There are 4 spawns of Limpwurt root in the resource dungeon located within Edgeville dungeon where the Hill giants are. To access the dungeon efficiently, a Brass key is needed to enter the hut just east of River Lum. On an averagely populated world, each limpwurt root respawns at a rate of 56 seconds per spawn. It takes about 90 seconds to run to the Varrock west bank and back, and 426 seconds for a round trip. That means one can gather about 225 limpwurt roots in 8.4 trips. It's possible to fill your inventory even quicker by collecting the four limpwurt roots, switching servers, collecting the four limpwurt roots on that server, and switching back. If done quickly enough, you'll be back in the first server just in time for the limpwurt roots to start respawning, effectively eliminating