Most preemergent herbicides do not kill dormant weed seeds. In most cases, the seeds must first germinate and contact the herbicide before they are killed. Some preemergent herbicides are absorbed by roots only, some by shoots only (at the hypocotyl in broadleaves and at the coleoptile in grasses, see diagram) and some by both roots and shoots. Weed seedlings will sometimes emerge and grow for awhile before they die or become uncompetitive with the crop. There are fumigants that do kill weed seeds. Some of these are metam sodium (Vapam), clorpicrin (Telone), dazomet (Basamid), methyl bromide, methyl iodide and calcium cyanamide. Flooding and solarization also can kill weed seeds. Fumigants, flooding and solarization are often used primarily to control diseases and have the added benefit of controlling some weeds.

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.