'Rock Mulch': Can You Use River Rock as Mulch?
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----Ryan in Wilmington Delaware
A. Well, this is an excellent question; but what really caught my eye was the subject line of their email: "Rock Mulch", which made me think of both the B-52's song "Rock Lobster" and J. I. Rodale, the founder of the organic gardening and farming movement in the United States in the late 30's and early 40's.
(No, J. I. was not in the band--although he would have fit right in.)
Back when he was establishing the first raised beds on his 'experimental farm' in Emmaus, PA, J. I. made use of the locally abundant fieldstone to frame some of the beds; a technique he called "rock mulching". You just keep setting and moving flat stones around the area that you've defined as your growing space until you make rocky raised bed frames. No worries about treated wood—or regular old wood rotting away.
Like me, J. I. was gardening in rural Pennsylvania, where the main growing season can be short. And he soon realized that the stone wasn't just local and free—it was also absorbing heat from the sun during the day and radiating it back out into the soil to keep the beds a little warmer at night; a real blessing in the chilly Spring and Fall.
So maybe river rocks aren't the worst idea?
Theoretically you could hoe them off to the side, install plants and then hoe them back into place. But in practice there are going to be problems. One big one is going to be weeds.
I'm guessing that the previous owner laid this stone recently—probably to try and make the place look more presentable for sale—because stone eventually does get weedy. Grassy weeds always find their way up—and those weeds are impossible to remove from a bed of stones.
And new weed seeds are going to blow in and take root around the stones. (There's always some organic matter in there to get them started.) And if any of the beds are under trees, birds will roost in the branches and poop fresh seed right down onto the stones. Like poison ivy—that's how that weed always gets started.
And the 'soil heating' factor is going to be in reverse here. J. I.'s 'stone mulching' along the perimeter of a bed surrounds the soil with natural radiant heaters. But stones covering a bed are going to keep the soil cold in the Spring by preventing the sun from hitting it directly and warming it up. That's the case with all mulches.
The right way to use mulch is to cover the soil with your mulch over winter to smother weeds, prevent erosion and keep nutrients intact and then rake or hoe that mulch away for a week or so in the Spring to allow the sun to hit the beds and warm the soil. Then you put the mulch back in place to prevent future weeds.
…Which would be a real chore with rocks. And unlike my preferred mulches of compost or shredded fall leaves, the rocks won't be adding any nutrients to the soil.
So away with the rocks!
But. My mind keeps going back to the potential benefits of J. I. 's original rock mulch. And these are rocks; very attractive ones in fact. So one option would be to hoe them to the edges of the bed and form them into frames. You could dig a little trench around the outskirts of the bed, fill it with some of the rocks and mound the rest overtop.
It would look great. But will they stay in place?
They will for sure if you confine them by making a long rectangular 'basket' out of chicken wire or hardware cloth and fill that with the rocks. I see structures like that filled with BIG rocks along highways; especially down at the bottom of steep slopes—and they look pretty cool.
Choose your 'basketing' material well and you'll see the natural beauty of the rocks, they'll stay in place, and you'll get that nighttime warming in the Spring and Fall.