By Barry Tickes, UA Area Agriculture Agent

Most pre-emergent herbicides do not kill dormant weed seeds. In most cases, the seeds must first germinate and contact the herbicide before they are killed. Some pre-emergent herbicides are absorbed only by the roots, some by shoots only (at the hypocotyl in broadleaves and at the coleoptile in grasses), and some by roots and shoots.

Weed seedlings sometimes emerge and grow for awhile before dying or becoming uncompetitive with the crop.

Some fumigants kill weed seeds including 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.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.