One of the problems we have with using pre-emergence herbicides in the garden is that they may affect some of the vegetables we grow.  In addition, don’t expect to control all weeds in a garden of mixed vegetables with one herbicide. Why? First, no single herbicide controls all weeds. Secondly, some vegetables are also sensitive to the herbicide, and if the wrong herbicide is used, the vegetable is injured along with the weeds. Finally, some herbicides have quite long residual activity and may have a long-term negative effect on subsequent garden plants.

Generally it is recommended that a gardener use a combination of hand weeding, cultivation, mulching and good garden hygiene to keep weeds under control. Most of us know what hand weeding involves. Cultivation can include tilling, hoeing, and mowing. Several types of mulches used in the garden include plastics, weed barriers, grass or straw mulches, or newspaper or cardboard. Garden hygiene means that we don't allow annual weed to produce seeds that build up the weed problem.

If you do decide to use a herbicide, here are a couple of suggestions. Remember, they need to be applied at the appropriate dosage, at the correct time relative to the weeds you are trying to control, they may need incorporation into the soil, and may only control some of the weeds in a garden

Dacthal (several brand names available) can be used on a wide variety of vegetable plants. Applied correctly, Dacthal gives good control of most grass weeds and a few broadleaf weeds. Dacthal controls weeds as the seeds germinate, so remove existing weed plants prior to application.

Trifluralin (several brand names available) is a pre-emergence herbicide used to control grass problems in the garden. Treflan should only be applied to vegetable tolerant to this herbicide. For the best weed control, mix Treflan with garden soil. After application, mix the herbicide in the top 2 inches of the soil. Read the package label for a list of approved vegetables.

Poast is a post-emergence herbicide that selectively controls grass weeds in several vegetables. Apply Poast to the grasses before they reach 8 inches tall. Mix a crop oil concentrate in the spray solution before application. Read the Poast label for specific instructions and approved vegetable crops.

Glyphosate (several brand names available) is a nonselective, post-emergence herbicide approved for limited use in the vegetable garden sites. Glyphosate is used to eliminating existing weeds before a vegetable seedling emerges from the soil and before vegetable plants are transplanted in the garden. Read the label for specific application instructions and limitations.
 
For more detailed information on weed control for the yard and garden, refer to the Utah State University extension fact sheet "LANDSCAPE AND GARDEN WEED CONTROL" http://extension.usu.edu/files/publications/publication/HG508.pdf