The use of pre-emergence residual herbicide provides a foundation for season-long weed management, along with tillage and other chemical management practices. It’s recommended to use pre-emergence when temperature are high enough for seed to take water, but before any germination. In Central Arizona, the end of fall and beginning of spring are the optimum times to use pre-emergence herbicides. But again, it’s judgment based on the temperature and amount of rain in certain areas. With above normal rain we received this summer/fall I would expect you have many emerged weeds in your property right now. If this is the case, and beside preemergence, you will need also to deal with these weeds using post-emergence herbicides. Near the end of winter when it starts to warm up, and before any germination, you’ll need to apply pre-emergence again. Besides standard orange pre-emergence, there are dinitroaniline (DNA or “yellow”) products that are helpful in delay or avoid developing herbicide resistant weed populations. Most of herbicides can be found at some retailers like Bug and Weed Mart, and some agrochemical companies like Helena, Fertizona and Wilbur-Ellis. With some weed resistant to Roundup and other postemergence herbicides in central Arizona, it requires the use of a diverse chemicals for weed control tactics.
When is the best time to apply pre-emergent for weeds?