How Late is Too Late to Control Broadleaf Weeds?

Many are asking is how late in the year can I apply a broadleaf herbicide to control dandelions, white clover, ground ivy and other broadleaf weeds. Generally, the best answer is that broadleaf herbicides can be applied in Indiana until mid-November and still provide effective control. This timing is +/- 2 weeks depending upon your location in the state. Broadleaf week control in Northern Indiana is best by November 1, whereas broadleaf weeks can effectively be controlled in southern Indiana prior to December 1. This window can be extended if temperatures are above average and days are sunny.
 
Many herbicides are effective in late fall because plants are more likely to translocate (move downward) herbicides into root and stem tissues as the day lengths shorten and the temperatures cool. Typically, this will occur near or following our first frosts which has recently occurred. Previous research shows that 2,4-D and/or dicamba were far more effective in controlling dandelions and Canada thistle in Nebraska when applied 1 to 10 days after the first fall frost than when applied 5 to 11 days before the frost (Wilson and Michiels, 2003). Other research in Michigan found that "good dandelion control can result from herbicides applied through late October, even when the plants are not actively growing." (Hanson and Branham, 1988). More recently, research at Purdue (Reicher and Weisenberger, 2007) found the following in regards to ground ivy control and application timing (also see Figure below):

• November applications of most herbicides resulted in ground ivy control similar to earlier (September and October) applications when rated the following June.
  
• Triclopyr was the most effective and consistent in controlling ground ivy.
  
• Long-term control of ground ivy from triclopyr was not affected significantly by application date.
  
• When using three-way herbicides (2,4-D + MCPP + diacmba) or products containing fluroxypyr, applications on 1 September through 1 November were most effective.
  
• Adding carfentrazone (FMC’s Quicksilver, PBI Gordons’ Powerzone and Speedzone) to 2,4-D+MCPP+dicamba dramatically improved short-term control of ground ivy from November applications, but had little long-term benefit on any application date. Products containing carfentrazone will aid in quick “burndown” and immediate customer satisfaction when making fall applications but they may not increase long-term control, especially with ground ivy.
  

The bottom line is that if the herbicide is rated to provide excellent control on the weed that you are trying to control, it may not bee too late to control broadleaf weeds this fall. 

Other notes:

• In all cases read the herbicide label before making an application. 
• Winter annuals. Many winter annuals like henbit, common chickweed, purple deadnettle and others have already germinated and are small and easily controlled in the fall. Applications in October and November will control these small winter annual broadleaves as well as the perennial broadleaves.
• Newly seeded areas. Read the herbicide label carefully if the area you want to treat has recently been seeded. Most herbicides require that newly seeded turf be germinated and mown 1-2 times prior to a herbicide application. 

For more information on weed control, search this blog and archived turf tip postings and check out our Turfgrass Weed Control for Professionals Publication.

Sources:

1. Hanson, K. V., and Branham, B. E. 1988. Broadleaf weed control research update: II. Late fall weed control. Proc. of the 58th Annu. Mich. Turfgrass Conf. 17:44-51.

2. Reicher, Z. J., and Weisenberger, D. V. 2007. Herbicide selection and application timing in the fall affects control of ground ivy. Online. Applied Turfgrass Science doi:10.1094/ATS-2007-0831-01-RS.

3. Wilson, R. G., and Michiels, A. 2003. Fall herbicide treatmets affect carbohydrate content in roots of Canada thistle (Cirsium arvense) and dandelion (Taraxacum officinale). Weed Sci. 51:299-304.

Aaron Patton, Turfgrass Extension Specialist