VALLEY — Fall is an excellent time to manage biennial and perennial weeds.  Control of these weeds works best when they are storing nutrients for the winter. 

In particular, biennials are much easier to kill while they are in the rosette stage of growth and prior to surviving a winter. Once they start growth in the spring, they rapidly develop with the goal of reproducing and it becomes more difficult to control them.

As you have heard many times before, late summer and fall is the best time to control most perennials with a systemic herbicide because herbicides are moved into the root systems allowing better control. In general, the application window runs from mid-August through September, or into October depending on what weeds you are targeting.

Applications to perennial species like groundcherry, and woody species like multiflora rose should be on the early side of this window, while cool-season perennials like Canada thistle, quackgrass, and dandelion can be effectively controlled after several light frosts. With both biennial and perennials species, adequate leaf tissue must be present and it should be reasonably healthy to absorb the herbicide.

Weeds such as Canada Thistle, Russian Knapweed and Leafy Spurge as well as others can be controlled by fall herbicide applications.

The most common herbicides used for broad-spectrum control of many weeds in the fall are glyphosate for grasses and broadleaves and 2,4-D or dicamba (Banvel, Clarity, etc.) for broadleaves. A combination of these products may be the best solution for a mixture of different perennial weeds. For most make applications from August through September and possibly into October or before a hard frost. In general, applications by September 15 may be more effective. An additional two-week application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.

Important considerations:

• Make sure that the foliage on the weeds appears relatively healthy and capable of absorbing the herbicide spray. Plants that have been damaged by insect feeding, drought, harvest equipment, frost, or autumn leaf senescence are not good candidates for fall applications. So, if that pokeweed you have been dealing with during the summer season is still standing and the leaves and stems are not too tattered after harvest, then there is still a great chance to control it yet this fall. Make sure to use adequate herbicide rates, high spray volumes, and get good spray coverage over the plant for effective kill.

• Favorable air temperatures should be a consideration immediately before, during, and after application. In general, the warmer the better, with daytime high temperatures in the mid-50s at a minimum. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications. The more active the weeds are growing, the better the herbicide performance.

For information on what fall weeds controls are most effective and a list of herbicides that are most effective, contact the Colorado State University Extension Office, San Luis Valley Area at 719-852-7381, or your local County Weed Control Office.

Visit the CSU Extension office for the San Luis Valley Area at 1899 E Hwy 160, Monte Vista, Co. 81144.  Please feel free to visit our website at: http://sanluisvalley.colostate.edu for information about services provided.

Extension programs are available to all without discrimination, Colorado State University Extension, U.S. Department of Agriculture and Colorado counties cooperating.

No endorsement of products mentioned is intended, nor is criticism implied of products not mentioned.