Lush green grass growing in a lawn is undeniably lovely and desirable, but the same grass plants creeping over into your garden beds can be one of the most stubborn weeds you can encounter. In additional to perennial turf grasses that can escape lawns and invade gardens, there are a variety of weedy annual and perennial grasses—sedges and crabgrass and quack grass—that can be ongoing enemies for the gardener. Weed grasses of all kinds require some special attention to eradicate them from the garden since it can be hard to kill them without hurting flower or shrubs as collateral damage.
Chemical weed killers (herbicides) of various types offer the best way to combat the various species of grasses that can invade a garden bed, but not all herbicides are created equal.
Broad-leaf herbicides: The most common weedkillers used are known as broad-leaf herbicides, and their chemistry is such that they are harmless to most plants with blade-like leaves (including all the grasses). This is the reason why these herbicides can be safely used on lawns—they don't affect the grass plants but are deadly to other types.
For this reason, traditional broadleaf weedkillers are worse than useless for combating grass in garden beds, since not only do they fail to kill grasses, but will kill any other plants or flowers they happen to contact.
Broad-spectrum herbicides: To kill grasses, you will need a weedkiller known as a broad-spectrum herbicide, which is chemically formulated to kill all herbaceous plants. Most of these products contain a chemical compound called glyphosate (RoundUp is one well-known brand). This compound inhibits certain critical plant enzymes and is absorbed through leaves to kill that entire plant, root and all. These products will indeed kill grasses, but they need to be applied very carefully, targeting only the grass plants and other weeds, since any drifting spray can also kill ornamental plants.
Selective grass killers: Some herbicides are chemically formulated to target only one specific plant or category of plant. Just as broad-spectrum herbicides are selective for plants that have wide, broad leaves, there are also herbicides that are selective for bladed grasses. Grass killers target only various grasses and do not affect most other ornamental plants. These are also a good choice for fighting grass in your garden beds.
Pre-emergent herbicides: Most types of herbicide are described as post-emergent products, since they are applied to plants that have already germinated, sprouted, and are actively growing. Another category of herbicides is per-emergent, which are formulated to control weeds by hindering germination of new seeds. These will inhibit the germination of all new seeds, not just grass seeds, so they need to be used carefully in a garden bed application. Applied at the wrong time, for example, they will prevent your planted vegetable or flower seeds from sprouting. And these herbicides do nothing to kills the roots of established perennial grasses.
The chemical glyphosate was once thought to be one of the safest of all chemical herbicides, and this is a claim still made by the manufacturers of products such as RoundUp and others containing glyphosate. It is true that glyphosate has less residual impact on the environment than some of the other chemicals used in herbicides since the active ingredient is rendered inactive soon after contacting common soil enzymes. But there is also recent evidence of health issues for industry workers who handle glyphosate, and there is now considerable controversy regarding this chemical. In March 2015, the World Health Organization's International Agency for Research on Cancer classified glyphosate as "probably carcinogenic in humans." But the United States EPA views glyphosate as having low toxicity to humans and concludes that glyphosate is not likely to be carcinogenic in humans. If you use glyphosate properly, according to label directions, it is probably safe, but individuals concerned about environmental chemicals will want to avoid it and other herbicides.
When to Kill Grass in Garden Beds
Most grasses spread both by underground roots (rhizomes) as well as by setting and dropping seeds, so efforts to eliminate grass in the garden becomes harder if you allow the grass plants to mature and set seed. Each plant can scatter hundreds of seeds in the garden bed. And any chemical or manual attacks on grass plants must also address the roots, as even a small bit of remaining healthy grass root can re-sprout into a full grass plant. Grass in the garden should be dealt with whenever you see it, but especially early in the growing season before seed heads appear.
What You'll Need
- Garden sprayer with wand
- Rubber gloves
- Garden trowel or hand fork
- Herbicide of your choice (broad-spectrum or selective, or organic)
You have several choices regarding which kind of chemical herbicide to use for killing grass in a garden area. There are also organic methods you can use.
Using Broad-Spectrum Herbicide
One choice and perhaps the most effective one is to use a broad-spectrum herbicide, such as one containing glyphosate. But be aware that broad-spectrum herbicides will kill any ornamental plant it comes into contact with, as well as grass and weeds; and it will also kill any lawn grass it comes into contact with. Glyphosate weedkillers are available both as liquid concentrates you mix with water and apply with a garden sprayer, as well as in pre-mixed forms sold in spray bottles.
- If using a concentrate, mix up a batch of weedkiller, following label directions exactly. Do not increase the amount of weedkiller in the chemical/water mixture, since maximum effectiveness occurs if you follow the recommended ratios exactly. Make sure to use rubber gloves when handling the chemical.
- On a calm, windless day where no rain is forecast for a day or two, apply the herbicide to the entire exposed grass plant, completely drenching the blades. A wand-style sprayer works best for this since you can closely target the grass plant with little or no overspray into the surrounding air. Where grass plants are in close proximity to ornamental plants, you can block the spray with a piece of cardboard or shield plants with a bucket or large flowerpot while you spray the grass plant.
- Clean your equipment thoroughly with soap and water, making sure to spray soapy water through the sprayer wand to cleanse it. Make sure to rinse thoroughly.
- Observe the grass plant over the next few days; you should see it begin to turn brown and die. If the blades are not completely killed, reapply herbicide.
- Avoid the temptation to dig up the plant too soon. It's important that the chemical is transported to all portions of the root. Once the grass plant has appeared completely dead for several days, you can remove the exposed portion from your garden.
Using Selective Herbicide
If you need to kill grass growing close to ornamental plants, you can apply a grass-specific herbicide that targets only grasses without harming your garden plants. These herbicides will kill most annual grasses such as crabgrass and foxtails, as well as perennial grasses such as nimblewill and quackgrass. Apply these exactly as you would a broad-spectrum herbicide (see above).
One type of "grass" isn't a grass at all. 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, and thus requires a different type of herbicide.
Using Pre-Emergent Weed Killers
Preen and other pre-emergent herbicides used to control weeds do their work by preventing germination of new seeds that fall on the soil. These products do not affect the roots of perennial weeds, nor can they prevent existing weed seeds in the soil from germinating. Thus, they are of limited effect as a measure to prevent perennial grasses from growing in your garden. Pre-emergents can be one measure among several used to help control grasses and broadleaf weeds, but you should be aware that they will also prevent desirable seeds from germinating. If your garden contains ornamental plants that routinely self-seed themselves, this herbicide will stop these seeds, also. Per-emergent weed killers usually come in a granular form that is blended into the garden soil.
If you are uneasy about using garden chemicals of any kind, then consider one of these purely organic methods:
- Vinegar: Diluted vinegar, sprayed onto grass plants by hand, will kill grass plants. It may require repeated application, however.
- Boiling water: Pouring water over the grass plant normally kills it, root and all.
- Flame: A variety of propane torch tools are available that allow you to kill weeds by hitting them with very high heat. This often (but not always) kills the roots, as well.
- Solarization: If a large area of the garden is infiltrated by grass, you can kill the grass over the whole area by laying clear plastic and allowing sunlight to smother and heat up the soil beneath it. This will kill the seeds and roots of all plants in the soil beneath the plastic. This can be a good method if you are creating a garden bed in an area that contains lawn grass, as the method will completely kill the grass without removing the sod.
Removing Grass by Hand
Small infestations of turfgrass or weed grass in a garden can be addressed by hand removal. But most grasses are perennial plants with rhizomatous roots that can easily re-sprout if even a small bit of root remains. So complete removal involves carefully loosening the soil and tugging out as much of the grass-root as possible. With some grasses, the root system can be many inches long, but these can be removed entirely with patient work. Loosen the soil thoroughly with a trowel or hand fork, then carefully tug the grass-roots out of the soil.
Hand removal is an ongoing process and can be done whenever you are performing routine weeding duties in the garden. It is not practical for large gardens that are heavily infested with grass, but for a small garden or one that has minor grass problems, this is a good, environmentally friendly solution.
Tips for Preventing Grass in the Garden
As is the case with many garden maladies, prevention is the best medicine.
- Once you achieve control, 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 make weeding duties easier.
- For stubborn and repeat-offending grasses, you may need to apply a pre-emergent product to prevent grass seeds from germinating. This also prevents flower seeds from germinating, so apply after any seeds for flowering plants have sprouted.
- Cornmeal gluten is an organic option for lawn crabgrass control that has exploded in popularity in recent years. It is normally applied in lawns to control crabgrass and other weeds. In theory, it can also work to hinder crabgrass in the garden, but it has no impact on turfgrasses that have crept over from lawns into garden beds. Timing of application is extremely important with corn gluten meal; if the timing is off, it can actually encourage 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. It works only on annual grasses and broadleaf weeds, not perennial plants.