Montana State University

Get ready for fall weed control

August 19, 2010 -- MSU News Service

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As fall temperatures begin cooling, growers across the region have an opportunity to control winter annual and perennial weeds. Understanding how weeds grow, and learning key control concepts will enhance producers' ability to reduce the spread and impact of weeds this fall. This article provides tips to manage winter annual weeds in small grain farming systems and reviews several concepts for managing perennial broadleaf weeds.

As a general rule, winter annual species germinate from late summer to early winter, overwinter as seedlings, grow rapidly as temperatures warm in the spring, and then flower, set seed and mature by early summer. These seeds lie dormant in the soil during the rest of the summer months.

"With more and more no-tillage acres planted with winter wheat, farmers across the region are seeing an increase in the abundance of winter annual weeds such as cheatgrass, jointed goatgrass, field pennycress, wild mustard, shepherd's purse, catchweed bedstraw, common chickweed and prickly lettuce," said Fabian Menalled, Montana State University Extension Cropland Weed Specialist

As a general rule, the best time to manage winter annual weeds is in the fall, and earlier is best. Timing of herbicide applications in the fall is critical for maximum effectiveness.

"Our research and experience has shown that winter annual weeds are easier to control in early fall when air temperatures are mild and weeds are still actively growing. Generally, treatments need to be made before the first killing frost," said Menalled.

Herbicides such as Stinger, WideMatch, Maverick, Olympus, and Osprey, can be used to control winter annuals in the fall. However, rotational crop restrictions vary between herbicides and producers should carefully read the specific restriction of the product they plan to use. This is particularly important in diversified rotations that include pulse crops such as lentils, peas, or chickpeas, as several small grain herbicides have restrictions of up to 36 months.

Fall also provides an excellent opportunity to control several problematic perennial broadleaf species. Cooler temperatures trigger the movement of food reserves down to the root systems, enhancing the movement of herbicides to the plant's root system and improving control. However, perennial weeds vary in their sensitivity to frost and the application window differs between species. For example, Canada thistle can survive light frosts and is effectively controlled with relatively late fall herbicide applications. Other perennial weeds such as hemp dogbane and common milkweed complete their life cycles by late summer and do not tolerate frost well, so fall herbicide applications should not be delayed when controlling these species. Finally, although fall application will not guarantee excellent control of field bindweed, late control practices can be quite effective provided there is re-growth of this weed.

"Regardless if you are targeting a winter annual or a perennial weed, it is important to know that if plants are stressed from drought or cold temperatures, applications will not provide satisfactory control due to poor movement of herbicide through the weed," said Menalled.

To secure active translocation, fall herbicides should be applied when temperature are expected to exceed 60-65 Fahrenheit during the day. Fall applications should be made only if plants still have green and pliable leaf tissue. As a rule of thumb, do not expect satisfactory control if less than 60 percent of the original leaf tissue remains.

"When designing your weed management plan, remember that the adoption of no-tillage systems has increased our reliance on herbicides," said Menalled. "While this approach to farming has benefits in terms of reducing soil erosion and energy use, it increases the potential for selection of herbicide resistant weeds."

The use of herbicides with different modes of action of action, applied as tank mixes, premix formulations, or sequential applications, can help in managing resistance.

According to Menalled, fall provides a great opportunity to clean up troublesome winter annual and perennial weeds. To secure success, scout your fields, identify plants that are still green and growing, make note of their growth stage, and select the best product that fits your crop rotation sequence.

Disclosure. Common chemical and trade names are used in this article 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.

Fabian Menalled (406) 994-4783 or menalled@montana.edu