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.
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
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.
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
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).
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.
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:
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.
||"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."
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 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.
||"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."
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
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 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.
||"By doing an effective job of blind
cultivation, you can achieve the biggest possible crop/weed
size differential from the start."
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
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.
2: Blind cultivation >>