Cover It Up!
Protect Your Organic Garden With Mulches and Cover Crops
by David Wann
 Mulches and green manures (cover crops) are like aces and kings in a poker hand. When the driest season in a century smacks us in the face, as it has recently where I live in Colorado, your stockpile of mulching materials and the organic content of your soil – bolstered over the years by green manures – are what keep your crops in the game. Mulches and cover crops are both composed of biological mass, either once-living or still-living, used to optimize soil conditions.
Mulches, which consist of dead plant material like compost, leaves, spoiled hay, grass clippings and pine needles, keep moisture in the root zone and also control weeds that would otherwise steal water from the crop. With lower water evaporation rates, soil moisture remains ideal, which makes nutrient uptake more effective.
Mulches also moderate soil temperatures, keeping the grow zone cool in the summer and warm in the winter. In the summer, the insulating qualities provided by mulch help protect roots from heat stress, resulting in stronger, healthier plants. In the winter, a layer of mulch protects the roots of perennial plants by keeping the soil from freezing and thawing.
My favorite mulch, especially for acid-loving crops like potatoes, is pine needles, which I scavenge from schoolyards and parks in the late fall. (The trick is to catch the grounds crew in the act of raking.) During our windy winters, other mulches like leaves, straw, and wood chips are blown into the next county, but pine needles stay put
When garlic shoots first emerge in early spring, I lay a thick layer of pine needles on top to keep the soil temperature constant. As May temperatures shoot up in the daytime, the pine needles also keep the root zone cool and the garlic happy.
Here are a few things to keep in mind about mulches:
- Never use material from the crop that is to be covered. For example, don’t use potato vines from last year’s crop to mulch this year’s potatoes, because the old vines might transmit disease.
- Use a light-colored mulch during the summer and early fall to reflect heat. Use a dark-colored mulch in winter and early spring to help warm the soil to permit earlier planting and hasten early growth.
- Older grass clippings, leaves, and sawdust laid down as a mulch can cause a temporary nitrogen deficiency in the soil, as soil microbes tap into soil nitrogen to break down the vegetation. Add a source of nitrogen, such as well-rotted manure, before you lay down the mulch.
Soil scientists refer to the “carbon-to-nitrogen ratio” as a key indicator of whether an organic material will add nitrogen or cause a deficiency of it. The carbon-nitrogen ratio of sawdust is 400 to 1, for example, while the C-N ratio for a cover crop like sweet clover is only 12 to 1. This is why cover crops are a valuable card to have in your poker hand.
Cover crops are living plant crops and are most valuable when they are incorporated into the soil, where they build soil structure and provide nutrients for upcoming crops. Instead of buying and bringing home bags or truckloads of compost or manure, bring home some seeds to plant a “green manure” crop.
The added organic matter from cover crops increases populations of beneficial soil microorganisms and earthworms, and also increases the soil’s ability to hold water. An active, diverse community of organisms such as bacteria, actinomycetes, fungi, centipedes, springtails, mites, millipedes, spiders, beetles and earthworms performs many critical functions, including:
- producing vitamins and other growth-enhancing compounds;
- increasing plant uptake of soil phosphorus;
- controlling outbreaks of soil pathogens;
- releasing carbon dioxide, that is then absorbed by plants to form new plant tissue;
- creating more soil aeration and distributing nutrients by continuous tunneling and burrowing. (Think of all the miles traveled by these busy, uncountable critters!)
There are several kinds of cover crops. The first kind is grasses and leafy plants like rye, winter wheat, buckwheat, barley, oats, millet and brassicas (kale, radish). These are generally fast growing and provide lots of biomass to aerate the soil and build soil structure when they are turned under.
The second kind of cover crop is the legume, which pulls nitrogen right out of the air and into the soil. Some legumes, like alfalfa, clover and vetch, are multi season crops, providing nutrients as they grow and also whenever they are turned under. Some cover crops,such as alfalfa, have roots that reach down into the subsoil up to eight feet, bringing valuable hard-to-reach nutrients up to the soil surface as the crops are harvested. You can sow cover-crop seeds like clover and winter rye in the fall and turn them under in the spring, or sow more tender seeds like buckwheat and millet in the spring and turn them under in time to plant fall crops like spinach, lettuce, radishes, and broccoli.
Both kinds of cover crops can become “too much of a good thing” if they are allowed to go to seed. So either mow or turn under cover crops before they seed. Sometimes mulches can be cover crops, and cover crops can be mulches. This year, instead of planting alfalfa and clover in certain parts of my garden, as planned, I’ve “planted” a cover crop of grass clippings. I’m careful to dry each layer out and add air as I spread the clippings out, which prevents matting and fast-acting, stinky decomposition.
Then I spread another layer of fresh clippings on top, stockpiling dried mulch for use around crops and at the same time, feeding the soil with the bottom layers of clippings. This fall, I’ll add a thick layer of horse manure to the clippings I haven’t used as mulch, and in the spring, I’ll dig the whole bed under, feeding next year’s crops with this year’s mulch.
Similarly, cover crops can be mulches when they are planted right under another main crop, such as melons, squash or tomatoes. In fact, recent Department of Agriculture research demonstrated that tomatoes planted in a cover crop of hairy vetch had fewer insect problems and were twice as productive as tomatoes grown without the cover crop. Vetch, a legume, fed nitrogen to the tomatoes’ roots, kept the crop cool and weed-free. In general, cover crops also reduce soil loss from wind and water erosion.
When you begin to garden holistically, feeding the soil rather than just the crop, you begin to work with materials that are close at hand and that don’t require heavy inputs of energy, like powdered fertilizers do. You begin to realize that cover crops and mulches are like aces in the hole.
This article is excerpted with permission from The Zen of Gardening in the High and Arid West by David Wann, published in April 2003 by Fulcrum Publishing. Wann is the author of six books, including the highly acclaimed book Affluenza, which he co-authored. He explores how holistic, nature-based design can prevent damaging environmental and social impacts, a philosophy which is prominent in The Zen of Gardening, which is based on years of sane garden practices, including those as head gardener in his cohousing community in Golden, Colorado.
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