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Whack these fall weeds now

This final 2010 Q&A series article targets what David Mortensen, a Penn State weed scientist, told members of a U.S. House Oversight Committee this summer was a weed resistance problem so serious that new strategies are needed to combat it. In fact, Mortensen proposed restrictions on use of herbicide-tolerant crops and a tax on biotech seeds to fund research and educational programs for farmers.

Key Points

• Perennial weed control is a make-or-break no-till issue.

• Perennials are easiest and cheapest to control during fall.

• Glyphosate-tolerant weeds may be bringing use restrictions.


Pennsylvania No-Till Alliance board member Russell McLucas and Penn State Extension grain crop specialist Del Voight also note the problem and worry about impending controls. But they stop short of such proposals. McLucas, past chairman of the Pennsylvania Corn Growers Association and a 30-year veteran no-tiller, farms near McConnellsburg, Pa.

Q: Why’s it so important not to let summer and fall weeds get out of control in no-till?

McLucas: With the absence of tillage, the weed spectrum changes markedly. Deep-rooted perennials become an issue around year three or four. Although there are multiple chemicals labeled for perennial control in growing crops, the best “cleanup” is still in a nonharvestable crop situation.

This removes many of the labeled restrictions. Rates can be increased.

Perennials are much better controlled if “hammered” between flowing to seed-set. And that’s normally after the growing crop passes the point of labeled application.

Many of today’s available chemicals have very interesting sidebars on the labels for early fall applications. Quackgrass, thistles, hemp dogbane, orchardgrass and alfalfa are all noted as “easier” (and cheaper) kills with fall application. It’s amazing what a 2-quart rate of generic glyphosate applied in early August can do to a nasty perennial weed problem.

Fall has been the best time to handle perennials. Translocating chemistries work much better when the plant translocates the chemical downward into the root. With this being said, I suspect we should look very hard at a more selective fall program, where the idea is to remove the more long-term perennials only, and not everything.

Here I tend to prefer to follow small grains with a legume cover crop. The postharvest, preplanting window gives a nice opening to go after various undesirables with nonselective chemistry. This has an interesting side benefit. Legumes will help “break” the grass root-disease cycle, fix nitrogen and add a taproot.

Voight: The reproductive potential and longevity of the weed seed affect your weed management. In general, perennials have few seeds compared to annual weeds. But to echo Russ, they can spread through their root structures and therefore require different tactics and timing. Canada thistle, for instance, produces about 680 seeds per plant. A common lambsquarters plant produces more than 72,000 seeds per plant.

Dandelions produce about 15,000 seeds per plant, while redroot pigweed produces more than 117,000 seeds per plant. You will begin to respect these weeds that produce a plethora of seed. Broadleaves tend to have more seeds than annual grasses. Black nightshade produces about 178,000 seeds per plant, versus about 34,000 for green foxtail.

Q: What weed uprisings really have to be hammered down — and why?

McLucas: [Regulatory] changes are coming in how we can control weeds. I’m very certain agriculture isn’t going to be pleasantly surprised. So let’s get the nasty perennials now, before the rules change.

Match label rates to target weed size and maturity through mid-October. Dandelions, thistles, quackgrass, orchardgrass, Italian ryegrass and hemp dogbane are all much more susceptible in this time frame.

But perennial control in a cover crop is a problem probably best left alone.

Voight: Perennials such as hemp dogbane, Canada thistle, quackgrass and wirestem muhly translocate or move sugars and carbohydrates from their leaves to their roots or underground storage structures. A higher percentage of a systemic herbicide such as glyphosate (Roundup or Touchdown or Glyphomax, etc.) will move with those sugars and carbohydrates to underground structures, where they can potentially kill these reproductive organs.

The most common herbicides used for this type of application, suggests Bill Curran, Penn State weed specialist, include glyphosate for grasses and 2,4-D or Banvel or Clarity, Status/Distinct for broadleaves. A combination of these products may be the best solution for a mixture of different perennial weeds.

Seed longevity brings in another factor. Dandelion seeds remain viable for only six years, while lambsquarters can last up to 40 years. That’s why you can’t let perennials establish a weed bank.

I also agree that cover-crop perennial issues are problematic. I think I’d get into some hot water if we began pushing fall-applied programs.

Don’t wait too long to treat

For most perennials, including hemp dogbane, horsenettle, common milkweed, pokeweed, hedge bindweed, multiflora rose, poison ivy and wild blackberry, make applications from Sept. 1 through Oct. 15, or before a hard frost.

Generally, applications by Oct. 1 may be more effective in Pennsylvania. In northern areas of Pennsylvania, consider making the application before Oct. 1.

An additional two-week application window can exist for Canada thistle and quackgrass. Make sure that the weed foliage appears relatively healthy and capable of absorbing the herbicide. Plants that have been damaged by insect feeding, drought, harvest equipment, frost or autumn leaf senescence aren’t good candidates for fall applications.

Del Voight


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THORNY ISSUES-TO-BE? Perennial control in cover crops and in biotech crops may turn into restricted-use issues.

This article published in the October, 2010 edition of AMERICAN AGRICULTURIST.

All rights reserved. Copyright Farm Progress Cos. 2010.