Eliot Coleman,Ā farm managerĀ of MiltonĀ Academy's Mountain School Program in Vermont,Ā has been a leading practitioner of organic growing for 20 years.Ā ThisĀ fall bringsĀ theĀ publication of his first book,Ā The New Organic GrowerāaĀ treatiseĀ ofĀ growĀing techniquesĀ that's theĀ best marketĀ-gardening guideĀ we'veĀ seenĀ in manyĀ aĀ year.Ā Coleman's book hasĀ much useful informationĀ thatĀ can be adaptedĀ to home gardensĀ asĀ well,Ā such as the following excerpt onĀ innovative waysĀ toĀ combineĀ bothĀ soil-building and edible crops.Ā
Not all crops are for sale. Green manure crops are grown not for cash but to contribute to the care and feeding of the soil.Ā A green-manure crop inĀcorporated into the soil improves fertility,Ā but the eventual benefits are even greater.
Green-manure crops help protect against erosion, retain nutrients that might otherwise be leached from the soil,Ā suppress the gerĀmination and growth of weeds, cycle nutrientsĀ fromĀ the lower to the upper layers ofĀ the soil, andāin theĀ case of legumesāĀleave to the following crop a considerableĀ quantity of nitrogen.Ā Other contributions of a green manure areĀ improved soil structure,Ā additionalĀ organic matter,Ā enhanced droughtĀ tolerance andĀ increased nutrientĀ availability.
TheĀ value of green manures has been apĀpreciated since the earliest days of agriculĀture, yet theĀ full potential of green-manure use is stillĀ underappreciated and unexploitĀed.Ā Growing green manuresĀ has traditionalĀlyĀ been viewed as an either-or situation. You grew either a paying crop or a green manure.Ā IfĀ the use of green manures means replacingĀ aĀ cashĀ crop,Ā then theĀ lack ofĀ interest inĀ themĀ is understandable.Ā However,Ā there are other options.
There are three waysĀ in which greenĀ-manure cropsĀ can beĀ managed: as overwinter crops, main crops and undersown crops.
First,Ā theyĀ can be sown for overwintering after a market crop has been harvested. For example, a leguminous green manure could be sown after pea harvest and would occupy the ground until it was tilled in the followĀing spring.Ā The benefit from a wintered-over legume that provides ideal growing condiĀtions for next year's crop is a strong incenĀtive for growing it.
In the second case, the green manure ocĀcupies ground during the growing season in place of a market crop.Ā If extra land is availĀable, this is a highly recommended practice,Ā and when the green-manure crop can be grazed by livestock, it serves a double purĀpose.Ā If a grower prefers to put all the land into market crops, however, a choice must be made between losing the future benefits of the green-manure crop to the potential inĀcome from a market crop.Ā Since this is a choice that usually goes against green maĀnures, often at the expense of the soil, I recommend a third management option, one that allows the grower to have a leguminous green-manure crop and the cash too: undersowing.
Undersowing, also known as overseeding or companion seeding, is the practice of growing a green manure along with the marĀket crop. When done correctly, undersowing provides the best of both worlds.Ā It is estabĀlished practice in small-grain growing. The clovers or other legumes are sown with or shortly after the wheat or oats, for example, and grow slowly in the understory until the grain crop is harvested.Ā In vegetable growĀing this practice is not common, to the best of my knowledge, and only recently has beĀgun to be seriously considered.
Timing: When to Undersow
The advantage of undersowing is that the green-manure crop is already established at harvesttime. In our northern New England climate, winter rye is the only green manure that can be seeded after fall harvest. A legĀume cannot be established that late in the seaĀson.Ā Since in my experience legumes are the most beneficial green manures, I try to use them whenever possible. The only way I can do that without taking land out of cash-crop production is to undersow them.
The practice of undersowing is something like planting desirable weeds between the crop rows. In a way that is very similar to the relationship between weed competition and crop growth, the effect of the undersown plantāthe deliberate weedāupon the crop plant depends upon the age of the crop.Ā Weeds can overwhelm young crops if they both start at the same time. Weed research has shown that crops will do fine if they have an adequate head start. If most crops are kept weed-free for the first four to five weeks afĀter establishment, later competition from low-growing weeds will have little effect on them.Ā If we interpret that correctly, then the best crops for undersowing would be low-growing, and the best sowing date for green manures would be four to five weeks after the establishment of the crop plants. My exĀperience bears that out.
Where timing is important, there is a tendĀency to err on the safe side. Why not wait six weeks or more before undersowing the green manure,Ā just to be sure? The problem is that then the balance is tipped too far in the other direction. Since the undersown "weed" is deliberate, I want to be sure it grows. If I wait too long before undersowing, the crop plants will be large enough to overwhelm the green manure. The trick is to undersow when the crop plants are well enough along not to be adversely affected by the undersowing, but not so well established as to hinder growth of the undersown green manure.
How does this timing work out in actual practice? Here in Vermont, where our springs are cool and crops such as corn, beans, squash and late brassicas are not set out until June 1, I find the Fourth of July to be just about perfect, year in and year out, as the date for undersowing to those crops. Obviously, later crops or succession crops will have their own dates.Ā In all cases, the four- to five-week delay has proven to be a reliable yardstick.
Preparing the Seedbed for Undersowing
Successful undersowing requires a clean, weed-free seedbed. Sowing the green manure is no different from sowing the crop: When seeds are planted into a weedy mess, they beĀcome the seeds for failure.Ā I have often thought that another side benefit of underĀsowing is that it motivates the grower to pay attention to clean cultivation right from the start, simply because there is one more reaĀson to do so.Ā Like any problem "nipped in the bud," weeds are easiest to control early in the season. The clean seedbed preparedĀ for undersowing is a by-product of early weed control.Ā AtĀ least three cultivationsĀ should beĀ madeĀ priorĀ to undersowing,Ā theĀ last one justĀ a dayĀ orĀ two beforehand.
The goalĀ ofĀ theĀ growerĀ isĀ to provideĀ every opportunityĀ for the undersowingĀ toĀ get wellĀ establishedĀ without weed competition.Ā UnĀlessĀ theĀ garden has aĀ lot of weed pressure,Ā the canopyĀ ofĀ the undersowing willĀ join with the crop canopyĀ to keep later weeds from germinating.Ā The few that do pop through should be pulled before they go to seed.Ā OccasionalĀ forays down the rows will keep theseĀ competitors from becoming a problem.
Seeding Undersown Crops
IĀ have seeded undersown crops both byĀ broadcasting and drilling,Ā and I emphatically recommend drilling.Ā If the undersown crop isĀ broadcast in the standing market crop,Ā there is no way to cover all the seeds to enĀsure their establishment and germination. In a hot,Ā dryĀ spell,Ā the green manure can be a total failure.Ā When I use a cultural practice,Ā IĀ wantĀ to beĀ ableĀ to depend onĀ its performance. If the undersown cropĀ isĀ drilled beĀtweenĀ the cropĀ rowsĀ with either a single-ĀrowĀ or multi-rowĀ drill,Ā the seedsĀ are planted at theĀ properĀ depth, inĀ contactĀ with moist soil whereĀ theyĀ are much more certain to germinate.
The single-rowĀ drill is theĀ same garden seeder I use for corn, carrots and peas.Ā ItĀ isĀ equipped with an appropriate plateĀ forĀ whatever green-manure seed I am planting.Ā The multi-rowĀ drill consistsĀ ofĀ five of these single-rowĀ seeders bolted together side byĀ side, with common axles and a common push bar.Ā That gives the tool a total width of 20 inches,Ā which fits nicely between the 30-inch row at which corn,Ā beans and brassicas are planted.
When usingĀ the multirow model,Ā all five hoppersĀ can beĀ filled with either the sameĀ seed orĀ different seeds.Ā Each hopper can be fitted withĀ its own seed plate.Ā Under some conditions,Ā the grower mightĀ want to alterĀnateĀ rowsĀ of different legumes or legumes and grasses.
Planning a Green Manure Crop
Sowing dates and equipment for underĀsown green manures should be as well thought out as those for the cash crops. SowĀing dates should be marked on the calendar.Ā The seeds should be ordered ahead. The equipment should beĀ efficient,Ā in good workĀing order and simple to use.
Obviously, green manures are most effecĀtive when they are considered as an imporĀtant component of the crop-rotation planning. There is another parallel between green manures and crop rotation that should be noted. Variety in green manures is as imĀportant as variety in the market crops. BeĀcauseĀ green-manure plants also have different faults and virtues that affect the soil and following crops in different ways, green manures should be rotated to include as many varieties as possible.
Principles of Growing Green Manure
Green-manure varieties and combinations are endless and are not limited to the ones listed here. The varieties I mention worked for me as I developed the biological producĀtion techniques for my particular soil and cliĀmate. But while I've shared my own specific, regional choices for the area of Vermont in which I garden (see the sidebar "Coleman's 8-Year Garden Rotation"), I want to emphasize principles that are more nearly universalānot only because different parts of the country require different green manures but because there are no hard-andĀ-fast rules.
Time of seeding.Ā Early, late, intercrop, unĀdersown, overwinter, year-round?
Establishment.Ā The ideal crop is easyĀ to esĀtablish and grows rapidly.
Time ofĀ incorporation into theĀ soil.Ā How maĀtureĀ is the green manure?Ā What is the followĀing crop, seed or transplant? Legumes turned under in fall lose 70% of added nitrogen but only 38% when turned under in spring.Ā With a winter-killed green manure it may be posĀsible to transplant the spring crop directly without incorporating the green-manure residues into theĀ soil.
RotationalĀ fit.Ā The green manure should not share diseases with the crop plants.
Soil microorganisms.Ā Rape, for example, stimulates the biological activity of the soil.Ā Soybeans improve scab control in potatoes.
BeneficialĀ insects.Ā Some green manures canĀ serve as nurseĀ crops for usefulĀ insects. ThisĀ is an emergingĀ field of knowledge with much to be learned!
Cost.Ā
Shade tolerance.Ā
Ability to grow with theĀ crop.Ā
Effects, includingĀ competition, on thisĀ year'sĀ crop.Ā
BeneficialĀ effects on nextĀ year's crop. Ā
Erosion control.Ā
Winter hardiness.Ā (In someĀ situations, a crop that is winter-killed is preferred,Ā to avoid having a vigorous residue in the way of an early spring sowing.)
WeedĀ control.Ā (Rapid growth and broad leaves are a plus.)
Remember,Ā although it is possible to present the broad outline of a biological sysĀtem inside a book, the fine-tuning that goes on within that outline is the province of the grower.Ā The best innovations and improveĀments usually come from the grower and not from anyĀ chart or list,Ā no matter how comĀplete it supposedlyĀ is. Whatever an expert does or does not sayĀ should not limit your opĀtions. The more active a grower becomes in taking charge of perfecting the system proĀposed here, the moreĀ independent, reliable and sustainable his or her own system will become.Ā
Spacing Undersown Crops: Examples
The garden is most easily visualized as a series of long strips five feet wide and 100 feet long. Forty-eight inches of that width is given over to the crops,Ā and 12 inchesĀ isĀ used as an accessĀ path.Ā For cropsĀ such as carrots,Ā onionsĀ and lettuce planted in l2-inch rows,Ā there are four rows side byĀ side with a l2-inch spacing.
12- to 18-inch spacing.Ā The one-row seeder can be used to drill three rows in the path. A single unĀdersown row can be drilled between each crop row. In this case, I usually sow dwarf whiteĀ clover in the paths. White clover or biennial sweet clover can be used between rows of onions or carrots.Ā The rows spaced at 18 inches are similarly undersown,Ā with three rows in the path and one row between the crops.
30-inch spacing.Ā For the 30-inch spacing atĀ which corn, beans, brassicas and so on are planted, the five-row drill is used. One pass is made down the center between each crop row.Ā Depending on the crop, dwarf white clover, sweet clover, red clover, hairy vetch and soybeans have all worked well between these crops.
60-inch spacing.Ā In the crops spaced at 60 inches (tomatoes and melons,Ā for example),Ā two passes are made with the five-row drill. Dwarf white, red, and sweet clover are all good choices here.
120-inch spacing.Ā At the widest spacing,Ā 120 inches for pumpkins and winter squash, everything except a strip about two feet wide on either side ofthe row is drilled (four passes with the five-row drill). My favorite undersown crop for squash and pumpkins is biennial sweetĀ clover.Ā
Coleman's 8-Year Garden Rotation
Undersown green manures are used extensively within my eight-Āyear crop rotation. The following sequence has worked out veryĀ well in practice.Ā It is described here both verbally and visually to show how all the pieces fit together.
Note that six of the eight rotational plots are undersown, a seventh is sown to legumes after early harvest, and only oneāĀpotatoesāis seeded to rye after fall harvest. The ground is never bare.Ā The soil is always growing either a market crop or nextĀ year's fertility. For much of the summer it is growĀing both!
Year 1.Ā BeansĀ are undersown to winterĀ vetch.Ā ItĀ is a dependable preceding green-manure crop for tomatoes.
YearĀ 2.Ā TomatoesĀ are undersown to oats or some other non-winter-hardyĀ grass crop. Certain grasses have been found to be excellent preceding crops for legumes such as peas, since they produce an allopathicĀ effect that suppresses grasses and other weeds but not legumes.Ā It is important to choose a non-winter-hardy cultivar so there will not be a mass of fresh growth in the spring toĀ impede earĀlyĀ soil preparation and planting of the pea crop.
YearĀ 3.Ā PeasĀ are not undersown but are followed byĀ a mixĀ of clovers asĀ soon as the peasĀ can beĀ cleared.Ā This combination of legumesĀ grows unĀtil it is turned under the following May, byĀ which time enough nitrogen has been fixed to ensure a splendid crop of brassicas.
YearĀ 4.Ā The cabbage familyĀ is undersown to sweetĀ clover, which is one of the best leguminous green manures to turn under for next year's corn crop.Ā It grows well under the cabbage familyĀ because it is a taprooted crop that does not seem to interfere with the moreĀ shallowlyĀ rooted brassicas.
YearĀ 5.Ā Sweet cornĀ is undersown to soybeans beĀcause research shows a soybean crop almost totalĀly inhibits potato-scab organisms in the soil. The soybeans also grow wellĀ in the understory of the corn and provide excellent weed suppression.
YearĀ 6.Ā PotatoesĀ cannot be undersown easily if the cultivation method used is hilling. I have grown potatoes without hilling by planting at a depth of six inches and filling the furrow partially at first, then completely after the potatoes emerge.Ā Vetch can then be planted as an undersown legume.Ā If the green manure is to be established following the potato harvest,Ā winter rye is probably the best choice.
Year 7.Ā SquashĀ is undersown to sweet clover in the emptyĀ strips between the squash rows.Ā Beets, carrots and other root crops grow veryĀ well followĀing sweet clover. The onion crop, on the other hand, has always grown best with no preceding green manure, so the onions are planted in the strips that were occupied by the squash plants themselves.
YearĀ 8.Ā Root cropsĀ are undersown to dwarf white clover (both in the paths and between the rows,Ā because theyĀ will grow in the crop understory and because theyĀ provide good erosion protection for theĀ soilĀ overĀ winter).