During a sarcastic
tirade, a former supervisor once called me an obsolete dinosaur. I guess the
dinosaur in me likes using information from ancient sources to help make a
modern technical point. Referring to the early agriculturalist Jethro Tull from
18th century England, he makes an interesting comment regarding perennial
weeds: “The other old remedy is what often proves worse than the disease
(sic.); that is, what they call weeding among the sown corn; for it by the hook
or the hand they cut some sorts (as thistles) while they are young, they will
sprout up again, like Hydras, with more heads than before; and if they are cut
when full grown, after they have done almost their utmost in robbing the crop; ‘tis
like shutting the stable door after the steed is stolen.”
Jethro Tull is referring to perennial
thistles growing in cereal grains (his term is ‘corn’), which are very
difficult to control, even today. If you mow thistles when they first begin to
produce a flowering stalk (called bolting), many other flowering stalks are
stimulated making the problem even worse. That is the basis for the simile Tull
makes to the mythological character Hydra. The point is that perennial weeds are
very difficult to control and effective control is based on an understanding of
how perennial weeds grow and propagate.
The basics of perennial weed growth.
Perennial weeds propagate primarily
by vegetative pieces of rootstock (rhizomes, stolons, tubers, and etc.) and
secondarily by seed. The perennating rootstock are the main reason for
perennial weeds being difficult to control. It is a well-established scientific
fact that successful control of any perennial weed targets the perennating
rootstock. However, do not forget about perennial weeds producing seed. The
perennial grass johnsongrass produces resilient rhizomes for long-term survival
in the field and copious amounts of viable seed, which can spread over significant
distances.
Mechanical controls — a blessing and
a curse.
Mowing and tillage are mechanical
weed control tools of immense value to food plot hobbyists. However, neither
practice used alone will adequately control perennial weeds. Jethro Tull mentions
thistles, but think about the perennial weed common bermudagrass. Improved
selections of the same species are commonly planted worldwide on golf courses,
athletic fields, and as a hay crop. The resilience of these selections, which
makes them desirable for those uses, is why the species is also a troublesome
weed. The aggressive lateral growth of the stolons and persistence of the
rootstock makes common bermudagrass a troublesome weed. Mowing may scalp above
ground portions of common bermudagrass, but the roots are obviously not
directly affected allowing recovery. Tillage may be a little more effective
than mowing in suppressing perennial weeds, but that difference is short-lived.
It is logical to expect that tillage may spread perennial weeds by cutting and
relocating perennial weed rootstock elsewhere. On the other hand, removing
periodic tillage from the food plot production system (i.e. no-till) allows perennial
weeds to quickly become problematic. That is why I am not a carte blanche
proponent of true no-till crop production, including food plots. It is worth
noting a sliding scale of effectiveness of mechanical control of perennial
weeds. One shallow tillage will, at best, do nothing for perennial weed
control. If you are going to use tillage to suppress perennial weeds, then till
with gusto. Repeated tillage during a fallow summer (no forage crop growing
during the hot summer months), will progressively weaken perennial weed
rootstock and perhaps expose those structures to desiccation causing mortality.
Chemical control of perennial weeds.
Effective herbicides for perennial
weed control must be systemic, meaning that the herbicide is actively
transported from foliage to the perennating rootstock. Contact herbicides are
non-systemic, do not affect the rootstock of perennial weeds, and are no more
effective than mowing. Common systemic herbicides that can be directly applied
to forage legumes for perennial weed control are Arrest Max and Slay. These two
herbicides work best for perennial weed control when applied in the latespring through
early-summer. Other systemic herbicides that can be applied to fallow sites are
triclopyr (Garlon), glyphosate (Roundup), and 2,4-D. Triclopyr and glyphosate
tend to work better on perennial weeds when applied in the autumn when weeds
are actively translocating to rootstock. Development of systemic herbicides
were weed control milestones in conventional agriculture and food plots alike. However,
these systemic herbicides are not fool-proof and not standalone.
Integrating mechanical and chemical
weed control.
Synergy can be defined as the final
outcome from combined factors is greater than the sum of the individual
factors. A classic example in agricultural science is the effect of integrating
mechanical control (mowing, tillage) with systemic herbicides to control
perennial weeds. This synergy must be captured for adequate control of any
perennial weed species. The synergy of mechanical and chemical weed control is
based on two general concepts, both affected by nuances of the weed species. (1.)
Mechanical controls (done correctly) weaken the survivability of perennial weed
rootstock and that improves overall performance of systemic herbicides. (2.)
Mowing alters the growth pattern of perennial weeds, specifically upright
perennial weeds. Mowing stimulates regrowth and those leaves tend to transport
systemic herbicides to the perennating rootstock. That is the case with
perennial pepperweed (a pasture weed in the western U.S.) and tall ironweed (found
in the U.S. heartland). A few years ago, Whitetail Institute received several inquiries
on how to control the perennial grass, tall fescue, in food plots. A search of
the agricultural scientific literature found the same general recommendations;
use mowing, shallow tillage or fire to weaken the tall fescue stand and then
treat succulent regrowth with glyphosate. These examples are the basis of our
standard perennial weed control recommendations. When it comes to perennial
weed control in food plots, satisfactory results are neither simple nor
immediate. Successful perennial weed control begins with some form of
mechanical weed control to weaken perennial weeds and stimulate regrowth,
followed by a systemic herbicide. Jethro Tull compared hoeing to control
perennial thistles with the Greek mythological character Hydra, a multi-headed
aquatic monster. If one head were severed, it was replaced by two. Hercules was
tasked with killing Hydra. Hercules used his sword to sever a Hydra head and
fire to cauterize the ‘stump’ to prevent regrowth and eventually killed Hydra —
synergy. Perennial weed control is no different; a synergistic system of
mechanical weed control and systemic herbicides. Just think, if Jethro Tull had
a whiff of glyphosate or any systemic herbicide in the 18th century, the Hydra
analogy in his essay would have a completely different meaning.
