The active ingredient in Roundup weedkiller is glyphosate, which interferes quickly with their growth. Plants absorb glyphosate through their leaves, and because the chemical is non-selective, the vegetable plants in your garden are just as vulnerable as the weeds. If you happen you get Roundup on the vegetables themselves, however, you won't cause them any damage, but you should definitely wash them thoroughly before eating them.
How Roundup Works
Roundup is mostly glyphosate, although it usually contains surfactants to prevent the spray from beading up around hairs on leaves and thus be absorbed more readily. Once inside the plant, glyphosate migrates to areas of new growth and blocks an essential enzyme, causing the plant to wilt and die. Glyphosate continues through the plant to the roots, stays close to the roots in the soil and breaks down after two or three weeks. Glyphosate is a post-emergent weedkiller, which means that it's only effective against growing plants. It has no effect on seeds or sprouts that haven't broken ground.
Planting After a Treatment with Roundup
Because glyphosate doesn't affect pre-emergent plants, the main consideration when clearing weeds from a vegetable garden that you haven't planted yet is leaving the weeds in the ground long enough for it to work. That's usually a matter of 72 hours at the most -- perhaps longer in rainy weather. Some Roundup products, however, contain a weed preventer called imazapic. These products, which are clearly labeled, can make the ground unsuitable for planting for up to four months and aren't suitable for use in vegetable gardens. It's important to read the label before using a Roundup product.
Spraying Weeds in an Existing Garden
It's possible to spray weeds in an existing garden without killing the vegetables, but it takes care and the proper tools. You need a sprayer with a nozzle you can adjust to a narrow width, and a garden hose readily available so you can wash off the spray from the vegetable plants immediately. Choose a windless day to spray your garden, and bring a piece of plastic or plywood with you to screen desirable plants. When the spray has settled on the leaves of the weeds, the rest of the plants in the garden are safe from it -- it won't soak through the soil and into the roots of your vegetables.
Environmental and Safety Cautions
The company that manufactures Roundup -- Monsanto -- characterizes glyphosate as nontoxic and harmless to humans, and the U.S. Environmental Protection Agency upholds this assessment. Not everyone agrees, however. Dr. Don Huber, a plant pathologist, tells alternative medicine activist Dr. Joseph Mercola on Dr. Mercola's website that Roundup persists in the soil and is as potent a toxin as DDT. Moreover, a team of French researchers from the University of Caen found polyethoxylated tallowamine, one of the surfactants in Roundup, capable of causing birth defects in humans. In light of ongoing research, the decision of whether to consume vegetables from a garden treated with Roundup is an individual one.