There are wonderful products on the market now that, when used according to directions, are safe and effective. But there are several ways we can unknowingly harm other plants in our yard or our neighbors'. The first major concern is the weather. On a windy day, you may aim your sprayer at one plant and hit plants downwind. If the temperature is very hot, a liquid or solid herbicide can turn to a gas, which also can spread to many more plants than those we wish to spray.
Rain also can play a part.
A heavy downpour can cause the herbicide to leach through the soil to roots of plants we want to encourage. Through the leaves is the most common way plants are harmed. There is evidence that barks can absorb more herbicides than has been suspected. If you spray poison ivy at the base of a red maple, the thin bark of the tree may absorb the herbicide and injure the tree. It is a good idea to shield or wrap the trunk when spraying other plants at the base of a tree.