There are some seeds which are immune to the Preen. One that I know from experience is Oenothera. I cleared an entire area of all vegetation, using the scuffle hoe to cut everything and went back to re-work the soil in case I missed anything. I spread Preen generously and watered it in. The only seeds which germinated all on their own were from the Oenothera which I had allowed to re-seed (on purpose to check if what I had read online was true).
I'm still testing, but I believe that White Sweet Clover /Melilotus officinalis is also immune to Preen; I would imagine that the Yellow Sweet Clover would react the same way if I could ever find seeds for that one.
