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Weeds
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Researchers at USDA ’s
Agricultural Research Service find that dense mulches of cover
crops, particularly grasses such as rye, provide excellent early-season
weed control. No herbicides were applied to this soybean
field – the rye was killed with a modified Buffalo stalk
chopper.
– Photo by Dr. Aref Abdul-Baki |
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Although they joke about it now, it probably didn't seem too funny
when Joe Rude and Wende Elliott thought they'd lost their first
year alfalfa crop to an invasion of weeds (see profile).
Yet weed management for organic and transitional farmers is a formidable
problem and ranked number one among research priorities in a national
survey of organic farmers.
Standard organic weed management strategies include:
 Smothering
weeds with cover and forage crops. A dense mat of cover
crops will prevent weed germination or crowd out weeds struggling
to gain a toehold. Residue from a grass cover crop decomposes
slowly, while legume residues break down faster. Grass-legume
mixtures also can control weeds while providing more nitrogen
to the cash crop. Forages serve a similar process; in his five-year
alfalfa, corn and oat rotation, "the three year alfalfa goes
a long way to getting the weed seed bank out of the field,"
said Joe Rude.
Managing
weeds selectively. Identify weeds and manage them according
to their lifecycle and reproductive strategies. For example, tilling
weeds like quack grass and Canada thistle is ineffective in the
short run, since tillage may propagate their rhizomes. Repeated
cultivation, however, forces them to draw upon their storage,
and can eventually weaken the population. Biennials, on the other
hand, must not be allowed to seed and persistent mowing can eventually
exhaust root reserves.
Conservation
tillage. Mark Davis, an agronomist working on a long-term
organic farming systems research trial run by the USDA Agricultural
Research Services Sustainable Agricultural Systems Lab in Beltsville,
Md., uses a regular no-till planter to sow corn directly into
a living stand of vetch, and then rolls and crimps the vetch with
a Buffalo Stalk chopper. "This system has great potential
for no-till organic cropping systems because it provides weed
control and nitrogen fertility at the same time," Davis said.
By allowing the vetch to grow longer in the spring, the researchers
are increasing the amount of nitrogen added to the system. At
the same time, the mat of vetch smothers the weeds long enough
for good early season weed control.
"It gives you the best of both worlds," Davis said. "Your no-till
practices help reduce soil erosion, improve soil structure and increase organic
matter, and you can still manage the system organically." The one caveat,
he cautions, is that in years with a dry spring, the late growth of the vetch
can deplete soil moisture for the following crop.
Using
living mulches. Inter-seeding one crop into another can
be done on a large scale by sowing rye from aircraft over corn
acreage, or from tractors or by hand. The second crop, which should
germinate after the first, will compete for nutrients and moisture
so this technique should only be used when crops are well established
or have ample soil fertility and moisture. Dutch white clover,
for example, is effective in corn or late season brassicas. Its
high density keeps out weeds, it fixes its own nitrogen, and it
is low growing so it doesn't compete with the crop for sunlight.
Many organic farmers also use some sort of mechanical weed control
in combination with the above strategies. (For more information
on mechanical weed control see Steel in the Field in "Resources")
Some organic farmers believe that weeds do not need to be eradicated, just
managed. Knowing when a weed is a threat and when it can be ignored, something
often gained by experience, remains a common strategy.
"Our farm becomes so much simpler all the time," said Dan Nagengast,
who farms five acres of cut flowers and mixed vegetables in Lawrence, Kan.,
and has been growing organically for 15 years. "We've learned from our
mistakes - it used to be if we were eight to 10 days away from harvesting lettuce,
we would hoe the weeds. Now we know when the crop will make it, and we don't
have to do all the extra things we thought we needed to."
In a bit of a radical departure from the conventional approach of "the
only good weed is a dead weed," some organic farmers choose to integrate
weeds into their cropping systems for the benefit of the whole farm. Steve Gilman,
who farms 15 acres in Stillwater, N.Y., and grows fresh market vegetables such
as lettuce, tomatoes and peppers in 4-foot-wide raised beds, decided there was
no need to spend the "time and energy needed to keep the two-foot wide,
permanent strips between the beds clean-cultivated."
As explained in his book "Organic Weed Management,"
Gilman was concerned about the susceptibility of the bare soil to
erosion. So he began planting "bio-strips." First, he
eliminated perennial weeds such as quack grass and thistle with
repeated cultivations before forming the beds. Then he sowed Dutch
white clover between them, allowing a mix of perennial grasses,
wild herbs and wildflowers to flourish.
Gilman sees numerous advantages of these bio-strips, including:
A
diverse, protective habitat and food supply for beneficial insects
and microorganisms in the field alongside the crops;
A source
of organic matter or mulch from the clippings of the plants (making
sure to mow before any wildflowers go to seed); and
Confinement
of potential compaction to bio-strips, where the soil is supported
by root system of this mix. The planted inter-spaces also provide
muck-free footing when Gilman needs to walk or drive a tractor
between the beds.
(To learn more about Organic Weed Management, a book produced
with support from a SARE grant, see "Resources")
The bio-strips enable Gilman to retain one-third of his farm acreage in permanent
no-till, preventing erosion and preserving soil organic matter. In the beds
themselves, Gilman quickly sows catch crops after each harvest to keep the soil
covered and to prevent weeds from taking hold. The increased growing capacity
of the raised beds, which can support much higher plant densities, offsets the
land lost to interspaces, while the high density planting helps prevent weeds.
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