Safely Using and Applying Glyphosate Herbicides
It is safer to use a strong herbicide to control a dangerous weed such as poison ivy than to engage in hand-to-hand combat. Mix a small amount of glyphosate herbicide according to label directions, and use a paintbrush to "paint" weed leaves on a warm day when rain is not expected for at least 48 hours. Be careful, because spills and splatters also will injure or kill other plants.
Ready-to-use glyphosate sprays are another option, but spraying with this chemical should be done only in still weather. A small amount of wind can carry the spray to nearby garden plants. As an extra precaution, surround the weed with a bottomless cardboard box before applying herbicide. Do not use glyphosate near water, and limit your use to dangerous plants, such as poison ivy. Glyphosate works by interfering with photosynthesis, so plants die slowly over a period of several days.