch lowers the air temperature in the fall and winter by insulating against heat loss from the soil. Tests during October and November showed that the air temperatures two inches above the mulch averaged five degrees lower than at the same level above nonmulched azaleas. In one case, a temperature of about 28 degrees 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 azaleas. Azaleas and other hardy plants prepare for the cold season in several ways. The new growth is hardened and drained of excess sap so that freezing will not burst the cells. If frost comes before these preparations are finished, injury results.

Two shows for flower fans:

The National Rose Show is at Twin Bridges Marriott Motor Hotel Saturday 2:30 to 6:30 and Sunday 1 to 5:30. $1. Call 256-0326.

And the National Arboretum exhibits gloxinias, columneas, sinningias and desneriads. Find out what that means Saturday, 1 to 5, and Sunday 10 to 5. Free. Call 472-9279.