How to Kill Grass Invading a Flower Bed

Use a Herbicide -- but Pick the Right Kind

Kill Grass Weeds in the Garden
Photo: Nicola Tree/Getty Images

Gardening is a deeply satisfying hobby -- you do it outside in the sunshine and warm breezes, and since it's usually done solo, it's a time for contemplation or losing yourself in gardening and forgetting about any problems in your life. Then you get a huge bonus: gorgeous flowers and/or delicious vegetables as the fruits of your labor.

But green grass, so lovely in the lawn, is an unwanted invader in the garden, as are weed-like grasses like crabgrass and other varieties. A few random blades can spread to form a thick mat in the loose, rich soil of a garden bed, diverting moisture and nutrients meant for ornamental plants. It's challenging for many gardeners to kill the grass without hurting the flowers as collateral damage. Regardless of whether the flowering plants are perennials or annuals, this is an issue.

Digging or pulling up the grass is a laborious task and generally not feasible in large garden beds. A herbicide is the best answer. 

Using Herbicide in a Flower Bed

You have several choices regarding which kind of herbicide to use to kill grass in a garden area.

One choice is to use a broad-spectrum herbicide. It will kill any ornamental plant it comes into contact with, as well as grass and weeds. It will also kill any lawn grass it comes into contact with. If the grass is close to but not touching your garden flowers, you can place a cardboard shield between the garden plants and the invading grass and use a broad-spectrum herbicide on the grass. Do this on a calm day to prevent herbicide drift from damaging your flowers. For any garden plants that are still small, you can protect them with a bucket or large flowerpot.

If you need to kill grass growing close to ornamental plants, you can apply a selective herbicide that targets only grasses without harming your garden plants. These herbicides will kill most annual grasses like crabgrass and foxtails, as well as perennial grasses like nimblewill and quackgrass.

If you have perennial nutsedge, also commonly called nutgrass, you need to apply a herbicide specifically labeled to kill nutsedge. You can identify nutsedge by the small tubers in the root ball. Nutsedge isn’t a grass but rather is a member of the sedge family.

You can kill broadleaf weeds that invade your garden in the same way as grasses. If you find you have certain weeds or grasses returning year after year in the same location, they are perennials. You can keep these pests from ever coming up by using a pre-emergent herbicide where you know they will grow several months before they emerge. This will break the growth cycle by stopping plants from forming seed heads or runners that increases the problem from one season to the next. 

Read the label carefully before you buy any herbicide to be sure it will kill the type of weed or grass that is invading your flower bed and that it is the type you want, whether it be a broad-spectrum herbicide or selective to grasses. 

Prevent Grass From Growing Back in Flowerbeds 

As is the case with many garden maladies, prevention is the best medicine. Once you achieve control with a herbicide, prevent the grass from coming back by covering the soil of the flower bed with a 3-inch layer of mulch to discourage new grass seeds from germinating. Although using geotextile landscape fabric seems like a reasonable shortcut, these products usually create more work in the long run when they tear, or when weeds germinate on top of the fabric and anchor it to the soil. Instead, use wood chips, shredded leaves, or compost, all of which will prevent grass seeds from germinating, and will make any invaders easier to pull.

For stubborn and repeat offending grasses, you may need to apply a pre-emergent product to prevent grass and other weed seeds from germinating. This also prevents flower seeds from germinating, so apply after your flowering plants germinate. Corn meal gluten is an organic option that has exploded in popularity in recent years. However, you must apply a thick layer for maximum efficacy, and if any seeds have already germinated the nitrogen content of the corn meal gluten will actually fertilize the weeds. A granular product that contains trifluralin is a synthetic alternative that can provide weed germination control in the garden for up to four months.