American Burnweed (also known as Fire Weed) is commonly seen in healthy lawns because those lawns have a thatch layer which is where this weed germinates. The preemergent chemical creates a protective barrier at the soil level which is below the thatch layer. Therefore, the pre-emergent is not effective against Burnweed and needs to be contolled with a post emergent broadleaf herbicide.  If you have questions or concerns, please do not hesitate to give us a call.