Q: I have an area with creeping thyme planted as a ground cover next to my lawn, and the grass is growing into the ground cover. How can I stop this from happening?

A: The bluegrass that most of us have in our lawns is a tenacious spreader and will generally out compete other ground covers if steps are not taken to physically separate the two areas. It will take some time and effort to change your situation. Here are some suggestions:

– First, make sure your the site where the thyme is growing meets its requirement for full sun. If not, consider using a ground cover more appropriate to the area so that it can grow as vigorously as possible.

– Next, take a look at how these areas are being watered. If they are getting the same amount of water and that amount is enough to keep the bluegrass green, consider changing your irrigation practices to water them separately, and water the ground cover less than the grass. Many groundcovers, such as sedum, creeping thymes, and vinca, require much less water than bluegrass, and they are perfect choices for using drip or soaker irrigation. The grass will not be as tempted to spread to a less frequently irrigated area. If the two areas are already watered separately, check and adjust sprinkler heads in the grass area so that they are not overspraying into the groundcover, and try reducing the irrigation in the ground cover.

– Now dig a trench about 10 inches wide and 6 inches deep along the edge where the two areas have mingled. If possible, separate out the ground cover so it can be replanted. Otherwise, compost all of the grass and ground cover that you’ve removed. Bury a 6-inch edging strip, preferably steel, in the trench. This will keep the underground rhizomes of the grass from spreading into the groundcover and vice versa.

– Finally, if you’d like a more defined transition zone between the ground cover and grass, refill the trench and then cover the soil strip you’ve cleared with at least 4 inches of mulch.

Further reading:
CSU Cooperative Extension Fact Sheet 7.400, Ground Cover Plants, www.ext.colostate.edu/PUBS/GARDEN/07400.html

Contributed by Carey Harrington, Colorado Master Gardener. For answers to your horticultural questions, contact the Master Gardener Help Desk at 636.8921 or CSUmg2@elpasoco.com. Access fact sheets and seasonal information on the El Paso County Horticulture website http://elpasoco.colostate.edu/horticulture/.