The best way to control weeds in your garden is without chemicals. This is especially true in vegetable gardens where you want to minimize the use of pesticides and rarely if ever use herbicides.
The use of herbicides can actually cause serious harm to your soil and you could ingest it but if you are wondering why your tomatoes are not growing and you use roundup anywhere near your beds that could be the problem because they are very sensitive to that chemical.
There are two basic types of weeds.
Annual weeds germinate in the spring and flower in the summer or fall this type is your grass varieties such as crabgrass, goosegrass and giant foxtail. Broadleaf weeds include smooth pigweed, common lambsquarters, purslane, galinsoga, common ragweed and tall morningglory.
When trying to remove these grasses and weeds it is important to remove the full plant and not leave the stems. Many weeds in this variety can propagate from the stem coming in contact with soil so if you turn them under your problem will remain.
Perennial Weeds include chickweed, yellow nutsedge, bermuda grass and bluegrass.Tilling the garden may spread these weeds by fragmenting and moving root pieces. Each of the root pieces may develop into a complete plant.
Mulches
Mulches can be divided into two groups Organic Mulch such as wood chips and Inorganic Mulch such as gravel.
Organic:
Straw
Peanut hulls
Chopped cornstalks
Ground corncobs
Pine needles
Broomsedge
Sudangrass
Grass clippings
Leaf mold
Compost
Newspapers
Sawdust
Bark
Synthetic:
Plastic (polyethylene)
Wax-coated paper
Aluminum foil
Kraft paper-polyethylene combination
Using Mulch and Cultivation in combination to Control Weeds
It is best to use a combination of cultivation and mulch to control any soil that you expect to plant in the near future.
You should stay away from herbicides because they can ruin your tomato plants causing small fruits and slow initial growth. Even very small amounts of herbicide such as 2,4-D or roundup can basically ruin your soil for vegetable gardening.