Q: Even after installing black plastic edging and applying several inches of mulch, I find that grass from the lawn still invades our vegetable and flower gardens. How can I keep the lawn from taking over?
—Doug; Hastings, MI
A: Roger Cook replies: Sounds like the edging isn't deep enough. I usually use steel edging that is at least 4 inches deep, and occasionally roots will even go under that. If I really want to be sure the grass won't invade the bed, I'll use brick standing on edge (8 inches) or cobbles (10 to 12 inches). These methods are a lot of work, and the materials are not cheap to buy.
The inexpensive way to stop the roots is to buy an edger — a hand tool that has a long wooden handle with a thick metal blade on the end. Work the edger along the lawn an inch or two away from the bed, cutting that much off the grass 6 to 8 inches deep, straight down. Leave this vertical edge exposed, with a slope on the other side back up to the level of the bed. Make sure when you mulch that you don't fill in this V-shaped channel, or the grass will grow into it.
Remove the cut-off grass from the channel and add it to your compost pile. If you don't, it will end up growing into the bed.
Once or twice a month, run down the edge with a string trimmer turned vertical and trim off any shaggy ends. Be sure to wear safety glasses and ear protection. Between the trimming and the air space in the channel, this will inhibit root growth.