The tolerance for weeds in vegetable crops grown in this area is very low and some fields are kept almost weed free. It is hard to understand why so any weeds appear the next season in fields that are kept weed free. There is no simple answer to this question. Part of the reason lies in the fact that weed seed, unlike crop seed, does not germinate all at the same time. Some may germinate immediately and some may not germinate for years. Another reason is that weeds produce millions of seeds and these move from field to field. In an area like this where irrigation is so intensive, water is moving into and out of fields all of the time. Most weed seeds are very small and float. There is just no practical way to filter small seeds out of large volumes of water. Filters that are capable of removing small seeds restrict water flow too much. Water would have to be diverted from the ditch or canal to successfully filter the seed. After water, wind is probably the next major means by which weed seed is dispersed from field to field. Some weeds have developed structural characteristics that allow them to blow long distances. The composites like sowthistle, groundsel, prickly lettuce and others, have fuzzy seed heads that blow in the wind. Some seeds have spinners, gliders and other structures that help them travel. All of the tillage, cultivation and spraying we do provide a mechanism for weed seeds to be moved mechanically. Sandbur, puncturevine, clovers and other seeds are hard to keep off equipment, boots and clothes and are difficult to control. Birds can disperse weed seed, tubers and bulbs. Some eat and defecate seeds, fruit or pond sediment that can be deposited in other fields. Seeds can attach feathers, claws and beaks. The physical damage that birds do to lettuce and Cole crops is far worse than spreading weed seeds. They pull young seedlings out of the ground and cause direct crop loss.

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.