It sounds like you're talking about Preen. You have to pull any live
weeds that are in the bed first.

Preen doesn't hurt established plants, just seed.


You can use Roundup to get rid of the existing weeds and plant a week
later. Preen will prevent new weeds from germinating for a few
months. You may want to check to be sure its ok for cactus, it's
intended for general use in landscape beds.