[jayerose]

Has anyone used in yard under mulch to help prevent weeds?

Has it worked?

[villagetinker]

IMHO, there is nothing you can put in to prevent weeds, over the seasons (years) dirt will accumulate above the barrier, and you will get weeds. I use a good liquid weed spray and go out and spend 15 to 20 minutes of quality time spraying the weeds. One other thought, if I understand what you are proposing you will accumulate water during heavy rains, and you will not be able to water plants correctly.