Ornamental Plants
Annual Reports and Research Reviews
2001

Special Circular 186-02

Understanding Failures in Ornamental Weed Control: Forget the Excuses!

Mode-of-Action

Herbicides have a specific target site, a place in the plant that herbicides bind to and inhibit function (Hall etal., 1999). Most target sites are enzymes; however, there are exceptions which interact with photosynthesis or are auxinic herbicides.

In North America, herbicides are divided into groups based on target site. Herbicides that affect the same target site frequently have the same symptoms, application method, constraints, and even toxicological profile (Hall etal., 1999). The commonly used ornamental herbicides can be divided into 10 different groups as indicated in Table 1, with different sites of action:

• Group 1, ACCase inhibitors such as the Cyclohexanediones
• Group 2, ALS inhibitors such as the Imidazolines and Sulfonylureas
• Group 3, Microtubule assembly inhibitors such as the dinitroanalines
• Group 4, Synthetic auxins such as the phenoxy acetic acids
• Group 5, Photosystem II inhibitors such as the triazines
• Group 6, Photosystem II inhibitors with different binding behavior such as the benzothiadiazoles
• Group 7, Photosystem II inhibitors again with different binding behavior such as the ureas
• Group 9, EPSP synthase inhibitors such as glyphosate
• Group 22, Photosystem I electron diverters such as bipyridiliums
• Group 15, Unknown site of action herbicides such as the chloroacetamides.