Preen is a preemergent killer which means that it affects seeds trying to germinate but not necessarily plant tissues after germination. In theory anyhow.

Ditto what macranthos said

But I did want to add that preen creates a barrier that deters germination. So, if you start hoeing around or otherwise disturbing the soil, you break the preen barrier which means you weaken or destroy Preens ability to do its job. Correct sequence s/b weed first, preen afterwards. Preen mfr recommends applying a 2nd very light sprinkling if you disturbed the soil after preening.

Oldbuck, you did in correct order but I posted info for benefit of other readers.