This can put the herbicide deep enough to be effective against weeds such as nutsedge, but may be too deep to be effective against some shallow rooting weeds such as black nightshade. In recent years, newer herbicide choices have permitted farmers to increase the use of post-emergent sprays and move away from some of the aforementioned risks faced with soil applied herbicides. Farmers can usually finish their planting and then start to spray, which adds convenience and allows some opportunity to see which weed species are present, to optimize herbicide choices. The problem is that if there's a week of rainy weather, or if it's windy every day, there's usually just a narrow window of time when the growth stage of the weeds and the crop are appropriate for optimum control. If you spray too late you run the risk of crop injury and lack of weed control, with limited options for rescue treatments.
Time management is the key to successful post-emergent herbicides.
You can also expect a couple more days per week when the winds will be a limiting factor. Pre-emergence may not fit into everyone's program. But it's an option for growers who are having problems scheduling all of their spraying for post emergence, or for those wanting more weed control early to make sure they don't miss the critical periods for weed control. For 2000, Ontario growers have two new pre-emergence herbicide options, Converge and Axiom. Converge is a new product from Aventis.