Yes, Preen works to keep seeds from sprouting. But it won't stop the seeds already growing.
Plastic and that landscape fabric are awful! Never use that stuff....ruins your soil!
Linda C

Apply 3-4" inches of mulch the newspaper can be recycled. I always preen on top of mulch.

One of the things in preen is corn gluten meal. Use that in sted of preen, no chems still a preemrgent.

Guess I'm a Plastic nut but even when transplanting thru plastic mulch it seems that the gerninating weed seeds fill in every square inch of exposed area. Antigermination products like Preen help eliminate hours of tedious weeding that everyone wants to avoid.

The placement of Preen or any herbicide over a non-porous mulch such as newspaper or plastic, where it can be washed by rain or irrigation, is a bad practice. The chemical has potential to concentrate to many times the recommended rate in runoff areas. Always apply under the mulch material or spot apply to only the exposed areas.

If you are using hay, straw or other porous plant residue for mulch all bets on effectiveness of the preen are off because the active chemical may be absorbed by the residue before it reaches the area where weed seeds are germinating. Chances are that weeds would not be a problem in this situation anyhow if residue mulch is adequately thick.

I hate to weed and have used Preen for years with great success. I start out every new bed after planting with a good sprinkle on the bare soil. I make sure it is as weed free as I can make it. This takes time, you must get rid of even the smallest weeds. I usually do a bed then wait a day or two checking on it and pulling up any little weeds that poke through.Then I mulch over the Preen and like magic,no weeding! If you haven't gotten up every weed and a few poke through it is still almost nothing to pull. Once when I was planting new beds on either side of a walkway, I forgot to sprinkle the Preen on the first side, so only the second side got "Preened" in the usual method. the first side was already mulched so I sprinkled the Preen over top of the mulch and hoped for the best. I had one side of a walkway I never had to weed and the other I had to visit daily , there was no comparision. I finally grew tired of the weedy side and raked up all the mulch, re-weeded and applied the Preen under the mulch. It was definitely worth the extra work. Needless to say I'll never forget again! DO NOT use the Preen in a newly planted bed if you are planting from seed. Your seeds will not germinate. Wait til after you have seedlings. Good luck

Does Preen interfere with bulbs?

Preen works well at stopping weeds from seed, it doesn't do anything for weeds that are there. It should not be put under mulch etc as it reduces the effectivness. Preen is a brand name and can be purchased for less as a generic at many garden centers. It does not prevent weed seed from sprouting it actually prevents them from emerging. It must be close to the surface as it forms a barrier that kills the sprout as it comes up through the layer of Preen, it doesn't have to contact the seeds etc. Read the package as it can be used with many seeds, the original use was as a farm pre- emergent herbicide (Treflan)

It is possible putting it under mulch reduces it's effectiveness, I can only speak to my personal experience and I ALWAYS put mulch over it and although I can't say I never have weeds, I don't weed much at all, just an occasional pull here or there. I can say that last year I put down Preen in a flower bed as usual, but didn't buy mulch until a couple of months later and that bed with Preen and no mulch was full of weeds and a real pain , I had to do a lot of weeding before I put mulch down. That's my experience.

Yes but you have to use it every 3 weeks or so. There is something that farmers use that will last up to 6 weeks, but don't remember the name of it. It comes in a long bag, narrow bag and can be found at feed and seed stores.