Carole Deppe parle aussi du mulch dans The Resilient Gardener. Sans surprise elle fait les mêmes constats, sûrement parce que le climat de l’Oregon ne doit pas être très éloigné de celui de l’Angleterre : le mulch abrite les limaces et protège leurs œufs du froid l’hiver, il retarde le réchauffement du col au printemps, rend difficile le désherbage à la houe, et surtout nécessite beaucoup de matières premières sur de grandes surfaces. En gros elle trouve que pour son cas le mulch ne représente pas forcément moins de travail que de désherber à l’outil, mais un travail différent.
I once had a conversation with a wannabe farmer who had just moved to Oregon from the East Coast. He wanted to try growing vegetables in permanent raised beds with deep mulch, as was often touted in the Organic Gardening magazines he had been reading. My response was: “But it doesn’t work here.” He touted it some more. I responded again with, “But it doesn’t work here.” And so it went, until we put in a test plot of mulched and unmulched potatoes side by side. The mulched potatoes took weeks longer to sprout and then grew slowly and were stunted and obviously miserable. In the maritime Northwest, you have to really screw up to make potatoes miserable.
Here in Oregon, any heat in the springtime is usually limited. Soil that lies beneath a deep layer of mulch stays soggy and cold. Pull back the deep mulch to make a row to plant something, and the something will take longer to germinate and will grow much more slowly than the same thing planted in bare soil, even though the soil over the seed is left exposed until the seeds have germinated. And the worst is yet to come. After the first couple of years of deep-mulched permanent beds of vegetables, there are so many slugs, sow bugs, and other such critters that everything is eaten up as soon as it germinates. Our winter freezes don’t go deep enough to kill pests in the soil under a deep mulch. Admittedly, the deep mulch does take care of the weed problem. Here, beds with permanent deep mulch are most useful for perennial and ornamental plantings, not for vegetables.
Deep permanent mulches have worked for some people in some places and certain situations, even in the vegetable garden. But they aren’t for everybody. However, mulches can be thin or deep, temporary or permanent. In the era in which I gardened with raised beds in the backyard, I often used a light or late-applied mulch as a way of adding to soil fertility or moderating water loss.
“Deep” mulch usually means a layer at least 6 inches deep after the mulch has consolidated. For loose material such as straw or leaves, this generally means a layer 8 to 12 inches deep initially. A thinner layer doesn’t do the intended job of preventing weeds from growing. If there are perennial weeds with large roots, even the deep mulch won’t work. You need to get rid of perennial weeds first.
Most people who use a permanent deep mulch successfully in their vegetable gardens seem to be located on the East Coast or in the Midwest, and also to have gardened for years or even decades using conventional methods, and have already eliminated all perennial weeds and built up soil fertility and tilth. Apparently, one can garden successfully using permanent raised beds with permanent mulches, even with vegetables, and even starting from scratch, at least in some areas. A good book on gardening in beds with permanent deep mulch is Lee Reich’s Weedless Gardening.
Once you have mulch on a garden, you can’t use an ordinary hoe any more. (With light mulches, I can still use my peasant hoe, but not light hoes.) So if you have perennial weeds coming up through the mulch, you may need to pull them all by hand. Ideally, with a deep mulch there are few or no weeds. Light mulches prevent some but not all of the weeds from germinating; those that grow have to be hand-pulled or hoed with a heavy hoe. So light mulches may either increase or decrease the weeding work.
Here in Oregon where it doesn’t rain in summer, it can be unworkable to try to provide water by overhead-watering with a deep mulch. You can’t get the water down through the mulch. So deep mulch tends to work best with permanent landscape plantings where drip irrigation lines are installed under the mulch. You can readily overhead-water through a 3-inch layer of mulch, however.
I have used mulches of various kinds with my raised garden beds, primarily to add fertility or reduce water needs. I used light mulches, not deep ones. My favorite mulched gardens were my tomato beds. Tomatoes need lots of water and are sensitive to changes in hydration, which cause the fruits to split. I planted transplants into raised beds and left them unmulched the first month or so, when the plants needed every bit of soil heat possible. After the plants were established and the soil had warmed up, I put on a layer of straw about 2 to 3 inches deep. With this light mulch, I could overhead-water my raised beds just once per week instead of twice a week. By the end of the season, most of the mulch had vanished into the soil, leaving just a little to be turned under.
Nate and I grow a 150-foot row of tomatoes these days. We grow them unmulched and level to the surface rather than in raised beds. They need to be watered only once per week. We could water even less if we added a thin mulch after the first month, as I did with raised beds. But where would we get the mulch? On large plantings, mulching is often impractical because there isn’t enough mulch available. We could get enough free city leaves to mulch our entire two-acre garden, but not without bringing in bindweed seeds. We leased our current garden land partly because it doesn’t have bindweed. We want to keep it that way.
Straw is commonly used as a mulch. On small garden beds, I often used a thin mulch of an inch or 2 of grass clippings, which served as both light mulch and fertilizer. (Deeper layers of fresh clippings turn to rotting goo instead of drying out into a nice layer of mulch.) Hay is fertilizer as well as mulch, but it is usually full of weed seeds. Ruth Stout, the Grand Lady of the permanently deep-mulched (East Coast) vegetable garden, used salt marsh hay. Salt marsh hay is from salty coastal marshes. It has few weed seeds of the kinds that matter in terrestrial gardens, and might have been an essential component of Ruth Stout’s success. When she added another twenty-five bales of hay to her garden each season, she was adding both mulch and fertility. If we try to do the equivalent with straw, we are adding an excess of carbon and very little nitrogen. If we try to do the equivalent with our hay, we are bringing in weed seeds, which turn into a solid layer of grass and weeds on the surface of the mulch. Mulches may or may not mean less total labor. As Robert Heinlein’s characters in The Moon Is a Harsh Mistress were wont to say, TANSTAAFL, or There Ain’t No Such Thing as a Free Lunch. You may get out of much or all of the labor of weeding as well as some of the watering. But you replace it with the labor of finding, hauling, and applying mulch (and tucking it up around each established plant).
In desert areas where every drop of water counts, mulches of some sort—deep or thin, permanent or temporary—may be obligatory. In areas with some but limited or erratic summer rain, even light mulching may mean you don’t need to irrigate at all, thus saving in both water and watering labor.
