Living mulch

You can grow your own fertilizer with living mulches. Living plants used as mulches have an advantage over dead mulches, such as straw and hay. They affect soils both above and below the ground. They grow with and around main crops and are usually green, succulent, and full of nutrients, with a well-developed root system. This root system works its way into the earth, opens up the soil, and feeds the soil food web all season long if living mulches are managed properly.

This article was kindly contributed by Helen, an agronomist who farmed and experimented for 17 years with living mulches in Montana. Learn more about Helen’s techniques at www.veganicpermaculture.com, including her short video about conservation farming.

Benefits of living mulch

HAPPY PLANTS PRODUCE MORE Over time, living mulches improve soils and build the skeletal framework which holds plant nutrients so that they are available when plants need them. This is because living mulches add organic material into the soil without disturbing it. When mowed regularly or tilled into the soil, living mulches add plant nutrients for free, including the big three: nitrogen, phosphorus, and potassium, as well as sulfur, calcium, and micronutrients. When compared to plants grown without mulch in bare soil, legume living mulches produce higher vegetable yields. Leaf cover and yields of bush green beans was higher when planted into a killed stand of hairy vetch. Corn yields were higher in red clover that had been killed in strips. Though fruit maturity was delayed, yields of tomatoes grown in hairy vetch were higher and fruit weights were greater than in bare soil treatments. The key is managing the competitive interrelationships between the living mulch and your crops. More on this below!

LESS IRRIGATION, MORE WATER CONSERVATION Because living mulches add soil organic matter over time, in the long term they help to conserve soil water. Soil organic matter acts like a big sponge soaking up water and releasing it slowly when needed. However, in the short term, most living mulches steal soil water from crops when both are actively growing, especially early spring through early summer. Living mulches hold onto and recycle soil water when NOT actively growing. In one study, corn grown in mowed hairy vetch struggled for water during the first 1 – 4 weeks after planting. But, by two to four weeks after planting, soil moisture levels were the same as in bare soil treatments. Soil water levels were higher than in bare soil after four weeks. In other words, over time living mulches can conserve moisture in your garden, but in the short term, especially right after planting, they compete with crops for soil water. We can design around this biological fact. Again, the key is managing the competitive interrelationships between the living mulch and your crops.

LESS PEST MANAGEMENT Living mulches provide diversity and a legume “crop rotation” which is the foundation of disease suppression in all organic gardens and farming systems. In my 17-year living mulch vegetable production system, insect and disease problems simply dropped off the radar – including cabbage worms. In a 2- year study, we discovered that the living mulch was providing a home for many kinds of predators who were controlling cabbage worms in our commercial plantings of broccoli, cabbage, and Brussels sprouts. In cold, wet soil, living mulches may encourage disease and some pests such as slugs or snails. Researchers from South Dakota found lower seedling emergence and survival and higher amounts of disease-causing fungi in clover and hairy vetch mulched fields, when crops were planted during cool weather. But many other studies indicate that several insect pests are controlled in living mulch as compared to bare soil plots. Several species of both specific and generalist predator and parasite populations have been shown to increase in living mulch plots.
Living mulches are also useful in managing many species of weeds. When hairy vetch and rye residues covered at least 90% of the soil in one study, weed density was decreased by 78%. Living mulches can fight weeds by "smothering" them, and by utilizing all the water and nutrients so that weeds are “starved” and cannot invade. Combinations of grasses and legumes are best for smothering weeds.

Some living mulch options for gardens and farms

Annual living mulches

Plant in the spring. In cold climates they are killed by below freezing winter temperatures.

Crimson clover (pH 6.5-7.5). Requires irrigation for germination and growth. Germinates best at 40-60o F and doesn’t tolerate high heat. Less vigorous growth than Berseem clover unless season is wet and cool. Flowers profusely and attracts early spring pollinators and predators.

Subterranean clover (subclover) (pH 6.0-7.5). A short, creeping clover. Requires irrigation for germination, then relatively drought tolerant. Germinates best at 40-60o F and a bit slower than Crimson clover to come up. Inconspicuous flowers do not attract as many predators and parasites, but also don’t compete with fruit blooms for pollinators in orchard and berry plantings. In zone 7 or warmer, can be planted in the fall and will overwinter. Reseeds itself well. Subclover dies naturally in early summer after blooming and setting seed. It is a good living mulch for late planted vegetable crops. It is relatively difficult to kill without deep tillage before mid-bloom stage. Subclover may inhibit the growth of weed seeds and seeded crops in the first few weeks after tilling into the soil (see allellopathy discussion below). In one study, plant toxic compounds from subclover mulch suppressed lettuce, broccoli and tomato seedlings for eight weeks, but not as severely or as long as did compounds from ryegrass mulch.

Berseem Clover (pH 6.5-8.0). Rapid germination if seed is raked in or rolled. Germinates best at soil temperatures of 40-60o F. Grows to 12-24", but tolerates mowing. Requires more moisture than the Medics and subclover. Fast growing. Doesn’t do well in heavy clay soils. Tolerates saline conditions better then red clover and alfalfa. Seeding rate: 5 lbs per 1000 sq. ft.

Snail Medic (pH 6.5-8.0). Germinates best at soil temperatures of 45-60o F. Does not re-seed itself well in cold climates; grows to 12", but tolerates mowing. Seed at 10-20 lbs per 1000 sq ft. Make sure seeds do not dry out during germination. Once established Snail Medic can tolerate high heat and dry periods. Parabinga Medic is similar to Snail medic but grows more viney, shorter, and matures later. Seed at 5-10 lbs./1000 sq. ft.
Nitro Alfalfa (pH 7.0-8.0). Annual in cold climates, usually, but may sometimes overwinter. Vigorous grower; reportedly out-competes Canada thistle. Excellent nitrogen source when tilled in the spring before planting. Drought tolerant once established.

Buckwheat (pH 6.0 -7.5 ) germinates fast, in three to five days, and flowers in approximately three to five weeks. Buckwheat has a short, fibrous root system, so does not tolerate drought well. It prefers moist soil. Mowed buckwheat decomposes very rapidly, perhaps too rapidly for good organic matter build up. Buckwheat is susceptible to frost and high heat/low moisture conditions. Plant after the last spring frost date and at least 60 days before the first expected fall frost date. Buckwheat can be managed to encourage flowering all summer long in cool climates. It attracts a wide range of predators and parasites. Planting border strips of buckwheat near zucchini decreased aphid populations in one study. Though not as good at suppressing weeds as the clovers, when seeded thickly, buckwheat produces a dense canopy quickly, shading the soil and acting as a good competitor against weeds. Some studies report effective control of Canada thistle, sowthistle, leafy spurge, and perennial peppergrass by buckwheat. Several scientists suggest allelopathy as the possible mechanism of weed inhibition and research in 2002 supports this idea. Buckwheat reportedly accumulates phosphorus and releases it when tilled into the soil in the fall.

Biennial living mulches

Plant in the spring. Will grow foliage the first season, overwinter and then flower, set seed, and die the next growing season.

Yellow Sweet Clover (pH 6.5 - 8.0). Drought-tolerant once well-established. If over-watered, sweet clover does not compete with weeds as well. Produces two- to four-foot tall flowering stalks the second year. Has become weedy in some native grasslands and along forest roadsides. To manage sweet clover, mow down at full flowering to prevent seed set.

Perennial living mulches

Plant in late summer/early fall or in spring.

White Clover (pH 6.2-7.5). There are many cultivars of white clover grouped by size. The shortest cultivar is Wild West. Intermediate height cultivars include Dutch White, New Zealand White, and Louisiana S-1. These cultivars are more heat tolerant and flower earlier than the tallest white clover cultivars such as Ladino and Alsike Clover. White clovers are very vigorous! They require moisture for establishment and to maintain good growth, prefer cooler growing temperatures, and germinate best at soil temperatures of 40-50o F. White clovers are the best choices for areas that receive heavy foot traffic. White clover stores 45% of the nitrogen it gives back to the soil in its roots. This is more than any other legume and is important to consider in managing white clover for nitrogen addition. Mowing the top growth of white clover will not give you a fast boost of nitrogen, but white clover is a great recycler of nitrogen.

Hairy Vetch (pH 6.2-7.5). Very vigorous, viney growth. Less vigorous growth has been observed in very cold climates. Needs irrigation for germination.

Perennial Ryegrass or Winter Rye (handles wide pH range) vigorous and cold-hardy (can be planted late in the season), good in combination with the above legumes for best weed suppression. Can be difficult to manage in small garden areas. Does not provide nearly as much nitrogen to the soil as do the legumes. May inhibit the growth of weed seeds and seeded crops in the first few weeks after tilling into the soil (see allellopathy discussion below).

Managing living mulches

Once you have chosen the right living mulch for your particular area and need, develop a management plan. You can keep living mulches from becoming too competitive by mowing, lightly tilling, rolling, or keeping them dry (withholding irrigation). Mulches should be mowed and left to sit on the soil surface for two days to two weeks before incorporating into the soil. Your management timing affects pest management. Keep mulches short in wet, humid weather to avoid disease. Mow mulches if annual weeds begin to pop their heads through in order to avoid weed seed production. For FAST plant nutrient cycling, till living mulches into the soil in late spring 2-4 weeks before planting.

Using legumes to add nitrogen

Legumes are prime living mulch choices because they “make” (fix nitrogen from the atmosphere) for other crops. But, they only give up that fixed nitrogen when they die or are tilled into the soil, or over time if they are mowed and the residue is left on the soil surface as a mulch. Actively growing legumes do not USE as much nitrogen as non-legumes, but they do not GIVE UP nitrogen to the soil or other plants when they are actively growing. Legumes do not fix nitrogen at equal rates, or under all conditions. Nitrogen fixation rates are decreased by low (< 40 – 50o F) soil temperatures and stop at freezing temperatures. Nitrogen fixation rates vary among legume species. For example, clovers, sweet clovers, medics, and vetch provide 0.1 to 2.5 lbs of nitrogen per 100 sq ft. Alfalfa provides six pounds of nitrogen per 100 sq ft. Legumes also contribute phosphorus, potassium, calcium, magnesium, sulfur and micronutrients to the soil (see below).

Nutrient content during active growth

How and when to control your living mulch

The way living mulches are managed in our gardens determines what benefits we derive from them. Legumes contribute most to soil fertility if they are mowed or tilled into the soil. Nutrient release is much slower if the living mulch is mowed and not tilled in. But, some studies indicate that nitrogen, calcium, magnesium, and potassium levels are best increased in the soil over the long term when crop residues are left on the surface, rather than tilled into the soil. Nitrogen release by cover crops depends on temperatures and humidity levels. The warmer the weather, the more quickly residues will release their nitrogen. Studies at North Carolina State indicate that 75% of the nitrogen in some legume cover crops is released within seven to ten weeks after mowing if residues are left on the surface. If the residues are tilled under, nitrogen release is quicker and may be accomplished within four to eight weeks. In cooler weather, nitrogen release can take much longer. In my experiments in Montana in a very microbially active soil, nitrogen release occurred two to four weeks after tilling red clover into the soil. There is a lag time of at least two weeks during which nitrogen and phosphorus will be tied up in the soil food web “digesting wheel” (This lag is called immobilization because soil microbes are using the same nutrients that plants need and thus “immobilizing” them, or making them temporarily unavailable to plants). Plan for this and wait two to four weeks after mowing or tilling the living mulch before planting main crops. Waiting at least two weeks to plant will also reduce the chance of increased disease organisms, which can be favored by an addition of fresh (particularly succulent and green) residue.

If left unmanaged, living mulches can sometimes out-compete a crop for light, nutrients, and water. Vigorous living mulches should be mowed in order to keep them manageable. Researchers report that unmowed grasses and clovers can compete with crops for available water in the root zone area (from 1-16 inches below soil line.) Mowing living mulches decreases their uptake of water and reduces their competitive ability. Mowed cover crop residue left on the surface to decompose needs to be kept moist (but not wet!) for at least the first five to seven weeks after mowing to enhance decomposition. Letting the residue dry quickly (like hay) may result in less nitrogen loss from the system into the atmosphere, and in certain cases, may be less favorable to the temporary buildup of disease organisms.

Some living mulches may need light tillage. Light tillage equates to walking your rototiller quickly over the surface of the living mulch. Do not let the tiller tines go into the soil more than one to two inches. If residue is buried deeper than several inches below the soil surface, decomposition time will be longer and anaerobic conditions may occur. Remember that soil microorganisms require oxygen to do their job.

Cover crops can also be managed with a rake. Rake the cover crop vigorously until the soil is exposed. Cornell University research indicates that disturbing living mulch cover crops by using light tillage is most successful in July. This is also the time when most summer crops are particularly resource demanding and hence it is a time when living mulches are most likely to compete with crops.

Inoculate legume cover crops

Inoculation of legume cover crops is suggested. Inoculants consist of species-specific bacteria that associate with legume roots and fix atmospheric nitrogen. Use the correct inoculants for the cover crop. Alfalfa and yellow and white sweet clovers share the same inoculants; true clovers share another; peas and hairy vetch share a third; garden beans and field beans share a fourth. Purchase inoculants when purchasing seed.

Allelopathic living mulches

A final note on managing the interrelations between living mulches and the crops in your garden:

Some living mulches are “allelopathic”. This means that they chemically inhibit the growth of other plants. Mustard family types, such as rape and black mustard, are good examples of allelopathic living mulches. Allelopathy can be used to help control weeds; on the other hand, crops can be adversely affected, particularly seeded crops. Seeded crops, like lettuce, can be inhibited by some living mulches, such as mustard, but usually only if the living mulch is tilled into the soil and a crop seeded immediately after. The allelopathic reaction dissipates in time. For example, the compound in plants from the mustard family that is most responsible for its allelopathic reaction loses 80 percent of its punch within two weeks. Plant main crops three to five weeks after mowing or turning under any living mulches suspected of exhibiting allelopathy. Living mulches exhibit greater allelopathic effect if they decompose without oxygen. Some growers take advantage of this to control weeds; they bury the cover crop deeply to decrease oxygen and encourage anaerobic (oxygen-free) decomposition. Studies at Michigan State indicate that weed inhibition is greater after tilling in cover crops of oats, barley, rye, or Sudan grass than after mulching with woodchips.


Final thoughts

With a little mindful planning, living mulch plants can be a great source of fertilizer and a way to design a pest-resistant garden.


26 July 2011
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