A late-summer 2015 issue of Farm Journal included a list of what its farmer readers considered their “worst weeds,” which are probably the ones they’re having the most trouble controlling. For if a weed problem is easily controlled by the farmer’s weed control program – either chemical, mechanical or by crop rotation – then it isn’t one of his worst weed problems.

Looking at the list is a good reminder of why regional farm publications like Farming will always have a place: None of the top four on the list – waterhemp, marestail, ragweed and palmer amaranth, comprising over 80 percent of the responses – are weeds that farmers in the Northeastern United States mention to me as being among their worst problems.

The 2015 survey included over 1,000 farms, and there was little change in the ranking compared to a previous survey done by the same publication in 2013. “Grasses” were cited as a worst weed by only 8 percent of the respondents, but in this region, annual grasses including giant, green and yellow foxtail, also smooth and tall crabgrass, are among the most troublesome of weeds.

Botanical shifts

Google the term “botanical shift” and you’ll get page after page of websites advertising women’s dresses. Very pretty, but that’s not the kind of botanical shift I’m referring to. In the field, a botanical shift occurs when a particular plant population either disappears and is replaced by another, or is forced out by a more competitive species.

Since the advent of glyphosate we’ve seen this occur in corn fields, not only regionally but nationally and internationally as well. (Glyphosate was originally sold as Roundup, but now there are many brands on the market that are similar, but not necessarily equal.) When I started advising N.Y. and New England farmers back in the mid-1960s, quackgrass, a tough, spreading perennial, was Public Enemy No. 1 in corn fields. Atrazine would kill quackgrass but only when used in a split application, something that required prior planning and two separate applications of the herbicide. A single application of atrazine would beat quackgrass back until the corn crop got a decent start but at the end of the crop year the “pestiferous perennial” would still be there. Roundup killed quackgrass in one application, and while atrazine (along with 2,4-D) was considered a “breakthrough” herbicide in that it resulted in permanent changes in cropping practices, glyphosate made such a difference that it resulted in a botanical shift.

Annual grasses don’t compete well with quackgrass since it’s a perennial while annual grasses are annual – germinating from seed each season. But remove quackgrass from corn fields and annual grasses can take over unless an herbicide is used that will control them. During the late 1960s I didn’t get nearly as many questions about how to control foxtail as I did during the 1980s after the use of glyphosate for quackgrass control had become common.

Herbicide resistance

Another type of botanical shift occurs when a species of weed develops a resistant biotype, usually due to the repeated use of the same herbicide, year after year. When a farmer finds an herbicide that does an excellent job at a reasonable cost he often sticks with it. That’s what happened with the use of glyphosate, and now there are so many glyphosate-resistant weed species that it’s become a global problem.

But before glyphosate resistance became widespread we already had weeds that had developed resistance to atrazine, particularly common lambsquarters. Lambsquarters is an easy weed to kill, either by use of a contact herbicide such as 2,4-D or a soil-applied one such as atrazine. In fact, atrazine provided such good control of lambsquarters and other annual broadleaf weeds that farmers used it every year on their continuous corn. (Stick with a winner, don’t argue with success, etc.) But after a triazine-resistant biotype of lambsquarters appeared, farmers soon discovered that unless they switched to a different herbicide program they experienced a “botanical shift” – from corn and few broadleaf weeds to corn struggling to compete with a solid mat of triazine-resistant lambsquarters plants.

“Worst weed” winners

“Worst weed” No. 2 on the Farm Journal list is marestail, which is an annual weed also known as horseweed. It’s received a lot of publicity in part because there’s a glyphosate-resistant biotype. According to USDA, marestail is more common in Canada, New York and New England than it is in Pennsylvania and the Southern United States.

Where the glyphosate-resistant biotype has developed, marestail is a very serious problem and has been for at least 20 years. Marestail is the same plant as horseweed, but it’s not horsetail, and, not surprisingly, this confuses many farmers. While marestail is an annual, field horsetail is a deep-rooted perennial spreading by rhizomes, and, once established, it’s virtually impossible to eradicate. It’s a primitive species (also called equisetum), and has been around since the days of the dinosaurs: Many of horsetail’s ancestors grew as primitive trees and probably contributed significantly to the formation of coal.

Horsetail is found in every state in the continental United States, also in Canada and Europe. It looks somewhat like a small pine seedling and has so little leaf tissue that there’s little uptake of post-emergence-applied herbicides. For this reason there are few, if any, herbicides that will adequately control horsetail; in my experience the best a farmer can hope for is to suppress it via pre-plant tillage followed by in-row cultivation during a corn or soybean crop, then establishing a hay crop.

Over 30 years ago when I started complaining about horsetail problems in New York corn fields, a weed scientist said to “tell the farmer to drain his fields” since horsetail was known to like wet soils. But when I saw it growing up the side of a pile of sand I realized that it had a much wider range of adaptation.

Some years ago while doing some farm consulting in Japan I saw horsetail emerging through the asphalt in a recently-installed sidewalk. Any farmer with a significant population of horsetail in his fields would be likely to name that as one of his “worst weeds!”

Cover photo: RuudMorijn/istock