We have installed lots of sheet mulches on the Lakewood farm. Sheet mulching involves laying an overlapping layer of cardboard to smother weeds, and then piling on a foot or more of organic matter, ideally weed free, and with a balanced ratio of nitrogen to carbon. There are lots of benefits to this: less weeds, more organic matter, and, after the first year, less work, since the beds are never tilled again. For more information, see this post.
However, we have discovered two potential problems with sheet mulching.
First of all, sheet mulch does NOT stop bindweed. Those long twisting roots easily wend their way around the cardboard and through every crack, to emerge triumphantly at the surface. However, nothing else really kills bindweed either, and sheet mulch at least makes bindweed easy to pull. And the sheet mulch killed off the grass, thistles, dandelions, mallow, and other weeds on our site fairly well.
The second problem is more serious. When we planted the tomatoes, we just opened a hole in the mulch, stuck the tomato in, and pushed the mulch back together. Some of the beds were full of a light, fluffy mixture of grass clippings and mulched leaves. We then got a lot of rain, and occasionally people would step on the beds (our paths are not that well defined.) The mulch compacted and sank. Soon the tomato root balls were sticking out of the mulch, where the peat in the potting mix acted like a wick to dry out the roots. The fact that the grass clippings were very porous did not help matters any. We first noticed the problem when a lot of the tomatoes looked sick, with curled and yellowing leaves on a hot day.
Hopefully, we have solved the problem by laying a layer of rough compost from the pile we built last year over the beds. This will cover the root balls and hold water.
So, if you want to build a sheet mulch:
Don’t expect it to kill bindweed,
Try to mix dried grass clippings with other, more compact and water retentive material,
Lay the mulch much thicker than you think necessary, and plant deeply,
Carefully pack mulch over the root balls of transplants,
And ideally, build the beds in the fall, so they can decompose and compact over the winter.
The tomatoes in our hugelkulture/ sheet mulch hybrids are doing splendidly so far. I will continue to report on the success or failure of our systems over the next year. And now we will know how to avoid this problem for the future.
We are almost done planting. We have also got a fence built on the perimeter of the property, set up plant supports, and done a lot of weed mowing on the site over the last few meetings.