Posted March 21, 2003: Effective weed control
involves more than good timing or having the right tools.
Biological Warrior: Gary Zimmer, farmer
and author of The Biological Farmer, shared
the secrets of weed control with the attendees of
The key to success revolves around soil management, according
to Gary Zimmer, a Wisconsin farmer who has authored the book
“The Biological Farmer.”
“Controlling weeds is all about soil building. Loose,
crumbly soil structures help control weeds, but hard, compacted
soil is ideal for weed seeds,” said Zimmer, who presented
the seminar “Biological Weed Control Strategies”
at the Upper Midwest Organic Farming Conference. The Feb.
28 seminar was standing room only, as more than 100 people
gathered to hear Zimmer’s insights.
||"Weeds are an indication that
something’s not right. Whether you are organic or
conventional, you need to start thinking long-term about
weed control and soil management. It takes three to five
years to improve soil structure."
Weeds are indicators of soil health, according to Zimmer.
“If you’ve got hard, tight soil that’s low
on calcium or sulfur and low in biological activity, foxtail
thrives in this. But weeds are nature’s tools to fix
the soil. If you left those foxtails there, they’d repair
the land in 50 or 100 years.”
But don’t misunderstand—Zimmer won’t leave
those foxtails alone.
“Weeds are an indication that something’s not
right. Whether you are organic or conventional, you need to
start thinking long-term about weed control and soil management.
It takes three to five years to improve soil structure,”
said Zimmer, who serves as president of Midwestern Bio-Ag
an ag consulting business that focuses on biological solutions.
Improving soil conditions
So what can you do to start controlling weeds better? Weed
control strategies can be grouped into four types--improving
soil conditions, rotating crops, growing smother crops and
using mechanical control.
“Success usually isn’t just about doing one thing
right. I’m a big believer in shallow incorporating residues
and subsoiling. I also mineralize my soils, adding what is
short and feeding the crop a mineral balance for its specific
needs,” said Zimmer, who operates a Wisconsin dairy
farm with his family and grows row crops on 400 of their 700
Adjusting these minerals involves a soil-balance approach,
Zimmer explained. “We start balancing with calcium and
phosphorus but also add sulfur and all the trace elements.
Calcium and sulfur do improve soil structure. Having lots
of organic matter shallowly incorporated does make the soil
surface loose and crumbly. This allows rain water to soak
in better and reduce weed pressure.”
When soil structure improves, conditions will allow crops
to grow vigorously and develop huge root systems that help
the crops shade out the weeds.
“Weakening and killing weeds are two different things.
Weed control is about competition. You can do a lot of things
to slow down weed growth and give your crop an advantage,”
To improve soil conditions, recycle organic matter like animal
manure, crop residues and green manures. But don’t add
too much at once, Zimmer said. When you spread livestock manure
on your land, make sure you follow some basic tips to control
“You obviously don’t want to fertilize weeds,
but that’s what soluble nutrients do—they boost
both your crops and your weeds. If you apply manure, do it
on grass fields or on a high mature residue crop like corn
stalks. Composting manure also stabilizes nutrients, and this
will control weeds.”
After a soluble manure application, plant a crop like oats,
Zimmer said. “These will grow and suck up a lot of the
soluble nutrients that really make weeds grow. They will also
stop leaching and erosion, and they provide green manure.”
Zimmer cited the example of an Iowa farmer who farmed conventionally
and developed a five-year soil management plan that included
oats. “He planted oats and then followed this with corn
and soybeans. He says this has made a tremendous difference
in his soil. It’s true that oats aren’t as cheap
as they used to be, but they’re still a good value.”
Include crop rotations
Growing a variety of crops on the same field will also keep
weeds off balance, because certain weeds grow best with certain
crops, Zimmer said.
“In a rotation, every crop does something different
and each provides some advantage. Crop yields are also better
in a rotation,” he noted.
In addition, a good rotation sequence can help improve soil
structure and fertility, which boosts weed control. Livestock
farms with rotations including two or more years in hay will
have easier weed control, because in two out of the five years,
no fresh weed seeds are added to the soil, Zimmer said.
“Without livestock, taking a year off in the rotation
really does break weed cycles. It also builds soil organic
matter and gives a way to build up mineral and soil biological
levels, setting the stage for a few years of excellent crops.
Even if you can’t take the whole field, maybe have a
full crop on 80 percent of the ground and don’t harvest
anything from the other 20 percent. It’s okay to let
land go fallow.”
Smother crops improve weed control
Farmers can also try growing smother crops, which control
weeds by shading them out and releasing natural chemicals
that kill weeds.
“My favorite smother crops are buckwheat and Sudan
grass. Rye, clovers, hairy vetch, alfalfa, barley, and oats
also work well to control weeds. Smother crops also improve
soil structure, prevent erosion and provide crop nutrients
when they are turned under as a green manure,” Zimmer
If you don’t grow these as a main crop, the smother
crops can be interseeded or overseeded into an existing crop,
Zimmer added. “Try to keep the soil covered all year
round. Every chance I get, I’ll be growing something
Machines add power to weed control methods
Finally, don’t overlook mechanical weed control. “If
you have trouble with weeds, the last thing you want to do
is quit cultivating or rotary hoeing. Rotary hoeing can give
excellent control, provided the weather allows it and it’s
done early, usually two to three days after planting and again
when the crop is three to four inches tall,” Zimmer
each his own: Gary Zimmer gave the basics
but explained that each farmer would have to decide
what works best for his farm.
Using more than one kind of tool for different conditions
can be helpful, Zimmer added. “The Buffalo cultivator
has worked really well on our farm. With the cut-away disks,
long ridged shield and huge sweeps, we can clean up most fields.
Disking, rotavating, harrowing and plowing can kill deep-rooted
or perennial weeds.”
Zimmer said he and his family also like the Howard rotavator.
“It can leave the field in better condition and can
kill almost any crop in one pass, which minimizes trips over
the field. It also leaves residues on the surface, protecting
Burning or flame cultivation provides another physical method
of weed control. “A set of propane burners can give
good in-row early weed control without harming crops. The
per-acre costs are low,” Zimmer noted.
Most mechanical weed control methods should be used early
in the season, when weeds are just sprouting. Keeping the
top inch or so of soil dry and loose will reduce the chance
of new weeds getting established, Zimmer added.
What about chemicals?
If herbicides are necessary for a biological farmer (of course
this isn’t allow if you are an organic producer), try
to reduce the rate and the amount you use per acre, Zimmer
“Maybe just spot-spray the worst areas of a field,
or only spray when weather conditions prevent non-toxic control.
Herbicide rates can often be cut by about half while still
getting effective weed control. Only banding the herbicide
in the row and using mechanical control between rows can reduce
rates even more,” he said.
Remember, weed control is all about improving soil structure.
Zimmer concluded, “Weed control really isn’t a
battle. It’s about learning to understand soil structure
and soil health. But remember that this isn’t formula
farming. There’s no one right way. Find out what works
on your farm.”