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College of Food, Agricultural, and Environmental Sciences


Ornamental Weed Control Involves Application Strategy

April 12, 2005

COLUMBUS, Ohio — Effective weed control in the garden or landscape involves more than simply laying down mulch, applying herbicides, or using a combination of both methods. An Ohio State University Department of Horticulture and Crop Science field mulch study found that the types of mulches or herbicides used, along with an application strategy are just as important.

First-year results of the two-year study revealed that herbicide applied between mulch and bare soil had benefits in reducing weed growth and was effective up to 120 days, compared to herbicide applied on top of mulch or with just mulch or herbicides applied alone. Additionally, some pretreated mulches (mulches sprayed with herbicides, dried, bagged and applied to the site afterward) offered even better control than those applied over or under mulches.

Further data determined that mulches without any herbicide treatment produced no adequate weed control, while herbicides alone provided some weed control but most lost effectiveness within 30 days. However, the herbicide mulch combinations provided better efficacy in the first 30 days and were still providing control at 120 days after treatment.

"What people want to know is what method of treatment in the landscape is most effective against weeds," said Hannah Mathers, an Ohio State University Extension nursery and landscape specialist. "We have been testing herbicide-treated mulches now for five years in containers. This study is unique in that this is the first time herbicide trials have been taken to the field."

Researchers tried 38 treatments. Of those, 20 had a rating of seven or above, which is considered commercially acceptable.

"Based on five years of mulch/herbicide trials, we picked the products that consistently excelled," said Mathers, who also holds a research appointment with Ohio State's Ohio Agricultural Research and Development Center. Pine nuggets and hardwood bark were the tested mulches, and the liquid chemicals used in the study included oryzalin (Surflan), flumioxazin (SureGuard), acetochlor (Harness), dichlobenil (Casoron), and a combination of oryzalin and flumioxazin.

Results indicated that in the 20 treatments that were commercially acceptable, pine nuggets provided the best performance with the herbicides used than hardwood bark.

"The hardwood bark didn't do well because it just disintegrates too quickly," said Mathers. "And the pine nuggets performed better with the herbicide under the mulch than on top. We expected the herbicide to bind better when applied over the mulch, but perhaps the herbicide is binding better to the soil than to the mulch, affording less leaching and greater efficacy."

Mathers said the study indicates the importance of not just picking the right herbicide but picking the right mulch that goes with the herbicide being used. For example, the herbicides used in the study worked effectively with pine nuggets, but nearly none of the treatments with hardwood bark provided a commercially acceptable rating.

"Now landscapers want to know how much mulch should be applied to a herbicide treatment, and whether or not granular herbicides are just as or more effective than sprays," said Mathers, "both of which we will look into in future studies."

The study is being funded by U.S. Department of Agriculture's Agricultural Research Services, Ohio State Department of Horticulture and Crop Science and OSU Extension.

Candace Pollock
Hannah Mathers