Why do weeds still emerge even after you apply a pre-emergent herbicide?

There are several reasons why weeds will emerge even though pre-emergent was applied. Here they are in no particular order:

• Weeds that spread via their root systems (creeping roots, bulbs, tubers, stolons, rhizomes) will not be affected by pre-emergent herbicide. They will keep spreading year after year if they are not properly addressed. Pulling them out by hand may not be effective due to their strong or expansive root systems, which will promptly regenerate new weed plants. Short of digging them up and replacing these sections of your lawn, the most effective way to eradicate them is to spray them with a post-emergent herbicide (preferably organic or reduced risk synthetic); and depending on how robust their root system is, it may take multiple applications throughout a growing season to get rid of them. 
• Some weeds that spread via seed are biennials (e.g., dandelions), which means that they don’t spread seed until their second year of life which is after a first year in which they become a well-established plant that is not affected by pre-emergent herbicide. If they are in their second year of life, they must be pulled out by hand or sprayed with a post-emergent herbicide. 
• The timing of when you apply the pre-emergent may not be in sync with when the weeds germinate. If you apply it too late, some seeds in your lawn may have already germinated and grown beyond the point at which the pre-emergent will kill them. If you apply it too early, the residual effects of the pre-emergent may be reduced before certain weed seeds begin to germinate. Well-timed application is critical. You need to know the soil temperature at which certain weeds begin germinating and apply the pre-emergent then for maximum impact. 
• Weed seeds that are deposited in your lawn by natural processes after the pre-emergent was applied may not be affected by the treatment. Weed seeds are being deposited in your lawn every day throughout the year by wind, birds, and other wildlife. Seeds may also be transported to our lawns inadvertently by our children, pets, and ourselves. 

With so many reasons why weeds might still grow despite applying a pre-emergent herbicide, you may be wondering if it makes sense to apply one at all. At Crownover Green, we believe that it is especially worthwhile to apply a pre-emergent on lawns that are thin or on which the weeds were not well managed the previous year in order to thwart certain aggressive and invasive weeds in a lawn. Your lawn may contain hundreds or thousands of weed seeds that are lying dormant during the winter. As the soil temperature rises, these seeds begin to germinate and grow. If a pre-emergent herbicide has been properly applied, many of these tiny plants will be terminated and you will never notice them.

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