Pre-emergent herbicides are used to kill crabgrass in an effective way. They are available in either granular or liquid form. They are applied on the crabgrass seedlings as they germinate. This means that they act as an invisible shield that kills the emerging crabgrass dead in its tracks. You should not practice core aeration on lawn after applying pre-emergent herbicides. You should have aerated the lawn before applying herbicide. The name of pre-emergent herbicides indicates its very nature; that it kill crabgrass before its seedlings emerge. Thus, you will understand the importance of timing in this method of weed killing. The right time should be calculated and the herbicide should be applied, you should not be early either. And in order for you to get a d=sign of when do they germinate, there is a common understanding that the blooming of lilac bushes coincides with crabgrass germination.
Edited by olcowhand, October 11, 2010 - 08:56 AM.