Weeds will grow in any patch that does not have anything else growing in it, even if the soil has been treated with compost, supercompost and ultracompost. Players can harvest them by using a rake on a Farming patch that has weeds growing in it. Doing so will yield up to three weeds per patch, giving 4 Farming experience each.
A fast way to obtain weeds is by raking the flax patch in Miscellania. However, you can only do this until the player's popularity reaches 100% (You can kill citizens to lower it again).
Weeds are generally used to make compost. Players can put up to 15 weeds at a time into a Compost Bin, close the lid, and leave the contents to rot. This will take around an hour, after which they can open the lid and retrieve their compost. Weeds are the fastest item to make compost with, because when you use a weed on the compost bin, it will put all 15 weeds at once. Note: this doesn't change the time it takes for it to turn into compost.
Weeds will stop regrowing on the player's farming patches if auto-weed from Tithe Farm is unlocked and enabled. This does not affect the patches on Miscellania that can be raked for approval.