Q.Can weed killer penetrate through mulch ?
My property was forest mulched recently and I was told to let it stay put for at least 1 year. I wanna prevent the weeds from returning to allow a nice landscape project and need advice. The area mulched is over an acre and my zone is 6, mostly shady part sun.

Certified GKH Gardening Expert
You can apply a preemergent, but that would prevent weed seeds from growing, while you still may have existing weeds that have been cut down.
The answer to your question depends upon what kind of weed killer product you are using. This is a good question to ask your local garden nursery expert when deciding what to use. This article will help: https://www.gardeningknowhow.com/garden-how-to/mulch/weed-growth-in-mulch.htm
https://www.gardeningknowhow.com/ornamental/flowers/fgen/planting-flowers-to-deter-weeds.htm