Weed control is a constant issue with farmers, whether their farm is a small vegetable plot or a large commercial operation. Weeds can choke vegetables by stealing light, nutrients and water from the vegetable plants. Many farmers cope with weeds by poisoning them using herbicides. But vegetables can absorb systemic herbicide left in garden soil. Herbicides can even contaminate your soil through runoff from nearby farms. You must grow vegetables cautiously in ground sprayed with herbicides.
Take a soil test to determine if there is herbicide present in your soil. A soil test will indicate what herbicide has been used in your soil and what quantity is left in the soil. To take a soil test, dig up 1 quart of soil from 10 locations across your garden using a shovel. Mix these soil particles in a bucket and dump them on a piece of newspaper to dry. Collect 1 cup of soil in a sandwich bag and take it to your county extension service. An agent with the county extension service will help you to send the soil to a soil testing laboratory. Test results typically come back within three weeks.
Check with your county extension agent for information on the herbicide indicated by the test result. According to the Extension Toxicology Network at UC Davis, a county extension agent is one of the best resources for information on chemicals used in the garden. There are three types of herbicides typically used to control weeds. Contact herbicides cause plant tissue to dry out and die. Growth-regulating herbicides disrupt a plant’s systems such as cellular division or photosynthesis. Soil sterilants are nonselective plant poisons that kill everything they touch.
Select vegetables to grow based on the type and concentration of herbicide left in the soil. Selective herbicides may affect one particular type of plant, but may not be absorbed by another type of vegetable. For example, if your soil contains a pre-emergent herbicide such as Trifluralin, you can plant vegetable transplants, but not seeds. Trifluralin prevents seeds from sprouting in your garden.
Fill 5-gallon plastic storage tubs with potting soil to create a raised bed garden for vegetables over soil that is contaminated with a soil sterilant. A soil sterilant will not allow any vegetables to grow in the soil until it completely breaks down. Continue to garden in containers until the herbicide breaks down in your soil. According to NC State University, some herbicides may take up to four years to break down in soil.