By Marco Peña, UA research specialist

On April 26, 2010, the “How Herbicides Work" field day was held at the UA YAC. Barry Tickes and students from the applied weed science class PLS 300 explained the different herbicide modes of action directly from the experimental plots.

We are frequently so busy spraying gardens and crops for weed control that it is easy to forget how these products really work. By attending this meeting, participants learned that seedling root and shoot growth inhibitors kill weeds by affecting mitosis, the process in cell division by which the nucleus divides - causing what is called "pruned roots" in the treated plants. This makes weeds suffer from lack of water and nutrients.

The students showed epinasty, a symptom caused by growth regulators and explained how weeds turn white by the action of pigment inhibitors.

Tickes also showed the symptoms of a cell membrane disruptor application which have mostly contact activity. He said these herbicides produce a dark green, water-soaked appearance on the leaves which means the cell membranes were destroyed and the leaking of intercellular fluids which causes necrosis.

Watch the videos on how herbicides work by visiting the Vegetable IPM Video Archive located at

This archive also has videos on insect management, evaluation methods, and insecticide trials conducted by researchers at the UA YAC. Plant pathology videos will be added in the future.

Contact: Marco Peña at (928)782-3836 or