Posted January 29,
2004: In no-till cover crop systems, the known benefits
of cover crops are maximized by allowing them to grow until
shortly before planting the vegetable or other cash crop,
and by managing the cover crop without tillage. The best cover
crops for this purpose have the following characteristics:
- They produce a lot of biomass, at least 3 tons above-ground
dry matter per acre.
- They are readily killed by mowing, rolling or other mechanical
means, forming a mulch or
- they are reliably winter-killed, leaving a mulch for spring
no-till planting, or
- they die down naturally in time to plant summer vegetables.
- Their residues are sufficient to provide effective weed
control in the subsequent vegetable crop.
- They provide habitat for natural enemies of vegetable
crop pests.
- They have favorable (or at least neutral) effect on levels
of available soil N, P and K.
- They do not suppress the vegetable through chemical (allelopathic)
or microbial effects.
- They do not present serious weed, pest, disease or other
management problems.
Often, a combination of a grass and a legume is used, since
this enhances biomass production and therefore mulch thickness,
weed suppression and organic matter inputs. The combination
also offers a balanced carbon to nitrogen (C:N) ratio, which
gives a gradual release of plant available N, in contrast
to the N-immobilization (tie-up) by an all-grass cover, or
the rapid N release and potential leaching losses from an
all-legume cover. The higher diversity of a two-species cover
crop can also enhance allelopathy (suppression of weeds by
natural chemical substances from the mulch), diversity of
beneficial soil microbes, and nutrient effects. For instance,
legumes tend to enhance availability of phosphorus (P), while
grasses, especially rye, enhance availability of potassium
(K).
The most widely-known and extensively researched organic
no-till systems are those based on hardy winter annual cover
crops, mostly combinations of cereal grain rye (or winter
rye), hairy vetch, crimson clover and Austrian winter peas.
These crops are planted in early fall, and mowed or rolled
after they flower the following spring, usually in May. Summer
vegetables like squash, cucumber, pepper, tomato, eggplant,
okra, sweet corn, beans, or (in cooler regions) mid-season
brassicas, are then transplanted or direct-seeded no-till
into the cover crop mulch.
In recent years, growers and researchers have begun experimenting
with a much wider range of annual cover crop species for no-till
vegetables planted at other seasons. Other cool-season annuals
like oats and fava beans can be planted in early spring, then
killed in mid summer for late plantings of cucumber, bean
or summer squash. Summer annual (frost-tender) cover crops
like millets, cowpeas or soybeans can be planted after the
spring frost date, then knocked down at the end of summer
to plant fall brassicas or other fall crops. Finally, cover
crops that are not winter-hardy in a given location can be
planted in mid to late summer and allowed to winterkill, forming
a mulch for no-till spring vegetables.
One of the basic tenets of sustainable agriculture is that
greater diversity yields greater agro-ecosystem stability,
more beneficial organisms, fewer pests and diseases, more
sustained crop yields, and more opportunities for farmer innovation.
We feel that this is true also of cover crops, and one of
the objectives of ongoing research is to develop a larger
cover crop “toolbox” from which growers can select
cover crops most suited to their regions and production systems.
The following table gives some basic information on a number
of cover crops, some tried-and-true, and some less-known experimental
species. The table is organized into legumes and non-legumes,
listed in order of cold-hardiness. The table is intended not
as a cover crop prescription for organic no-till vegetables,
but as an information resource for farmers and other experimenters
to use in selecting cover crop combinations for their specific
vegetable crop rotations and cropping systems.
Cover Crop Information Sources:
The information in the table is based on the written
references listed below and on the first-hand experience with
cover crops of the three researchers at the end of this list.
Managing Cover Crops Profitably, 2nd Edition,
Sustainable Agricultural Network, Handbook Series No. 3, National
Agricultural Library, Beltsville, MD 20705-2351. 212 pp.
Peaceful Valley Farm Supply, P.O. Box 2209,
Grass Valley, CA 95945; tel 888-784-1722; www.growingorganic.com.
Products and Seed Catalog.
Overview of Cover Crops and Green Manures,
Fundamentals of Sustainable Agriculture series bulletin, 16
pp. Available through Appropriate Technology Transfer for
Rural Areas web site or by phone order. ATTRA, P.O. Box 3657,
Fayetteville, AR 72702; tel. 1-800-346-9140; www.attra.ncat.org.
Pursuing Conservation Tillage Systems for Organic
Crop Production, Organic Matters series bulletin,
28 pp. Available through ATTRA web site or phone order.
Summer Cover Crops, by N.G. Creamer and
K.R. Baldwin, North Carolina Cooperative Extension Service,
Horticulture Information Leaflet 37, 1999, 8 pp.
Professor Ron Morse, Department of Horticulture, Virginia
Tech, Blacksburg, VA 24061. Tel. 540-231-6724; e-mail morser@vt.edu.
Dr. Keith Baldwin, P.O. Box 21928, North Carolina A&T
State University, Greensboro, NC 27420. Tel. 336-334-7957;
e-mail kbaldwin@ncat.edu.
Dr. Mark Schonbeck, 439 Valley Drive NW, Floyd, VA 24091.
Tel. 540-745-4130; e-mail mark@abundantdawn.org.
Cover Crops for Organic No-Till Mulch Systems
Hardy
Legumes |
Bigflower vetch |
<-10 |
20-40 |
0.5 - 1.5 |
pea/vetch |
early fall |
Fls or Sk |
1-2 |
N, B, P, TS |
Comments: Sets seed &
dies May, germinates in fall |
Hairy vetch |
-10 |
20-40 |
0.5-1.5 |
pea/vetch |
early fall |
Fls (May) |
1-3 |
N, B, P, TS, W |
Comments: Best winter legume
for cooler regions |
Sweetclovers |
-10 |
6-20 |
0.25-1 |
alfalfa/ sweet clover |
Apr-Aug* |
Fls (May) |
1.5-3 |
SS, P, N, W, NR,
TS |
Comments: Several varieties,
most biennial w/ deep taproot; some semi-hardy annual
|
Crimson clover |
0-10 |
15-30 |
0.25-0.5 |
clover |
late sum. |
Fls (May) |
1.5-3 |
N, NR, B, P, W |
Comments: Slow-release
N; can self-seed well; best winter legume for milder regions |
Subclovers |
0-15 |
10-30 |
0.25-0.5 |
clover |
late sum. |
SK |
1.5-4 |
B, W, N, NR, P |
Comments: Drought tolerant, may inhibit
small-seeded crops |
Austrian winter peas |
5-10 |
70-120 |
1-3 |
pea/vetch |
late sum. |
Fls (May) |
1.5-3 |
N, B, P, TS |
Comments: Rapid N release & mulch breakdown |
Semi-Hardy
Legumes |
Lana vetch |
10-15 |
10-60 |
0.5-1 |
pea/vetch |
early spr. late sum. |
Fls (~July) WK |
1.5-4.5- |
N, B, P, TS, W |
Comments: Similar to hairy vetch, less hardy;
very high biomass if it overwinters |
Spring field peas |
10-20 |
70-120 |
1-3 |
pea/vetch |
early spr. |
Fls (July) |
1-2.5 |
N, B, P, TS |
Comments: Better than Aus. peas for spring
planting |
Lupines |
15-20 |
70-120 |
0.75-1 |
lupine |
early spr. late sum. |
Fls
WK |
|
N, P, B, SS |
Comments: Easy to mow-kill. Bitter var.
disease resistant but unsuitable for livestock |
Purple vetch |
20 |
30-80 |
0.5-1 |
pea/vetch |
early spr. late sum. |
Fls (~July) WK |
1.5-4 |
N, B, P, TS, W |
Comments: Fastest growing & maturing
spring |
Berseem clover |
20 |
8-20 |
0.25-0.5 |
clover |
mid-late sum. |
WK |
2-4 |
N, NR, B, P, W, TS |
Comments: Late flowering, hard to mow-kill;
fresh residue may inhibit small seeded crops |
Bell/ fava bean |
20 |
80-150 |
1-3 |
pea/vetch |
early spr. late sum. |
Fls (June-July)
WK |
|
N, B, P, SS |
Comments: Excellent nectar source for beneficials;
small-seeded (bell) varieties best. |
Tender
Legumes |
Soybean |
tender |
40-100 |
1-2 |
soybean |
after frost |
Fls or WK |
1.5-2.5 |
N, B, TS |
Comments: Forage varieties give greatest
biomass |
Cowpeas |
tender |
30-100 |
0.75-7.5 |
cowpea |
when soil is >/= 65°F |
Fls or WK |
1-3 |
N, B, W, P, TS |
Comments: Drought tolerant, requires heat,
suppresses nematodes, easy to grow |
Sunn Hemp |
tender |
10-50 |
0.5-1 |
cowpea |
after frost |
Fls or WK |
2.5-4 |
N, W |
Comments: Fibrous, persistent mulch, tolerates
acid or droughty soil, slow-release N |
Lablab Bean |
tender |
10-40 |
1 |
lablab |
after frost |
Fls or WK |
2+ |
N, W |
Comments: Drought tolerant, easy to mow-kill |
Hardy
Non-Legumes |
Winter Rye |
-40 |
60-160 |
0.75-2 |
fall |
Fls (May) |
2-5 |
W, NR, K, TS, B |
Comments: Tolerates poor & acid soils,
may tie up N, can initially inhibit small crop seeds |
Winter Wheat |
-25 |
60-120 |
0.5-1.5 |
fall |
Fls (June) |
1.5-3.5 |
W, NR, K, TS |
Comments: Later and harder to mow-kill than
rye |
Triticale |
<-10 |
60-120 |
0.5-1.5 |
fall |
Fls |
1.5-4 |
W, NR, TS |
Comments: Wheat/rye cross, taller than most
wheat |
Little Barley |
0-10 |
15-35 |
0.25-0.5 |
early fall |
SK |
|
W, TS |
Comments: Self-seeding winter annual, experimentally
grown with subclover |
Semi-Hardy
Non-Legumes |
Barley |
10-15 |
50-125 |
0.75-2 |
early spr. late sum. |
Fls
WK or Fls |
1.5-5 |
W, NR, B, TS |
Comments: Drought tolerant, likes light
soils, deep-rooted if grown over winter |
Spring Oats |
15-20 |
80-140 |
0.5-2 |
early spr. late sum. |
Sd. (milk)
WK |
1-4 |
W, NR, TS, B |
Comments: Tolerates acid soils, less N tie-up
than rye; may slightly inhibit small seeds |
Black Oats |
20 |
15-20 |
0.5 |
late sum. |
WK or Fls |
2-4 |
W, NR |
Comments: Experimental; easy to mow-kill |
Fodder/oil radish |
20 |
10-20 |
0.5 |
late sum. |
WK |
|
SS, W, B |
Comments: Not recommended before brassica
veg. |
Tender
Non-Legumes |
Buckwheat |
tender |
60-80 |
0.5-1.5 |
May-Aug |
Fls or WK |
1-1.5 |
B, W, P, TS |
Comments: Short life cycle, residues decay
rapidly; can become weed by reseeding |
Sorghum-sudan hybrid |
tender |
35-50 |
0.5-1.5 |
when soil is 70°F |
Sd or WK |
3-5 |
W, SS, NR |
Comments: Mow at 3-4 ft to promote deep
rooting allelopathy may inhibit some crops |
Foxtail millet |
tender |
25-30 |
0.25-0.5 |
after frost thru July |
Sd or WK |
1.5-2 |
W, NR, TS |
Comments: Fast growing, drought tolerant,
no hard seed, thus low potential to become weed |
Pearl millet |
tender |
5-30 |
0.25-0.5 |
mid sum. |
WK best |
1.5-2 |
W, NR |
Comments: Hard to mow-kill, very tall |
Japanese millet |
tender |
20-30 |
0.5-1 |
after frost |
Sd or WK |
1.5-2.5 |
W, NR |
Comments: Quick maturing (45 days) |
|