I Will be starting to use Pre- emergent this coming spring. I already have purchased it so, your bad luck will not stop me from using it, It needs to be used anyway or it will sit in my barn forever. I hope it helps make the spring new growth more prolific in desired plants, rather than the less desired invasives.

It actually needs to contact the seeds when they sprout, so it depends on where the seeds are. If you had weedy soil, then put down mulch, then applied the herbicide, it may have less of an effect than if you had put it on the soil where the seeds are.

Also, any chemical applied will begin to degrade and/or leach away as soon as you apply it. So very early spring is better than fall, generally speaking. Unless the directions for your product say differently.

A pre-emergent weed control product needs to be in contact with the sprouting weed seeds - wherever they are.

If you have weeds sprouting in a mulched bed, it's probably because they blew in or birds pooped them into the mulched area, and they are growing above the landscape fabric. You might do better to just hand-pull the few that sprout instead of using a pre-emergent.

Pre-emergents are best in areas where you have a lot of dirt and a heavy weed crop you want to prevent. After a couple of years, if they are successful, you can switch to post-emergent killing with herbicides or hand pulling.

Thank you

Also keep in mind that while many pre emergents are "broad spectrum" a good number of them are selective to either broad leaf weeds or grasses, meaning they will only control one type but not the other. Make sure you're buying a product that is doing what you want it to do depending on your situation.