Getting the most from fall perennial weed management
Fabian Menalled, Cropland Weed Specialist, MSU-Bozeman
Fall provides an excellent opportunity for weed control, particularly for 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.
Putting into action a few key concepts will enhance your ability to control perennial weeds this coming fall.
If plants are stressed from drought or cold temperatures, applications will not provide satisfactory control due to poor movement of the herbicide through the weed. Thus, 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 still remains. When estimating the extent of the frost damage, make sure to exclude green brittle leaves.
Be aware that perennial weeds vary in their sensitivity to frost and that 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. Thus, fall herbicide applications should not be delayed when controlling hemp dogbane or common milkweed. Finally, best control of field bindweed is obtained sometime in July at the full flower stage. 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.
The following is a summary of control guidelines for several perennial weeds that are problematic in Montana.
Canada thistle is one of the most difficult to control perennial weeds as its extensive root system stores large quantities of food reserves. Effective control can only be achieved after these food reserves have been depleted. Glyphosate (Roundup) provides good to very good control when applied at the bud to bloom stage. Banvel and 2,4-D ester can also be applied at the bud stage. Do not till fields infested with Canada thistle as root fragments can produce new shoots.
For perennial sowthistle, fall applications of 2,4-D ester or Banvel provide good control when applied at the rosette or bud stage. This thistle is less sensitive to glyphosate than Canada thistle and applications of this product should be made at rosette stage to enhance effectiveness.
For field and hedge bindweed, glyphosate and Banvel provide acceptable fall control, but herbicides should be applied only if bindweed is not under drought stress. Due to the lack of excellent control, periodic fall applications should be considered to secure long-term control. Tillage prior to herbicide application is not recommended, because it can stimulate vegetative reproduction due to the increase in the number of buds produced at the nodes of crowns, roots, rhizomes and stems.
For prickly lettuce, fall applied glyphosate can be used for control. Unfortunately, prickly lettuce geminates over an extended period in the fall and spring, particularly when temperatures remain warm. Thus, pre-plant spring control may be more effective than fall applications.
With common milkweed, glyphosate and Banvel provide fair to good control when applied in the fall at the late bud to flower stage. If possible, pre-harvest applications should be used, because once the combine header cuts down milkweed it will not generate enough foliage for good quality post harvest applications.
For dandelion, glyphosate, Banvel and 2,4-D ester are among the several products than can be applied for control in the fall. Dandelions tolerate moderate frost and remain actively growing in late fall (mid September). However, opportunities for post-harvest control are sometime limited by the inadequate weed re-growth.
Fall provides a unique opportunity to clean up some troublesome perennial weed problems. To secure success, scout fields for perennial weeds that are still green and growing, make note of their growth stage, select the best product that fits your crop rotation sequence and good luck!
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.
Categories: Herbicides, Dandelions, Canada Thistle
Date: 08/17/2006