Manage noxious weeds this fall

By Ted Schaaf
Gardens on Spring Creek

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Don't give up on weed control for the season. Perennial weeds and cheatgrass can be effectively controlled in the fall.

While biennial weed species such as musk thistle, hounds-tongue, common mullein, diffuse and spotted knapweed are best managed in the spring or early summer prior to seed production, fall is a great time to get after Canada thistle, leafy spurge, Dalmatian toadflax, Russian knapweed and cheatgrass.

Perennial weeds can be controlled spring, summer or fall with an herbicide application, but late summer or early fall is an opportune time to attain control.

Perennial species transport carbohydrates from aboveground plant tissue to storage in extensive underground root systems prior to winter dormancy. Application of an herbicide during this translocation process places the herbicide deeper within the root system, allowing for more effective control.

Best timing is two weeks prior to the first hard frost, but don't wait too long. As autumn weather advances, aboveground plant tissue will die back following a hard frost of 25 degrees Fahrenheit or colder. At this point it is no longer worthwhile to apply an herbicide.

Cheatgrass (downy brome, Japanese brome) is a troublesome species prevalent in the Rocky Mountain region from montane elevations to the prairie. Cheatgrass follows a winter annual life cycle, meaning the plants germinate in August or September following late-summer rains and then over-winter as short flattened clumps. They green up early in the spring and set seed in May or June. By July the prickly seed heads become a nuisance to hikers, pets and livestock. The dry grass is also a fine fuel and fodder for the spread of wildfires.

The best time to control cheatgrass with an herbicide in the fall is at the early emergence stage, soon after the initial flush. Emergence will not occur until significant moisture is received, usually one-half inch or more. Glyphosate (Roundup and others) can selectively control cheatgrass mid-winter through early spring when applied at low rates that don't injure desirable perennial grasses that are dormant.

For assistance in identifying weed species and for management recommendations, call the Larimer County Weed District office at 498-5768 or visit the web site at www.larimer.org/weeds.