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Title: Thistrol Herbicide Registered in Mint

item Boydston, Rick

Submitted to: Washington Mint Drops
Publication Type: Popular Publication
Publication Acceptance Date: 4/22/2009
Publication Date: 5/15/2009
Citation: Boydston, R.A. 2009. Thistrol Herbicide Registered in Mint. Washington Mint Drops. Washington Mint Growers Association. Pg 2. Spring 2009.

Interpretive Summary:

Technical Abstract: MCPB (Thistrol) is labeled in peppermint and spearmint after many of years of anticipation and frustration. The new label allows for early postemergence application of MCPB at 1 to 2 pts/acre. Lower rates of 10-12 fl oz/acre is adequate for field bindweed suppression in the spring and results in less mint injury than higher rates. MCPB was the least injurious of six growth regulator type herbicides tested in 2005 and 2006 studies in peppermint and spearmint, including clopyralid and fluroxypyr. MCBP should provide slightly better suppression of field bindweed with less mint injury than previously labeled 2,4DB. For field bindweed suppression, MCPB should be applied when bindweed runners exceed 6 to 8 inches. Usually, this coincides with mint that is less than 6 or 8 inches tall, which occurs in early to mid May in the Columbia Basin. Applying MCPB to smaller, younger field bindweed results in less translocation of the herbicide to underground rhizomes and also increases the possibility of incomplete emergence of bindweed. When applications are made to mint over 6 inches tall, injury increases and the possibility of reducing oil yield increases. Fall applications of MCPB for field bindweed suppression can be utilized, but can result in delayed emergence of field bindweed the following spring. Once the later emerging bindweed is large enough to treat in the spring, mint will be larger and more susceptible to injury by MCPB. MCPB has not provided consistent control of prickly lettuce, common groundsel, Western salsify, or flixweed at 1 pt/acre. MCPB at 12 fl oz/acre in the spring tank mixed with bromoxynil improved suppression of tall hedge mustard compared to bromoxynil alone. MCPB at 1 pt/acre in the fall did not control tall hedge mustard that was 4 to 8 inch rosettes. MCPB applied at 2 pts controls pigweed, lambsquarters, smartweed, and sow thistle under 3 inches tall. MCPB at 1 to 2 pts/acre will suppress Canada thistle.