The washing of orchard trees is an old practice. It usually results in making a tree more vigorous. One reason is that it destroys insects and fungi that lodge underneath the bark; but probably the chief reason is that it softens the bark and allows the trunk to expand. It is possible, also, that the potash from the soap or lye eventually passes into the ground and affords some plant-food. Trees are ordinarily washed with soap suds or with a lye solution. The material is usually applied with an old broom or a stiff brush. The scrubbing of the tree is perhaps nearly or quite as beneficial as the application of the wash itself.

It is customary to wash trees late in spring or early in summer, and again in the fall, with the idea that such washing destroys the eggs and the young of borers. It no doubt will destroy borers if they are just getting a start, but it will not keep away the insects that lay the eggs, and will not destroy the borers that have found their way beneath the bark. It is perhaps quite as well to wash the trees very early in the spring, when they are starting into growth.

It is an old practice to wash trees with strong lye when they are affected with the oyster-shell bark louse. The modern method of treating these pests, however, is to spray with some kerosene or oil compound when the young growth is starting, for at that time the young insects are migrating to the new wood and they are very easily destroyed (p. 204).

The whitewashing of the trunks of trees tends also to relieve them of insects and fungi; and it is probable that in hot and dry regions the white covering affords protection from climate.