Fall can be a good time for growers and custom applicators to handle certain weed problems in pastures and other non-crop areas, thereby avoiding the spring field work rush. We often receive questions on how late fall treatments can be made while maintaining their effectiveness. The questions often focus on what impact frost has on the susceptibility of weeds to herbicides.
Most perennial and biennial weeds found in Iowa are relatively cold-tolerant and can be controlled with applications made following a few light frosts. I conducted studies in 1992 and 1993 in central Iowa to determine the effectiveness of fall 2,4-D applications for musk thistle control. The experiment was conducted in a bluegrass pasture with a heavy musk thistle infestation. Musk thistle rosettes were 4 to 12" in diameter at the time of application. The pasture was being grazed so the sward height was approximately 4" and allowed good coverage of the rosettes.
Table 1. Musk thistle control with fall 2,4-D applications.
|Date of application||Temperature at application (oF)||Days below 32oF1||1 qt LVE (4lb/gal)||2 qt LVE (4lb/gal)|
-- % musk thistle control --
Source: B. Hartzler, ISU.
1Number of days in fall with temperatures below 32oF prior to application.
2Musk thistle control was evaluated in early May.
Effective control of musk thistle was achieved with applications made after several nights when temperatures fell below 32 degrees (Table 1). The growth habit of musk thistle provides protection from freezing temperatures since the leaves are close to the soil surface. Increasing the 2,4-D rate to 2 qts/A did not significantly improve musk thistle control in these studies.
Musk thistle normally acts as a biennial in which plants require two years to complete their life cycle. In the first year the plant produces a basal rosette, in the second year the stem elongates, produces the seedhead, and then the plant dies. Under certain environmental conditions musk thistle can act as an annual, and will flower during the year in which it germinates. Due to this trait, flowering musk thistle plants may be evident in fields treated in the previous fall. However, the convenience of fall applications may be worth this potential for reduced control since only a small percentage of plants display this trait.
We evaluated only straight 2,4-D in these studies; however, we would expect similar results from other products registered for use in pastures. In most situations it would be advantageous to use a combination treatment such as 2,4-D + Banvel, Crossbow, etc. to provide more consistent results or a broader spectrum of control.
In summary, many perennial and biennial weeds can still be effectively killed after a few hard frosts. Research with quackgrass and Roundup actually found greater translocation of the herbicide after the first frost than before frost. Plants having a prostrate growth habit such as musk thistle will be more tolerant of frost since they are protected somewhat by heat released by the soil. With most plants it is possible to determine whether the foliage has been severely affected by frosts, thus scouting the field prior to application is important to ensure that active foliage is still present.
Prepared by Bob Hartzler, extension weed management specialist