Preemergence herbicides can be very effective in the low deserts where weeds germinate with every irrigation or rainfall. It is essential, however, that they be at the right place in the soil at the right time. The right place is in the soil around the weed seeds and the right time is hen they are germinating. Only fumigants kill seeds. For most other herbicides, the seed must germinate and are then killed. When weed seeds germinate is dependent on a great number of interdependent variables. These include weed species, soil type, moisture, soil temperature, condition of the seed, depth, organic matter and many other variables. Many seeds have biochemical mechanisms that allow them to germinate at different times. Some may germinated in a few hours and some may not germinate for several years. If they all germinated at the same time, they would be easier to control. The methods used to predict when weed seeds will germinate vary from very sophisticated numeric equations to Farmers Almanac type lore involving things like the thickness of gopher fur, the cuticle thickness of plants or the condition of bird feathers. Some methods will give you a precise day while others will give you just a general idea. In general, as the precision goes up, the accuracy goes down. Regardless of what technique you use, now is the best time to apply preemergence herbicides for the control of summer annual weeds. We know that they can begin germinating in February and will continue through September. It is better to be a month early than a day late.

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.