Q: What’s the best way to prevent aggressive lawn grasses from taking over my perennial bed? — Lori C. Conway, Chamblee, Ga.
A: Digging a narrow, steep-sided trench between grass and flower bed is the traditional method. Grasses are slow to cross the bare soil in a cleft that’s roughly 4 inches deep, and when carefully dug, the sunken boundary creates visual definition as precise as the leading in a stained-glass window. Execution is simple, though it helps to have a steady eye and a steady hand. Using a square-bladed spade, cut into the lawn in a continuous line all along the bed’s perimeter, carving a miniature cliff — as close as possible to vertical — out of the topsoil under the grassy edge. As you slice off pieces of sod, shake them over the bed to dislodge the soil from the roots (save discarded sod for composting). Rake the front of the bed into a slope, angled toward your flowers and away from the turf-topped trench. To hold the grass at bay, you’ll need to trim those edges again in midseason (using either a spade or a crescent-bladed lawn edger); in warmer regions a fall touch-up may be necessary, too. A practiced gardener can trim at a rate of between 50 and 70 feet an hour. With rocky soil like mine, this can take much longer.
Purists pride themselves on edges where nothing shows but soil and mulch, but after the first round of digging, many gardeners decide to put in a durable, weedproof barrier and reserve the spade for other tasks. Unobtrusive, dark-finished metal lawn edging, about 4 inches deep, is anchored with steel spikes that fit through tabs in the sides. Rarely offered for sale in retail garden centers, metal edging is usually installed by professional landscapers. Rolls of black, brown, or green plastic edging are widely available, however. Generally about 6 inches deep, these strips are held in place by plastic spikes or have horizontal ridges that prevent the strips from being heaved upward when frozen soil thaws. Both metal and plastic barriers are best situated with their tops flush with the soil surface so that the rims are nearly invisible.
A third option is the mowing strip, a flat band of paving brick or stone, 6 to 18 inches wide, laid between bed and lawn. This allows garden plants to spill gracefully over the front of the bed without risk of damage from the lawn mower. It’s also nice to have a hard, level surface for the mower to ride on: no more tire ruts in the flower bed or chewed-up turf. Nevertheless, a mowing strip, even one that’s 2 or more feet wide, isn’t impenetrable or deep enough to block invasive grass roots from eventually probing the bed. If you're going to the effort of installing a mowing strip, why not dig a bit deeper and place a plastic or metal berrier under the edge next to the lawn?