The Riddle of Understanding and Controlling Perennial Weeds

By W. Carroll Johnson, III, PhD.,, Weed Scientist and Agronomist

 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.