Weeds would be much easier to control if they all came up at the same time. They could all be germinated, killed and we’d only have to worry about new migrations into the field. Crop seeds has been developed to have uniform germination, but weeds survive by having multiple germinations. When and how for how long weeds will emerge are complex questions. There are a large number of interdependent factors that determine this. These include species, depth, number present, crop, soil temperature, soil moisture, age of the seed, seed coat, light, salinity, soil compaction, oxygen level, soil texture, cultivation, presence of herbicides and other factors. These variables all interact to create a complex system and every seed can be different. It is easiest to generalize and say that summer annual weeds can begin to germinate in February and continue when moisture is present until October. Winter annual weeds will begin to germinate in October and continue to March and Perennials will germinate in spring or fall but remain viable year round. These generalizations are often wrong, however. Especially here in the desert where mild winters and summer irrigations can allow summer annuals to survive the winter, winter annuals to survive the summer and both to live for 2 years as biennials or longer as weak perennials.

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.