Weeds

Weeds will grow in any patch that does not have anything else growing in it, even if the soil has been treated with compost or supercompost.

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 are also needed in the Ratcatchers quest.

Upon completion of the Fairytale III - Orks Rift quest, patches will no longer need to be weeded. Talk to any farming assistant to activate this effect.

Trivia

 * Some players were banned for saying "Buying weed" in situations where they had a good reason to do so, and Jagex mistook it for the drug itself, which is against the rules of RuneScape. However, through several appeals and complaints, Jagex unbanned most of these accounts during the Wilderness update of 2007.
 * Players will sometimes try and sell weeds to unsuspecting players as "weed."
 * The price of weeds increased dramatically from 24 March 2010 up until 19 July 2010 when they were no longer able to be bought or sold on the Grand Exchange.
 * If you start to rake Weed away from your farming patch with a full inventory, the weed itself will not appear in your inventory (due to no space) and will dissappear. (You still get the XP).