Mulching

If reading what I’ve written about watering and weeding makes you feel like just giving up on this whole gardening endeavor, take heart—mulch helps with both! Once I get my mulch down, usually in June, I find I can relax knowing that my garden isn’t going to dehydrate overnight. And I can stop worrying about weeds almost entirely if I use a thick enough layer—one that excludes light to keep those weed seeds from germinating in the first place. The occassional escape is easily pulled. Additional benefits of mulch include that they keep my vegetables cleaner and prevent some diseases, by stopping soil from splashing onto their leaves, and over time mulches, at least organic ones, break down and build soil organic matter. I won’t go into detail about all the kinds of mulch available, but would like to recommend three:
Grass clippings are my favorite mulch for vegetables. Grass clippings are available for free from the Ramsey County Compost Site just over a mile from the garden on Pierce Butler. Go there towards the end of the day on a weekend and you can scoop up truckloads discarded by home-owners who don’t appreciate how valuable it is. According to Organic Gardening Magazine (“How to Fertilizer Your Garden”, July/August 2000, pp 46-51), a 1 to 2 inch layer of grass clippings per year, applied as a surface mulch, is enough to sustain your soil after its fertility has been built to an ideal level. The soil at SAP community garden is already high in both phosphorus and potassium, so nitrogen is our primary concern. Grass clippings are especially high in nitrogen, and supply it in a slow controlled release way which matches your vegetables’ need for it.

I am often asked whether I am concerned about the possibility of herbicide residues on the grass clippings I collect from unknown gardeners. Although I cannot completely discount this concern—I have heard of problems with herbicide residues on grass clipping mulch in other places -- I can say that I have never personally had a problem. My impression is that most home-owners here apply pre-emergent herbicides, that work by inhibiting weed seed germination when they are washed into the soil, but I don’t really know since I don’t believe in using herbicides on lawns. At any rate, residues from pre-emergent herbicides in grass clippings should be negligible since they are applied to the soil and are not taken up by the grass. However, if you are really concerned, you can stick with grass clippings from known sources, or look for grass clippings that contain clover seeds, a sign that herbicide was probably not used. Clover clippings are an even richer source of nitrogen than grass clippings!

Wood chips are a good mulch in perennial gardens or in places which will never be plowed, such as in pathways in extended season plots. I do not, however, recommend their use in plots which will be tilled every year, such as in the main part of SAP garden. The reason is that they can tie up soil nitrogen. Here’s how that happens: soil microbes require a diet that is balanced between nutrients, especially between carbon and nitrogen. Normally lack of carbon in the soil is what limits them. If you apply a carbon-rich and nitrogen-poor mulch, such as woodchips, it stimulates a population explosion of the soil microbes, which balance their diets by soaking up the free nitrogen in the soil, which takes it away from your crops. Fortunately, this is only a temporary problem, because eventually the microbes die, and when they do the nitrogen in their bodies is re-released to the soil, in a slow controlled-release fashion. But it may take a year or two for this to happen.

The reason why woodchips are okay for perennials or pathways is that this nitrogen tie-up only occurs at the interface between the soil and the woodchips. (Microbes don’t have big jaws and can’t work very effectively at the surface of large chips of wood.) So it isn’t a problem until the woodchips are incorporated into the soil. If you’ve already used a woodchip mulch in your annually-tilled vegetable plot, don’t worry. At the end of the season simply rake it off, bag it up, and save it for next year. If you or the gardener preceding you applied woodchips last year, and they are already incorporated, you might consider supplementing your nitrogen fertilization this year. (I recommend bloodmeal, or fertilizer made from fish, alfalfa or soybeans.) Take heart—in the long run your soil will be better for it.

Autumn leaves are intermediate between grass clippings and woodchips in their ratio of carbon to nitrogen. They are fine as a mulch in an annually-tilled garden, though if you use them exclusively you might have to supplement nitrogen. Leaves break down more slowly than grass clippings, so don’t have to be renewed as often. Depending on what kinds of trees they come from, they may also supply some other nutrients besides N, P and K, such as calcium and magnesium. Over a period of years they can build an excellent soil, especially if mixed with grass clippings.

I like to use leaves in the fall to cover tender perennials and overwintering annuals such as spinach. In the spring I rake them off my beds into the pathways. After several years I shovel the decomposed accumulation of leaves and grass clippings from my pathways back onto the beds so as not to let that good fertility go to waste where there are no plants to take advantage of it.
posted by Lois