Long before there were herbicides, diesel tractors, or rotary cultivators, smart farmers learned to manage their weeds. How did they do it? Here’s how:
Living mulches suppress weeds, reduce soil erosion, enhance soil fertility, attract beneficial insects, and help retain soil moisture. The best living mulches are low-growing, nitrogen fixing legumes. Dutch White Clover (Trifolium repens) is a good example.
Before seeding clover or any other living mulch, remember that two crops are growing on the same land at the same time — the mulch crop and a cash crop. Success requires careful management or both crops may fail.
All living mulches compete with their companion crops. The extent of competition and consequential yield loss vary with management and crop type. For example, under drought conditions shallow rooted crops generally show more yield loss than deep rooted crops. Low or slow growing crops may be overwhelmed by more aggressive companion crops.
As a general rule, living mulches are not recommended where drought is expected because yield losses are too high. However, many crops benefit from clover mulches during dry conditions — the clover shades the soil, retards evaporation, and increases humidity around the cash crop.
Transplanting Vegetables into Clover
Dutch white clover makes good living mulch for TRANSPLANTED vegetable crops provided: (1) Crops are irrigated, (2) Crops are fertilized, and (3) Crops are protected for the first 4 to 6 weeks from competition by the clover.
1 to 2 inches of water are needed weekly to grow both clover and vegetables without undue competition for moisture. If water is limiting, it is best to drip irrigate the cash crop rather than water the entire field.
Nitrogen fertilizer is not often required for small grains but is recommended for maize, fruits and vegetables. The reason is that clover fixes about 100 pounds of nitrogen per acre but these nutrients are not immediately available — they are retained by the living mulch. Phosphorous and potassium should be applied according to crop requirements along with lime to correct soil acidity. Dutch white clover needs sulfur and responds well to powdered agricultural gypsum at 2 to 3 tons per acre.
Dutch white clover grows only 6 to 8 inches high so there is little competition for light except when crops are young. Mow a narrow strip where transplants will be set, or apply a circle of mulch around transplants to give crops a head start. Once crops are established they will overgrow the clover and produce normal harvests.
Aggressive, fast-growing crops like tomatoes, peppers, okra, melons, squash, sweet potatoes, gourds & pumpkins all do exceptionally well when transplanted into Dutch white clover. Cucumbers are slower growing and require extra mulch to protect them from early season competition with the clover cover crop.
Stake-less = self-supporting tomato varieties (with thick upright stems) grow well in Dutch white clover. The living mulch keeps fruits clean and allows easy harvest even in rain-soaked fields.
Once established, Dutch white clover is an aggressive mulch crop that blots out most weeds. Walk the fields and hand pull any weeds that escape the clover. Alternatively, thin weeds to at least 1 yard or 1 meter apart. Thinly spaced weeds will not significantly affect quality or yields of cash crops (but will provide food and shelter for beneficial insects). Weedy fields often require little or no insecticides to control crop pests.
Direct Seeding into Standing Clover
Dutch white clover is not well suited to direct-seeded crops, especially those with small seeds or slow germination.
Common potatoes are an exception, especially if whole tubers are planted to establish the crop. Roto-till a narrow strip just wide enough to get the seed potatoes in the ground. After planting, over seed tilled rows with additional clover seed to maintain soil coverage. The potatoes grow through the clover without trouble. Fall potatoes (planted after hard frost in November) averaged 22.8 tons per acre when grown in irrigated Dutch white clover. Adjacent non-irrigated fields averaged 16.4 tons per acre, the yield loss due to water competition.
Costa Rican Indians grow dry beans by broadcasting seed into the weediest fields available. The weeds are then hand cut and left as mulch to protect the germinating beans. Yields are low, only 400 to 500 pounds per acre, but there are no costs other than labor for planting and harvesting.
The same technique works with Dutch white clover. Spring turnips broadcast into standing clover averaged 10.8 tons per acre when the clover was intensively grazed for 3 days and the seed stomped into the soil by sheep. Adjacent plots mowed 1-inch high averaged 14.3 tons per acre. Control plots (no grazing or mowing) averaged only 0.90 tons per acre because of intense competition from the clover. In comparison, winter turnips (sown after the first snow) averaged 13.1 tons per acre.
These results demonstrate the importance of timing when sowing any small-seeded crop into Dutch white clover. Ideally, seed should be sown when the clover is dormant. The next best choice is “sow and mow” (or sow and graze).
Direct seeding into standing clover is not recommended unless the clover is knocked back to reduce competition with the primary crop.
In non-irrigated, non-fertilized fields, flint corn transplanted on 40 inch centers into mown Dutch white clover averaged 68 bushels per acre (along with 1,300 pounds of dried beans and 9,600 pounds of pumpkins). Adjacent fields transplanted into Red Clover (Trifolium pratense) were overwhelmed and failed to make a crop.
Careful timing is essential when planting mixed crops into living mulches or bare soil. For example, in a maize-bean-pumpkin polyculture, the primary maize crop should be at least 18 inches high (4 to 8 leaves) before beans or pumpkins are sown, otherwise the grain will be smothered by the companion crops.
Strip cropping combines the pest control advantages of polycultures with the high efficiency of mechanized agriculture. For example, fields seeded into mown Dutch white clover with 4-row strips of maize alternated with equal width strips of dry beans and winter squash (maize-beans-maize-squash, et cetera) out yielded individual crops grown as monocultures. The yield advantage for maize alone averages 15% when grown in narrow 4-row strips with other companion crops. Yield increases from strip-cropping are attributed to better light penetration into the maize canopy, and reduced pest populations in the beans and squash.
Living mulches work especially well with intensive horticulture systems like truck farms and market gardens where careful management and judicious cultivation (including mulching and mowing) prevent the companion crops from overgrowing the cash crops. When crops are planted into living mulches, entire farms (up to 25 acres) can be run with only a small rear tined roto-tiller and common lawn mower. Leaving strips of hay, wildflowers, and clover between cash crops and around field borders creates a sanctuary for beneficial predatory insects that help keep pest populations under control.
Seeding Small Grains into Clover
Seeding small grains into living mulches works best when: (1) The companion crop is dormant or its growth retarded by mowing, grazing, or rolling, and (2) The grain crop is selected for a competitive growth habit. Heirloom (non-dwarf) varieties usually pair well with understory legumes like Dutch white clover. Alternatively, clover can be broadcast into standing grain that is well established (8 to 12 inches high). Again, careful timing is essential to prevent the cover crop from overwhelming the cash grain.
In non-irrigated, non-fertilized fields, fall seeded wheat averaged 28.1 bushels per acre when broadcast into dormant clover. Spring seeded wheat averaged 21.6 bushels per acre when the crop was “frost seeded” (planted in frozen soil). Late spring “sow & mow” wheat averaged 19.9 bushels per acre while wheat broadcast into standing clover barely made a crop, only 3.4 bushels per acre. In comparison, broadcast planted spring wheat top-seeded with clover when the wheat was 8 inches high averaged 15.4 bushels per acre. To put these yields in perspective, conventionally drilled & cultivated spring wheat (without clover) averaged 39.7 bushels per acre (without irrigation) and 78.5 bushels per acre (with irrigation).
Extra water and fertilizer reduces competition for moisture and nutrients resulting in higher yields. In irrigated, fertilized fields, fall seeded wheat averaged 70.4 bushels per acre when broadcast into dormant clover. Frost seeded spring wheat averaged 56.5 bushels per acre, while late spring (sow & mow) wheat averaged 61.9 bushels per acre. Spring wheat broadcast into standing clover failed to make a crop, while clover sown into standing 12 inch high wheat averaged 74.7 bushels per acre.
Sometimes Old Ways are Best
The clover-wheat-turnips rotation common during the Renaissance is a good example of how cover crops and living mulches can be integrated with modern low-till and no-till agriculture. Typically, the clover cover crop was “hogged down” (uprooted by foraging pigs); this eliminated the need to plow and harrow. Wheat was then broadcast by hand and the seed trod into the ground by sheep or cattle. Turnips were broadcast into the wheat as the heads were filling out, and clover was broadcast over the turnips a few weeks before harvest. This rotation reliably averages 40 bushels of wheat per acre under European weather conditions without the need for irrigation, synthetic fertilizer, machinery, fossil fuels, or agrochemicals. (Favorable rain or irrigation boosts this average to 80 bushels per acre). Low production costs more than compensate for modest yields, a primary consideration for most farmers operating on slim profit margins.
Thoughtful Weed Management
The key point to intelligent weed control is to disturb the soil as little as possible, just enough to get a crop into the ground.
Remember that weeds have evolved specifically to rapidly colonize bare soil. The more soil is tilled, the more weeds are stimulated to grow. Conventional bare earth agriculture invites weed invasions. In order for crops to coexist with weeds and living mulches, a different approach is needed. Ideally, crops should be over seeded or transplanted with the minimum possible disruption to both soil and surface vegetation. Often, specialized equipment is needed. For example: Why dig a long furrow when only a few discrete holes are needed for seeding?
Without irrigation and fertilization, competition between living mulches and cash crops can reduce yields 50% or more. Poor judgment (such as seeding at the wrong time) can result in crop failure.
Clearly, there is significant competition from living mulches; the question is whether the savings from reduced tillage and other costs are outweighed by observed yield reductions. These differences may not be significant depending on how the crops are marketed. For example, the premium for “organic” produce and the profits from artisan breads are substantial. In this case, lower yields are offset by higher margins from specialty products sold to niche markets.
>>> Dutch white clover and winter wheat can be seeded at the same time. Remember to plant only after the Hessian Fly Date for your area. This technique works well with all winter grains.
>>> Top seeding Dutch white clover usually requires a separation of 7 to 14 days between plantings (about the time it takes for the cash crop to germinate). Slower growing crops need more time to become established. For example, sweet corn should be at least 6 inches tall before over seeding with Dutch white clover. Rule-of-Thumb: Maize should have 4 to 8 leaves (16 to 24 inches tall) before top seeding with Red Clover (Trifolium pratense) or any other type of tall growing clover.
>>> Organic herbicide may be used instead of mowing, grazing or cultivation to control Dutch white clover prior to planting a cash crop. For example, a narrow strip of clover can be killed with herbicide before transplanting vegetables. Use spray shields to prevent herbicide drift. It is important to disturb as little of the living mulch as possible — kill just enough clover to get the crop established. Removing too much plant cover favors weed growth.
>>> If clover seed is unavailable or too expensive, use weeds as living mulch. This technique works best with fast growing vine crops. For example: Choose the weediest field available then transplant melon seedlings on 10 to 12 foot centers. Mulch each transplant liberally with straw or any other convenient material. Mulch is necessary to keep weeds at bay only until vines begin to run. Once started, vines will overgrow the nurse crop. Melons thrive in the light shade of weedy fields. As an added benefit, vines growing among weeds rarely have insect problems.
>>> Red Clover (Trifolium pratense) seed is usually less expensive than Dutch white clover (Trifolium repens). Sweet corn, popcorn, flint corn, flour corn, pod corn, and dent corns all grow well when planted with red clover. Top seed = over seed maize with red clover at the last cultivation or when plants have 4 to 8 leaves. The corn plants are tall enough (about 1 1/2 to 2 feet high) so that competition with the living mulch is minimal.
>>> Any type of maize can be seeded directly into standing red clover using a no-till planter with a fluted coulter. Two weeks later the field should be closely mowed with a swathing board and divider to keep the clover from falling on the planted rows of corn. Alternatively, clover can be mowed directly before seeding. Watch regrowth carefully; a second mowing may be required 2 weeks later. No herbicides are needed if maize is planted into standing clover; nitrogen fertilizer is not required if clover has grown on the land for 1 or more years.
>>> Maize is sensitive to drought, especially during pollination and when ears are filling out. For highest yields apply 1 to 2 inches of water weekly to prevent moisture competition between crop and living mulch.
>>> Planting hybrid sweet corn into standing red clover yields about 415 sacks per acre on average when sweet corn is seeded 8 inches apart within rows and 30 inches between rows = 25,979 seeds per acre. Actual plants per acre is approximately 21,000 (17% field loss rate is common). 1 sack = 52 ears = 4 baker’s dozen = 21,580 marketable ears per acre. Note: Yield figures are discounted 50% for typical losses to crows, deer, groundhogs, coons, earworms, undersize or poorly pollinated ears, and other causes.
>>> It is best to use pelleted seed when hand dropping or broadcast seeding into living mulches. This is especially true for large-seeded crops like peas, beans, maize, melons, and squash. Pelleted seeds greatly increase germination and stand establishment rates.
>>> Seedling survival and stand establishment are optimal when planting is done with no-till equipment. Expect 20% to 25% loss rates when broadcasting naked, unprotected seed into living mulches or other standing vegetation such as hay or weeds.
>>> Biological agriculture is all about managing little details, for example, choice of companion crop: Flour corn top seeded with sweet clover (Meliotus officinalis) was overwhelmed and failed to make a crop. Flour corn planted with standard (tall) red clover yielded 37.4 bushels per acre. Flour corn planted with medium red clover yielded 41.8 bushels per acre. Flour corn planted with Dutch white clover yielded 47.6 bushels per acre. Yield differences were entirely due to living mulch height. Taller clovers compete more strongly with maize cash crops, especially when corn plants are young.
>>> Every farm has different soil and micro-climate. Agronomic practices that work in one field may fail in another. For best results, every farmer should maintain one or more research plots so that new methods can be tested and adapted to local conditions.
For More Information
Readers who have any questions or require additional information about living mulches should contact the Author directly:
Please visit: http://www.worldagriculturesolutions.com — or — send your questions to: Agriculture Solutions, 413 Cedar Drive, Moon Township, Pennsylvania, 15108 United States of America — or — send an e-mail to: Eric Koperek = firstname.lastname@example.org
Most agricultural universities publish extensive literature on cover crops, nurse crops, living mulches, green manures, and crop rotation. Contact your County agricultural extension agent or search the Internet for relevant publications.
About the Author
Mr. Koperek is a plant breeder who farms in Pennsylvania during the summer and Florida during the winter. (Growing 2 generations per year speeds development of new crop varieties).