|How long is Preen weed preventor effective? I've used it with transplants with great results for the past three years. Last application was June 1998, but now I plan to direct-sow annual wildflower seeds later this month (April 1999). I don't see any Preen granules left, and in fact, quite a number of cosmos, poppies and violets self-seeded from last year's garden in certain areas where there's no run-off. I'm wondering if it's OK to go ahead and direct sow, or will some remaining Preen from last year still prevent seed germination?