I've done it the first time but I can't trigger the event 2-3 weeks later. Several times I tried raiding it again but the farmers don't come back.
- Wikia says it's probably a unique event: http://gta.wikia.com/Braddock_Farm
- IGN says it's available every tuesday with any character from the 2nd time onwards: http://www.ign.com/wikis/gta-5/Raiding_The_Weed_Farm!
- Someone claims it used to work before a patch fix: http://gtaforums.com/topic/605948-weed-farm-not-respawning/#entry1064050226