Perennial lawn grasses are bred for their ability to naturally spread and fill in bare spots. It's this quality that makes for the thick, lush turf that everyone wants for their lawn. But that same feature can cause turf grasses to go where you don't want them—into gardens and planting beds, over sidewalks and edgings, and up through paving cracks. To understand how to keep lawn grass in its place, it's necessary to know a little about how these plants reproduce and spread.
How Turf Grasses Propagate
Turfgrasses, like all plants, can reproduce (propagate) in one of three ways. They can set flower and set seeds, which then fall into adjacent soil or are spread by wind or water to nearby areas where they germinate and take root. Or, the plants can spread through roots that travel underground and sprout up in nearby areas. These traveling underground roots are known as rhizomes. Some types of turfgrass spread primarily by this method—Kentucky bluegrass is one such grass variety.
Finally, a plant can propagate and spread by surface runners, called stolons, that stretch out over the surface of the earth and send down roots when they find suitable soil. A plant that spreads with way is said to be stoloniferous. St. Augustine grass is a turfgrass variety that spreads primarily by stolons. And some grass varieties use both stolons and rhizomes to propagate—Bermudagrass and zoysia are examples of this.
There are also grasses that produce no rhizomes or stolons, but simply form self-contained bunch structures wherever the seeds sprout. These grasses are known as bunchgrasses, and include varieties, such as perennial ryegrass, fine fescues, and some tall fescues. These grasses have the least tendency to spread into unwanted areas, but to keep a thick, healthy lawn, you will need to over-seed regularly to make sure bare patches get filled in.
The typical turf lawn planted from seed usually contains a mixture of grass varieties. The lawn may be dominated by rhizome grasses or stolon-forming grasses, but more likely it will contain several grasses of both types. Close inspection of your lawn may reveal different colony areas where different types of grasses have found their ideal growing situation and have come to dominate certain sections of the lawn.
Ways to Stop Grass From Spreading
The primary culprits when grasses overrun garden beds and other areas are those turfgrass varieties that spread through rhizomes, stolons, or both. Combatting them can be an ongoing battle, but there are ways to control them:
- Weed by hand. This is not what you wanted to hear, but systematic and regular weeding by hand is one of the best and the most environmentally responsible way to remove unwanted grasses from garden beds. This can be a bit laborious with grasses that spread by rhizomes, as it is necessary to dig out the spreading roots carefully to prevent new grasses from sprouting up almost immediately. This is a method for truly devoted gardeners who enjoy digging in the soil.
- Use a torch weedkiller. Like other plants, extreme heat will kill turfgrass plants. A convenient propane torch tool—a long wand with a small propane tank attached to the handle—can be used to kill grass plants with no harm to surrounding vegetation.
- Use a broad-spectrum herbicide. Many so-called weed-killers are designed only to kill broadleaf weeds while leaving narrow-bladed plants such as grasses untouched. To kill grass plants, you'll need to use a broad-spectrum herbicide, such as a product containing glyphosate (Round-Up, etc.). Be aware, though, that these herbicides will kill anything they touch, so spray them very carefully, applying them with spot treatment on a windless day rather than spraying broadly. Handle all chemicals carefully. Glyphosate becomes inactive shortly after contacting the soil, but recent research indicates that it may have some of the same cancer-causing properties that other, more notorious weed-killers are known for.
- Reseed with bunchgrass. Because bunchgrasses such as perennial ryegrass and fine fescues do not spread by rhizomes or stolons, they tend to stay put. If spreading grasses truly annoy you, it's possible to kill them off and reseed with grass varieties that don't spread. This is a drastic measure, but you can also do it somewhat gradually by using bunchgrass seed whenever you are top-seeding the lawn. Over time, the bunchgrasses will begin to dominate the mixture of grass varieties in your lawn.
- Install deep and tall edgings. Lawn and garden edgings can be used to keep turfgrasses within their boundaries, but the best edgings must be quite deep and tall, and they must be made of impenetrable materials, such as aluminum or steel, in order to do their job perfectly. Wood timbers eventually rot away, and bricks have cracks that can easily be penetrated by rhizomes. Ideally, solid bands of metal edging at least 1 foot deep are necessary to halt rhizomes. The perfect lawn edgings also need to be tall enough to block stolon-forming grasses from creeping over them. Stolon-forming grasses can defeat ground-level edgings rather easily. Edgings do, though, make it easier to cut back these matt-forming grasses to where they belong during weekly lawn-care chores.