Welcome to
the first installment in this three-part series--a
basic primmer about weeds, tools, and techniques--which
will be followed by a more in-depth look at equipment
and cultivation techniques.
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Organic weed control is not rocket science,
but it does take understanding the anatomy and physiology
of the crop plants, the weeds and the soil, as well as a cultivated
anticipation of how each will respond to the implement used.
Weed control strategies must consider the prevailing weed
species; their size, condition and age; the soil condition;
the available equipment; the species of crop; crop plant size;
and the weather.
There
are three main ways to kill weeds by cultivation:
- Burying them
- Uprooting them so that they desiccate (dry up)
- Severing or damaging the weed enough so that neither part
can regenerate
Weeders and cultivators generally do a combination of these
three. It is important to be aware of exactly what the cultivator
is doing to the weed seedlings while it is operating in order
to time the operation correctly and make proper adjustments
effectively.
Weed species vary widely in their susceptibility to cultivation
equipment and in the length of time after germinating during
which they are most easily controlled. Ragweed sends down
a taproot very quickly, making it difficult to uproot almost
as soon as it comes up. Mustard has very shallow roots at
first and is easily plucked out until it is quite large. Summer
annual grasses form small seedlings with few reserves that
are easily destroyed by burying or uprooting them. Large-seeded
weeds like velvetleaf that can emerge from deep in the soil
are very resistant to shallow cultivation with weeders. Redroot
pigweed is very difficult to kill by burying or uprooting
once it gets a few inches tall, because it can push up out
of fairly deep soil when buried and can re-root. Pigweed can
also grow even if pulled all the way out and left lying on
top.
Dessication
High humidity, cool temperatures, cloudy skies, and rain
reduce the effectiveness of desiccation. The easiest time
to kill weeds by desiccation is on a sunny, windy, hot afternoon.
Weeds will often wilt and die under these conditions even
if they are only partially uprooted.
In periods of drought, weeds may go into a semi-dormancy,
leading farmers to stop cultivating or to set cultivators
less aggressively. Soil often becomes hard under these conditions,
making it difficult for equipment to penetrate to the proper
depth. In a drought, deep-rooted weeds are tightly held in
the soil and have large root systems relative to the size
of the tops. A cultivator that is run too shallow can bury
these weeds without disturbing the roots significantly. Such
fields may look very clean at first but, if drought persists,
the weeds will push back out of the dry soil ready to grow
rapidly from a large well-developed root system. It is important
to uproot and desiccate these weeds thoroughly, because burying
them in the loose dry soil is not very effective (unless they
are buried quite deeply).
Burying Weeds
During wet weather, burying weeds becomes the more effective
approach, particularly if rain follows shortly after the cultivation.
A rain on freshly cultivated soil will make it stick together
and become tight. Often the soil surface will crust slightly
as it dries. Under these conditions, weeds that are buried
will die quickly and will seldom manage to push back out.
It is important to note that any crop plants that are buried
by weeders or cultivators just before a rain are usually lost,
as well.
Tillage
Organic mechanical weed control consists of 4 distinct phases,
each one very important to the overall success of your weed
control program. These phases are:
1. Tillage
2. Planting
3. Blind cultivation
4. Row cultivation
The goal of early mechanical weed control is to eliminate
the bulk of the weed population before it competes with the
crop and to create as large a crop-to-weed size differential
as early as possible. When crop plants are bigger and more
vigorous than the weeds, the weed pressure will usually not
jeopardize the crop. Therefore, effective early weed control,
before weeds present a visible threat to the crop, is absolutely
essential.
Appropriate tillage of fields is critical to:
• Create a good seed bed for uniform, vigorous crop
emergence
• Prepare the ground adequately for successful subsequent
mechanical weed control operations
• Kill weeds that have already emerged, including
tearing up and burying perennial weeds with large underground
root systems
The sun on the soil surface brings the shallower weed seeds
out of dormancy in the spring, preparing them to sprout. The
warm soil, full of weed seed ready to grow, responds to tillage
quickly with a new flush of weeds. Moldboard plowing inverts
the soil, bringing deeply buried dormant weed seeds to the
surface and burying germinated weeds down below where they
can’t grow. When this surface soil is turned under cleanly
with a load of germinating weeds, deeper soil is brought to
the surface. The newly surfaced weed seeds that had been laying
dormant deep in the soil will often not begin to grow until
after the crop gets started. Chisel plowing does not invert
the soil and can result in a heavy flush of weeds that will
compete with the crop early in the season.
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"The goal of early mechanical
weed control is to eliminate the bulk of the weed population
before it competes with the crop and to create as large
a crop-to-weed size differential as early as possible." |
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Another approach, called the stale seedbed technique, works
well if there is enough time before planting. The soil is
plowed early, encouraging as many weeds to sprout as possible;
then they are killed as the ground is tilled again. If several
cycles of weed emergence and tillage occur before planting,
we will have greatly reduced the weed seed bank, thus eliminating
most of the weeds that were likely to germinate to compete
with crops.
There are many microbial species in a biologically active
soil that attack weed seeds and the rhizomes of perennial
weed species. Tillage adds air to the soil and stimulates
biological activity as microbes feed on organic materials
and break them down. This accelerated decomposition is often
said to be burning organic matter. This is not neccessarily
bad. Destroying weed seeds and helping crop residues break
down are important benefits of tillage. Tillage helps mineralize
nitrogen and phosphorus, cycling it from less available forms
into ones that crops can readily use. It is only when tillage
is excessive or poorly timed, or combined with poor rotations,
lack of cover crops, high usage of nitrogen fertilizer, and
other related poor farm-management practices that tillage
actually damages the soil. When this happens, more organic
matter is used up each year than is replaced, and soil degradation
results.
In seedbed preparation, the goal is to prepare an environment
that helps the crop to emerge as quickly and uniformly as
possible without encouraging weeds. The seedbed should be
smooth and level to allow for effective weeding and cultivation
latter in the season. Large clods, rough spots, sod clumps
and debris at planting will interfere with subsequent cultivating
and weeding.
All perennial weeds need to be plowed under completely when
the field is prepared. Rhizomous weeds such as quackgrass
can often be killed by pulling the rhizomes to the surface
with a spring-tooth harrow to dry out on a sunny day.
Planting
Planting equipment must be adjusted carefully to insure that
the seed is planted at the proper depth for the crop and that
it is planted uniformly for even emergence. Planting when
the soil is too damp can cause a large flush of weeds to germinate
very quickly from the moist packed soil at the surface. A
dry, crumbly or even slightly cloddy surface with moist, fine
soil at seeding depth gives the crop a good head start over
the weeds. Basket rollers can produce finer soil at seeding
depth with a looser and coarser surface than cultipackers
or rollers.
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"Clay soils are often worked
slightly wet to get them fine. This can lead to crusting
and a heavy early flush of weeds . . . . A dry, lumpy
soil with just enough moisture to get a stand of soybeans
started will often produce a perfectly clean crop with
minimum effort." |
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It is important that planting equipment is in good repair.
Disc openers worn past the point that the manufacturer recommends
should be replaced. The gauge wheels must contact the disc
openers exactly as the owners manual suggests, and the press
wheels must follow straight behind the openers with the proper
amount of down-pressure for the soil conditions.
Planters often plant seeds deeper when the soil is soft and
slightly damp and shallower where it is dry and a little cloddy.
The planting depth should be checked in lumpy spots as well
as where the seed bed is ideal. Older John Deere 7000 planters
sometimes put seeds almost on top in lumpy spots while dropping
them in just right where the soil is softer. This can happen
when there is a big enough gap between the disc opener and
the gauge wheel for dry clods to push up in between and then
drop into the seed trench ahead of the seed. The seeds are
then placed into dry soil from the surface wherever the field
is somewhat dry and lumpy. Replacing worn parts and proper
adjustment of the planter can eliminate this problem. One
after-market company builds replacement seed tubes that insure
seed placement at the bottom of the seed trench while others
sell attachments to better push seed down into the V left
by the seed openers.
Corn and other crops with axillary roots must be planted
deep enough to allow the plant to set roots above the seed.
The tiny radicle on a corn seed only provides a small start
for the plant. There has to be good soil contact with the
stem to allow roots to form above the seed. Corn should usually
be planted a minimum of 1.75” deep to allow for normal
root formation. If corn is planted too shallow, it will have
poor rooting and be prone to lodging. Soil hilled up around
the corn plant as it grows stimulates further axillary root
formation.
Clay soils are often worked slightly wet to get them fine.
This can lead to crusting and a heavy early flush of weeds.
A slightly rougher surface doesn’t look as nice, and
care must be taken to avoid uneven emergence, but weed control
is usually much better. Soybeans can germinate and emerge
from much dryer soil than most weeds or even corn can. A dry,
lumpy soil with just enough moisture to get a stand of soybeans
started will often produce a perfectly clean crop with minimum
effort. When a field gets too hard and lumpy at planting,
running over it with a cultipacker or roller right after planting
will often firm it enough to make soybeans emerge well but
not the weeds.
Blind cultivation
Blind cultivation is the easiest and best opportunity to
destroy the weeds that would be growing within the rows and
presenting direct competition to the crop. In blind cultivation,
the entire field is tilled shallowly with the implement, paying
little attention to where the rows are.
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"By doing an effective job of
blind cultivation, you can achieve the biggest possible
crop/weed size differential from the start." |
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The point of blind cultivation is to stir the top 1/2 to
1-1/2 inches of the soil, breaking the contact between the
weed seedling roots and the soil and burrying the tiny weeds.
This adds air to the soil, causing the millions of tiny germinating
weed seeds to dry out and die. The larger crop seeds germinate
below the level of the cultivation and are not usually damaged
by this operation. Weed seedlings are very vulnerable to drying
out and to burying at this stage. By doing an effective job
of blind cultivation, you can achieve the biggest possible
crop/weed size differential from the start. Blind cultivation
also can break a soil crust, allowing crop seedlings to emerge.
Usually, the first blind cultivation pass is done right before
crop emergence, with a second pass done about a week later,
depending on conditions. The most effective blind cultivation
is done when the soil is fairly dry and the sun is shining.
A wind also improves the effect.
Blind cultivation equipment includes rotary hoes, tine weeders,
spike tooth harrows, springtooth harrows and chain link harrows.
One resourceful farmer we know even drags a set of old tire
chains over his fields for blind cultivation. 
Part
2: Blind cultivation >>
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