All grasses have the potential to spread to an untended garden, but some grasses grow so rapidly that they take over cultivated plots in one to two growing seasons. Bermuda grass and bahia grass remain the chief offenders and spread easily into garden beds via aggressive runners. Grasslike weeds, such as crab grass and goose grass, produce and spread through vast quantities of seed. Tilling the soil is not the best option because the stems and seeds re-establish. Gardeners use other mechanical and chemical methods to kill grass growing over a garden, but they often have to wait a season to sow seed or transplant.
Water the vegetation prior to removal. Moist soil loosens up quickly and releases roots more easily.
Pull grasslike weeds from the crown, making sure to remove the entire root mass. Collect and destroy any broken pieces that may potentially regrow.
Cut a trench around the unwanted grass to separate it from the permanent grass or garden crops. Push a spade vertically into the soil to sever the grass roots and stems and prevent chemical treatment from drifting to permanent vegetation.
Apply a post-emergent herbicide with glyphosphate to grass growing over an empty garden. Use a low-pressure, coarse-droplet applicator or wick-type applicator to control the spray. Use the herbicide according to the manufacturer's rate of application and on non-windy days when it is not expected to rain for at least 48 hours. The application may take up to two weeks to be effective.
Use solarization to kill grass growing over a garden with crops. Spread 3 mm black plastic sheets over the unwanted grass in July and August. Plastic sheeting traps the sun's heat and weakens the grass so that it dies. Seal the edges of the plastic with soil or heavy boards laid end-to-end. Remove the plastic when the grass is dead, usually six to eight weeks later.
Separate the dead grass from the soil. Shove the spade horizontally under the grass and push upward on the handle to pull the sod away from the soil. Continue removing the dead grass until the garden is clear.