What is the definition of a weed? Depends on who you ask. For me, the best answer is any plant growing where you don’t want it growing. This means that even your beautiful, lush lawn can be a weed if it ends up somewhere it wasn’t intended. Your lawn may become a weed when it invades your garden beds, driveway or even your neighbor’s lawn. In fact, I once received an email describing a situation where one neighbor decided to establish a zoysiagrass lawn next to his neighbor’s Kentucky bluegrass lawn. The problem started when the zoysiagrass, an aggressive summertime spreader, crept into the bluegrass. These two grasses have different texture, growth habits, and color. Soon the relationship between the neighbors become contentious as the bluegrass neighbor felt wronged by the zoysiagrass neighbor. Not only was it a silly reason to be offended, but it was also avoidable in the first place.
Spreading, Sod-Forming Grasses
To contain a lawn and keep it from spreading to unwanted areas, first you need to understand how grasses spread after being grown from seed. Many grasses spread horizontally by producing specialized underground roots called rhizomes. This group is known as the sod-forming grasses and includes Kentucky bluegrass, creeping red fescue, bahiagrass and some of the newer turf-type tall fescues. Some grasses produce specialized above ground roots called stolons. Buffalograss is an example of this, and is also considered a sod-former. Still other grasses, such as bermudagrass and zoysiagrass, will produce both rhizomes and stolons. These sod-formers are very aggressive in their spreading growth habit.
Some grass species produce no rhizomes or stolons. Once established, they don’t spread horizontally the way the sod-formers do. These grasses are known as bunch-type, or bunchgrasses. This group includes perennial ryegrass, fine fescue (with the exception of creeping red fescue) and some varieties of tall fescue. These grasses can still make beautiful lush lawns, but should be overseeded regularly to maintain a thick sod. Bunchgrasses also pose the least threat of creeping into undesired areas.
Stopping Rhizomes and Stolons
By knowing how your lawn spreads, you’re now able to take the proper steps to prevent it from becoming a weed. The most effective way to achieve this is with a physical barrier. These barriers stop below ground rhizomes and above ground stolons from spreading to other areas. Many gardeners make the mistake of thinking a railroad tie, rocks, bricks or concrete edging will keep grass from invading their gardens. While these borders may prevent the above ground stolons from spreading, they do little to stop rhizomes. Rhizomes can grow deep, up to six inches or more below the soil surface. To stop rhizomes, you’ll need to install edging material that goes at least eight inches deep into the soil.
Install Impenetrable Barrier
Not all edging material is created equal. Wood will decompose over time, bricks will crack and form spaces, rocks will easily allow rhizomes to creep around them and plastic edging can look tacky. Remember, rhizomes can be very aggressive. I’ve seen a bermudagrass rhizome completely penetrate a tennis ball that happened to be buried under the soil surface. I recommend using aluminum or steel barriers. These materials are impenetrable, fairly easy to install and often look the most professional. In fact, once installed they pretty much disappear. Depending on the type of grass you have, both below ground barriers and above ground edging may be necessary, or only one or the other may be needed.