As the name suggests, pre-emergent herbicides must be applied before weeds emerge to have any real effect, since they work by preventing weed seeds from germinating in the first place. If you wait until after the weeds emerge, the herbicide won't have any effect and you are defeating the purpose of it altogether.
Timing is critical. A common misconception about the ideal time for pre-emergent herbicides is that application should coincide with certain events, such as daffodils or forsythias in bloom. In nearly every area of the country, the weeds will have already emerged by that time. Pre emergents are usually applied around Feb 14th and the following two to three weeks.
If you missed the window of opportunity for applying a pre-emergent herbicide, you can apply a post-emergent product. Post-emergents work by destroying already established weeds. However, take care when applying post-emergent herbicides. Some are selective, meaning they target specific weeds, while others are nonselective, which means they destroy anything and everything it comes in contact with. Therefore read the product label.
Keep in mind that you'll never get rid of all the weeds in your lawn. The wind will blow weed seeds from nearby lawns into your lawn, birds will deposit them and kids running from one lawn to the next will transport weed seeds on their shoes. So do what you feel you must to battle the weeds in your lawn, but do it wisely. Raise the height of your mower to help prevent weed seeds from germinating in the first place and fertilize as needed with a natural product to promote vigorous grass.
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