Q: I am planning to change about 550 square feet 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. — Multnomah County
A: 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 to 12 inches, 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 now, when the cardboard and mulch are soaked, the grass will be covered just when it wants to start growing, and it will die. By the next fall it will be good and dead.
For garden beds, the answer is somewhat more nuanced. Washington State University’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 Chalker-Scott’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. — Signe Danler, OSU Extension horticulturist
Can I use concrete blocks for garden beds?
Q: What material is best for building a raised garden bed? Are concrete blocks toxic for growing food? — Josephine County
A: First, here is a general article on raised bed gardens from OSU Extension, which mentions them as a possibility.
Second, here is a second Extension article, which says this about concrete blocks: “Cement block, cinder block and concrete block, all are made with cement and fine aggregates such as sand or small stones. Fly ash is also often included. Fly ash is a byproduct of burning coal and so contains heavy metals and other hazardous waste. Labels do not give specific information on exactly what aggregate is used in the manufacture of the block. There is also little research data on this topic. Ultimately, this becomes a personal choice based on your comfort level. If you plan to use block as a raised bed material — and many people do — and you are concerned about potential risks, you could seal the blocks with polymer paint. Or you can choose to use another material. — Kris LaMar, OSU Extension Master Gardener
Now’s the time to turn to feel-good activities like gardening. And, as you shelter in place, it’s a good opportunity to get your questions answered. Turn to Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type in a question and the county where you live.