Our garden center sells quite a bit of bulk stone mulch, particularly to landscapers who do commercial projects. Washed river stone of one-inch (pebbles) and two-inch diameter (cobbles) has been popular for many years in places like office parks and fast-food restaurants, because it reduces the need for regular weeding and mulching for the first few years after it’s installed. Personally, I would never use it on my own landscape, however it’s worth a discussion of the pros and cons.
Stone mulch should always be applied over weed barrier fabric. If care is taken to keep it free of organic debris like grass clippings, dirt, leaves, bird droppings etc. it will remain weed-free for several years, and won’t need re-mulching each year like wood mulches. It keeps its color, doesn’t wash onto pavement or lawn areas, and looks quite sharp in certain types of plantings. But is it good for plants?
Let’s assume that, before planting and mulching, you take the trouble to thoroughly till your beds. We call this “adding air” or “making fluffy dirt”, and it’s the secret ingredient for healthy plants. Spreading stone mulch adds tons and tons of weight on top, quickly squeezing the air out of the soil and suffocating plants. Simply spreading weed barrier fabric on hard-packed soil and cutting planting holes in it isn’t healthy for plants, and their growth will be restricted.
Years from now, once stone mulch has become polluted with wind-blown weed seeds, it will be twice as hard to pull the weeds. Some stone mulches, particularly lava rock and white marble chips, are only practical in very clean setting like pool decks and parking lot islands. Lava rock attracts clippings and debris like Velcro. Once the stone is dirty and full of weeds, the fix is to dig up and remove tons of stone. It can’t be simply tilled into the soil like wood mulch. Colonizing perennials and groundcovers won’t take over and cover it.
We like to say that whatever you add on top of your landscape each year, that’s what your soil will become. Some wood mulches, pine bark in particular, are very beneficial to plants and improve your soil over time, provided you don’t put a layer of plastic fabric between the mulch and the soil below. Weed barrier fabrics will prevent existing weeds or sprouted weed seeds from coming up, however they do nothing to prevent windblown weed seeds, bird droppings etc from sprouting on top of the mulch. They also prevent the natural process of soil mixing with mulch, so mulch will pile up on top of the fabric and have to be removed.
So, what’s the answer to weed control in landscape beds? First and most important, prepare your beds by killing all the existing weeds and grasses beforehand. Glyphosate (Roundup) will kill most weeds in a week or so. Stubborn weeds like wild violet, nutgrass, ground ivy and clovers may need special weed killers. Only when every last weed is dead toast is it time to till and plant. After planting, you need to apply enough mulch to smother any leftover weed seeds. The best weed control is complete darkness, so three inches of mulch is a minimum. Then you need to renew your mulch every year in March or April, before weeds start to take over.
Sound complicated? Then perhaps stone mulch over weed barrier fabric is for you.
Steve Boehme and his wife Marjorie own GoodSeed Nursery and Landscape, located 9736 Tri-County Highway, near Winchester. More information is available at www.goodseedfarm.com or call 937-587-7021.