For the best timing on when to apply Quali-Pro Dithiopyr 2EW Pre-Emergent Herbicide  we would recommend contacting your Master Gardener at your local cooperative extension office as they will have information for your specific region. Please be sure to review the product label for more information and complete application instructions before use.  Defender Herbicide is a post emergent herbicide best applied when weeds have just started to emerge and can be applied around the same time if listed weeds are actively growing. The general rule for spring is to apply your pre-emergent herbicide treatment after the ground has thawed from the winter but before the temperature of your soil reaches 55 degrees, as that is when crabgrass begins to germinate. Other weeds will germinate when the soil temperature is between 55 and 65 degrees. You can use a soil thermometer to determine the temperature of your lawn/area. You will want to measure the temperature of the soil a few inches below ground. The ideal time to apply a fall pre-emergent herbicide to your yard is between August and November when the soil temperature is below 70 degrees and dropping.