Herbicides are defined as a chemical substance that is used to eliminate unwanted plants. This is a very general description and it is important to remember that herbicides differ with respect to when they are used (for example, pre-emergence or post-emergence), their activity (for example, contact or systemic), their selectivity (for example, grasses or broadleaves), and their mechanism of action (also known as: mode of action, site of action). (See this blog post about basic herbicide terminology:http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=5973)

What is a mechanism of action? The...