Montana State University

Control perennial weeds this fall

August 27, 2009 -- MSU News Service

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MSU News Service
Tel: (406) 994-4571
BOZEMAN -- Field bindweed, Canada thistle, and dandelion are among the most difficult perennial weeds to control. Understanding how they grow and reproduce can help manage their spread.

"Although we often joke that some weeds are perennials because they emerge every year, the term perennial refers to plant species that live for longer than two years, and may reproduce several times before dying," said Fabian Menalled, Montana State University Extension Cropland Weed Specialist.

As fall approaches, perennial weeds start pumping food reserves to the roots to increase root mass, the number of reproductive buds, and the ability to over-winter. Fortunately, fall applied herbicides used to control perennial weeds follow the flow of food reserves from the leaves down to the roots. Although managing perennial weeds requires attention throughout the growing season, it is important to consider management in fall to maximize the amount of herbicide movement from leaves to the root system and vegetative buds.

"While simple herbaceous perennials such as dandelion reproduce from seed, other perennial species such as Canada thistle or field bindweed can reproduce asexually using their own root systems. If the root system of these creeping perennials is injured or cut, each piece can regenerate," said Menalled.

Perennial weeds store nutrients in underground structure such as taproots, tubers, or rhizomes. These reserves make perennial weeds particularly difficult to kill. Therefore, the key to managing perennial weeds is to know how to kill the root system.

Herbicides that move to the root system on perennial weeds include 2,4-D, dicamba (Banvel, Clarity and other trade names), picloram (Tordon), clopyralid (Curtail), and glyphosate (Roundup, Glyphomax, and other trade names). Not all these products are labeled for use in lawns, and herbicide selection should depend on the cropping system, specific weed targeted, and rotational crops.

Fall herbicide applications to perennial weeds may free up time in the spring, and they are usually more effective than spring treatment. To be most effective, fall herbicide treatments require application after significant rainfall and in adequate vegetation. Drought conditions occurring during summer may decrease herbicide activity and eliminate the potential benefit of fall herbicide applications on perennial weeds.

Fall herbicide applications should be combined with cultural practices such as timely tillage or establishment of competitive crops. Developing an integrated approach that stresses perennial weeds throughout the growing season and minimizes their reproduction over time is the best strategy in dealing with these problematic species.

Disclosure. Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval.

Contact: Fabian Menalled, (406) 994-4783 or