Montana State University

MSU pesticide expert warns that lingering herbicides can contaminate gardens, ornamental plants

May 3, 2012 -- MSU News Service

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MSU News Service
Tel: (406) 994-4571
BOZEMAN - Herbicides can linger in grass clippings, compost and manure, so Montanans should be sure to read product labels to keep from contaminating gardens and ornamental plants, says Montana State University Pesticide Education Specialist Cecil Tharp.

Many pesticides that target broadleaf weeds can damage other broadleaf plants, such as peas or tomatoes, Tharp said.

MSU's Schutter Diagnostic Laboratory reported 103 plant samples that exhibited symptoms of pesticide toxicity between 2009 and 2011, Tharp said. The plants came from Montana gardens and had symptoms that made it appear they had been exposed to a class of herbicides known as "plant growth regulators. " Almost 80 percent of the contaminated samples were thought to be linked to compost, manure and grass clippings that had been introduced into the soil.

Plant growth regulator herbicides include the common active ingredients 2,4-D, dicamba, picloram, aminopyralid, clopyralid and the new active ingredient aminocyclopyrachlor. Of greatest concern are picloram, clopyralid, aminopyralid and aminocyclopyrachlor because they can remain active in hay, grass clippings, manure piles and compost for an unusually long time, Tharp said.

To avoid contamination, producers using those active ingredients should pay special attention to the pesticide product label requirements, Tharp said. Instructions vary slightly between products, but they often contain re-cropping , haying, composting and manure restrictions

One restriction that may be found on an aminopyralid product indicates that producers shouldn't use manure in compost or mulch if the manure comes from animals that have grazed forage or eaten hay harvested from treated areas within the previous three days. It takes about three days for the forage or hay to run through the animal's system, Tharp said.

Applicators should also be aware of new requirements regarding the products Milestone, ForeFront and Chaparral, Tharp said. Hay from grass treated in the preceding 18 months cannot be distributed or sold off the farm or ranch where harvested unless allowed by supplemental labeling. Montana, however, doesn't have supplemental labeling to allow off-farm distribution. As a result, applicators must wait 18 months to cut and distribute hay off treated sites, thus allowing adequate time for the grass to metabolize the pesticide product.

Hay also cannot be used for silage, haylage, baylage and green chop if treated within the previous 18 months. Producers cannot use manure from animals feeding on treated hay in compost. Applicators may follow the less restrictive pesticide product label language of earlier purchased stocks of Milestone, Forefront and Chaparral products until stocks are exhausted.

For more information, contact your county Extension agent or go to and select the "Non-Target Plant Toxicity Around the Home and Garden" link. For other questions, contact Tharp at (406) 994-5067 or

Evelyn Boswell, (406) 994-5135 or