Pre-emergent herbicides: These herbicides do not kill growing weeds. They should be applied to weed-free soil, either before weed seeds sprout in the spring, or after weeds have been removed. Pre-emergent herbicides attach to soil particles in the top inch of soil, where most seed germination takes place, and kill weed seeds as they germinate. If the soil is cultivated after application, the herbicide will be diluted and its effectiveness diminished.

Postemergent herbicides: These herbicides control weeds that are actively growing. When spraying them, be careful not to allow the material to touch desirable plants. This can be most easily done by following these guidelines:

1. Spray on a windless day.

2. Use low pressures when spraying.

3. Keep the nozzle as close to the weeds as possible.

4. Use a shield of cardboard, plywood, or sheet metal to protect garden plants when working near them.

Clean sprayers carefully before using them for fungicides or insecticides. Some gardeners keep separate sprayers for herbicides.