by Sally Colby

Glyphosate is a hot button issue for both consumers and producers, so AmericanHort brought together several experts in the field to discuss the topic. AmericanHort represents the entire horticulture industry and has been following the glyphosate debate in the interest of dispersing the science behind the glyphosate issue.

Dr. Scott Senseman, Department of Plant Science, University of Tennessee, has done extensive studies on pesticide residue. He explained that herbicide activity from glyphosate was first observed in 1971, and it was non-selective and highly effective for crop burn down – especially in no-till systems. Farmers have noticed increased herbicide resistance, especially with aggressive weeds such as palmer amaranth. Senseman said that as of autumn 2018, agronomists listed 42 weed species resistant to glyphosate.

Senseman said a report by IARC (International Agency for Research on Cancer) lists glyphosate as a “probable human carcinogen,” but added that the same list includes processed meat, all alcoholic beverages, sunlight, engine exhaust and outdoor pollution.

Since the IARC report, there have been multiple evaluations and reports on glyphosate. “The European Chemical Agency looked at data and reported that glyphosate was safe,” said Senseman. “The European Food Safety Authority also reported glyphosate as safe. The EPA has designated it as safe, and the joint WHO and UN evaluation also came back and said there is an unlikely cancer risk.”

Dr. Jeff Derr, professor of weed science at Virginia Tech, explained how glyphosate works. “It inhibits amino acid synthesis of aromatic amino acids tryptophan, tyrosine and phenylalanine in plants,” he said “Animals and people do not make these amino acids – we have to get them in our diet. The result is a chemical that controls plants with low acute toxicity to animals and people.” Derr added that some formulations (such as Roundup PROMAX) include a caution on the label due to low acute toxicity.

Derr said glyphosate moves from foliage to the underground portions of roots, rhizomes and tubers to control perennial weeds. It has essentially no soil activity, and a field can be planted a week after application.

For those interested in glyphosate alternatives, Derr said there are no easy replacements. However, he suggested several non-selective post emergence options for growers who want to replace glyphosate.

The product Derr most often recommends is glufosinate, initially introduced by Bayer under the name Finale. “Cheetah Pro is a new farm product, and there are other products that contain glufosinate,” said Derr. “We have to use products labeled for nursery and landscape use – others are labeled for crop use.”

Derr explained that glufosinate is contact material with some systemic action. It works more slowly than pure contact herbicides, but faster than glyphosate. “Within two or three days, we see the effects of glufosinate application,” he said. “For weed control effectiveness, it’s between a contact herbicide and glyphosate – it’s more effective on perennial weeds than a contact material but overall less effective on perennials than glyphosate, especially on perennial grasses.”

Some weeds, including cutleaf evening-primrose, doveweed, dayflower and white clover, are more effectively controlled with glufosinate than glyphosate. Derr cautioned growers to use directed sprays because glufosinate can cause bark splitting and cankering on the bark of young trees.

Derr discussed contact nonselective herbicides (herbicides that injure or damage all plants). “The problem with contact herbicides is they have no direct effect on the underground portions (rhizomes, bulbs, tubers) of perennial weeds,” said Derr. “If we’re trying to control a perennial weed, we’re going to burn off the tops, and it will regrow from what’s underground and we’ll have to repeat applications. For controlling annual weeds, coverage is important. We have to cover all the foliage to get effective control.” Derr added that if weeds are tall, it’s difficult to get thorough coverage of lower leaves because the upper leaves intercept the spray. Multiple applications may be required, especially on taller annual weeds.

Derr’s top contact herbicide recommendation is diquat (Reward). “On a warm sunny day, if you treat in the morning, by afternoon or evening you will see results,” he said. “We need thorough coverage of weed foliage so we need a nonionic surfactant to spread the droplet across the leaf surface.”

Herbicidal soaps are another option for non-selective weed control. Pelargonic acid is the active ingredient in Scythe, and a related fatty acid product is AXXE, which is OMRI-listed. Scythe is sometimes used as an adjuvant to speed up the action of glyphosate, and is labeled for greenhouse use.

Dr. Joe Neal, professor of weed science and Extension specialist at North Carolina State University, discussed some glyphosate work-arounds, noting that mulches are the number one defense against weeds in landscape plantings. “A well-maintained landscape bed will include top-dressing mulch when it becomes too thin to prevent weed emergence,” he said. “But sometimes mulches aren’t enough.”

Neal said numerous products are labeled for pre-emergence weed control in landscape and nursery settings. “The real challenge is to select the most appropriate ones,” he said. “First and foremost, we select for plant safety – safety to the ornamental plants we’re growing or maintaining in the landscape bed. Secondarily, for the most effective products that will control the weeds that are important in these areas.”

In general, annual grasses are easy to control with most pre-emergence herbicides labeled for landscape plantings, but control of broadleaf weeds is more variable. For landscape uses, granular formulations are preferred because they’re better for plant safety. Granular formulations are often chosen because they seem easier to apply, but Neal said it takes a lot of extra effort to achieve uniform application.

Neal said grassy and perennial weeds are generally not as well controlled by contact products. “But there are alternatives,” he said. “For example, Bermuda grass encroaching in a bed can be selectively controlled with a selective grass herbicide.”

Annual bluegrass is not well controlled by most post emergent grass herbicides, but Neal says clethodim (Envoy) is effective as a post-emergence annual bluegrass control treatment. Several products are effective for managing nutsedge, and labeled products can be used as directed applications around woody ornamental plantings.

Several herbicides are effective for controlling perennial broadleaf weeds in landscape plantings. “I use the term ‘selective’ in the context that they control certain broadleaf weeds and not others,” said Neal. “They cannot be applied over ornament plants. These are synthetic auxin herbicides, designed to control broadleaf plants, and the only safety factor for using Lontrel (clopyralid) or triclopyr in a landscape planting is the operator. The applicator must avoid applications on ornamental plants.” Neal said these products control legume and aster weeds (vetch, clover, thistle) but because they are highly active in these plant families, operators must take extra care to avoid applications around desirable legumes (redbuds) or desirable asters such as anything in the daisy family.

“There are alternatives to glyphosate, but it requires more planning,” said Neal. “Most of these herbicides are more expensive, and remember these products are not necessarily ‘safer’ than glyphosate.”