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Herbicide-treated Mulch Effective A Lo-o-ong Time

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Written Friday, October 11, 2002  

Weed control is among the biggest challenges the nursery and landscape industries face in managing plants and providing top-grade products to consumers.

Not only is weed control costly, but also not every product used to inhibit weed growth, like standard mulches, is completely effective. Plus, multiple chemical applications tend to raise environmental red flags.

However, Ohio State University researchers in the Department of Horticulture and Crop Science have found herbicide-treated mulches may be the ticket to an improved, inexpensive and safe way to control weeds.

In recent studies, Hannah Mathers and graduate student Luke Case discovered some products controlled weeds for more than 300 days with a single application. That's more than 170 days longer than what was found in previous trials.

"To get nearly a year of weed control with one application in a nursery container is just mind boggling," said Mathers, an OSU nursery and landscape specialist. "Imagine how long the application would last in the landscape. The clients of landscape professionals have very low tolerances for weeds, and the landscaper needs to have access to a one-time application product. However, that product is not currently commercially available. So providing that product would be a big deal to the industry."

Mathers and Case analyzed Douglas fir and pine nugget mulches treated with a single application of three herbicides -- oryzalin, glumioxazin and acetachlor -- for their effectiveness against common chickweed, annual bluegrass and prostrate spurge. They found that the Douglas fir mulch treated with oryzalin and acetochlor provided efficacy for 303 days with at least 70 percent weed control -- a commercially accepted level. The yearlong study was an improvement over previous studies that showed some herbicide-treated mulch provided 130 days of weed control.

"In studies conducted in 1998 and 2000 to 2002 we have found that different mulches worked differently with different herbicides," Mathers said. "So we are trying to determine which herbicide-treated mulches work the best -- which ones last the longest and provide the best weed control."

In 1998, Mathers found that herbicide-treated Douglas fir bark nuggets were extremely effective for weed control, regardless of whether oxyfluorfen (Goal), oryzalin (Surflan) or isoxaben (Gallery) were applied to the bark. However, bark treated with oryzalin had significantly greater efficacy than bark treated with other herbicides, such as oxyfluorfen and isoxaben.

Extended weed control would not only be beneficial in the landscape industry, but also provide a marketing advantage for nursery container producers, Mathers said.

"You would not only have weed control through the growing season, but also during winter," she said. "Then come the following spring, you could pull those container plants out and they would be clean and saleable.

"Spring is a busy time in nursery production. In some nurseries, 50 percent or more of the yearly sales occur during spring. Weed control can take a low priority under this circumstance."

Herbicide-treated mulch that provides nearly a year of weed control is an added benefit to what the product already brings to the ornamental plant industry.

The No. 1 reason mulches are used in landscaping is for weed control, but in most cases mulch products are effective no more than 50 percent of the time, Mathers said.

"As a result, too much mulch is applied and done so incorrectly, creating more problems than providing benefits," she said.

Additionally, multiple herbicide applications made to provide weed control in container plants during the year are costly and raise environmental concerns. The herbicides can leach through the pots.

"Most herbicides only last 35 to 40 days per application," Mathers said. "Many container stock nursery growers prefer pre-emergent granular materials that are applied with cyclone spreaders or belly grinders. Granulars work well when treating rectangular areas, but become more problematic with irregular-shaped areas that are difficult to reach from both sides.

"In container production, three to five applications of pre-emergent herbicides may be required to keep the chemical barrier on the container surface, due to the large amounts of water that are applied to containers each season."

Granular pre-emergents also are expensive, costing the average grower $315 per application per acre.

"If you consider four applications at $315 each, $1,260 is spent on chemicals per acre, and then there's no guarantee of weed control," Mathers said.

Nursery growers estimate they spend $500-$4,000 per acre -- or $1,235-$9,880 per hectare -- for manual removal of weeds, depending on the weed species. Economic losses due to weed infestations have been estimated at approximately $7,000 per acre -- $17,290 per hectare.

Mathers said herbicide-treated mulches with extended weed control would help cut costs, since only one application would be required. In addition, herbicide-treated mulches help reduce the leaching potential into the environment and, as a result, are more effective than mulches alone because more herbicide is available to help provide weed control over a longer period of time.

Mathers, Case and research associate Jenny Pope are studying how herbicide-treated mulches provide weed control. They also are studying the storage life of bagged herbicide-treated mulches to determine if and when they begin to deteriorate.

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