A Mulch That Gives Nearly A Year's Worth of Weed Control

October 10, 2002

COLUMBUS, Ohio - Weed control is one of 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 not all products used to inhibit weed growth (like standard mulches) are completely effective and multiple chemical applications tend to raise environmental red flags.

But Ohio State University researchers in the Department of Horticulture and Crop Science have found that herbicide-treated mulches may be the ticket to an improved, inexpensive and safe way to provide weed control. In their latest studies, Hannah Mathers and graduate student Luke Case have discovered some products controlled weeds for more than 300 days with a single application, over 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. Imagine how long the application would last in the landscape," said Hannah Mathers, an Ohio State University nursery and landscape specialist. "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."

Case and Mathers analyzed Douglas fir and pine nugget mulches treated with a single application of three herbicides (oryzalin, glumioxazin and acetachlor) on 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 year-long study was an improvement upon previous studies that showed some herbicide-treated mulch provided 130 days of weed control.

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

Mathers first found in 1998 studies 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 or isoxaben.

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

"You would not only have weed control through the growing season, but also during winter. Then come the following spring, you could pull those container plants out and they would be clean and saleable," said Mathers. "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 just another added benefit to what such a product already brings to the ornamental plant industry compared to what mulches or herbicide applications provide by themselves.

Mathers said that the number one reason why mulches are used in the landscape is to provide weed control. However, in most cases, mulch products give no better than 50 percent efficacy, well below commercially acceptable standards.

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

Additionally, multiple herbicide applications to provide weed control in container plants during the year are costly and raise environmental concerns due to leaching through the pots.

"Most herbicides only last 35 to 40 days per application. 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," said Mathers. "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 preemergents are also 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," said Mathers.

Nursery growers estimate that they spend $500 to $4,000 per acre ($1,235 to $9,880 per hectare) of containers for manual removal of weeds, depending upon weed species being removed. This is the manual removal cost when the $1,260 spent on herbicides fails to provide the control needed. 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 back on costs, since only one application would be required.
Additionally, 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 have begun additional studies to determine exactly how the herbicide-treated mulches work in providing weed control. They are also studying the storage life of bagged herbicide-treated mulches to determine if and when they begin to deteriorate.

Author(s): 
Candace Pollock
Source(s): 
Hannah Mathers