Answer: Preen is a pre-emergent herbicide that will stop weed seeds from germinating, but will not control already established weeds. Even a tiny blade can grow into a full sized weed in a Preen treated bed. You might want to dig and divide your daylilies and remove all weeds in the process. Replant, then lay down a 3" layer of organic mulch over the soil surface to eliminate light, which will keep weed seeds from germinating. Any weeds that grow on top of the mulch can be easily pulled out, roots and all. Once you remove the offending weeds and keep them from flowering and setting seed, you'll get the upper hand in the weed war.
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