Changing from grass to rock/bark with plants

Asked March 17, 2020, 5:04 PM EDT

Hello, I am planning to change about 550 sq ft of my yard from grass to an area of rock path with shrubs, flowers and bark. I have been instructed to put down a layer of cardboard and then put about 4 inches of bark or mulch on top as a first step. Is this the correct (or at least one good) method? We will be gradually planting, but don't have time (or money) to do it all at once. Thank you RaeAnn

Multnomah County Oregon

1 Response

Hi, RaeAnn,

Thanks for your question about using cardboard as the first step to kill grass and prepare a garden bed. You may be surprised to hear that this is a somewhat controversial technique! First I will comment that I have used the method myself, on several occasions, with good results. It is especially effective for a casual path, where many of the problems I will mention later are not an issue.

There are several key things to keep in mind for this method to work. First, cover the area thoroughly, overlapping your cardboard by 8-12”, and blocking any gaps made by box flaps. Grass spreads by rhizomes and is terrific at finding a way to the light. Top the cardboard with several inches of organic matter to hold it down and complete the seal. You can use wood chips, leaves, compost, straw – whatever you have plenty of and want to add to the area. You can use soil, too, if you want to raise the area.

Seasonal timing is important too. If you cover grass going into the dry season, it will go dormant and be ready to spring into action when the rains start next fall. If you cover it in fall or winter, and make sure the cardboard and mulch are soaked, the grass will be covered just when it wants to start growing, and will die. By next fall it will be good and dead.

For garden beds, the answer is somewhat more nuanced. WSU’s Dr. Linda Chalker-Scott opposes it on the basis of research that shows problems with lack of air circulation into the soil, and a subsequent decrease of soil life, which of course is a bad thing. You can read her report here “The Myth of Paper-based Sheet Mulch”.

In addition to her concerns, even dead turf remains a thick, fairly impenetrable barrier to digging, and other plant roots, for several years. So I don’t recommend this method for any bed that you want to plant quickly, unless you plan to till the whole thing (after you are sure the grass is good and dead). Bear in mind that tilling will bring up many buried weed seeds.

It is also more problematic for a vegetable garden or perennial border, where you use small plants. However, for a shrub/tree border, where the plants will be large and well-spaced, it can be quite useful. Make sure the grass is dead before planting, and in light of Linda’s research, I would also recommend penetrating the cardboard with a fork in numerous locations, after the grass is dead, so oxygen circulation can resume more quickly.