Knobweed is a type of plant on Roshar. Its sap is used as a powerful antiseptic. It grows in various locations, including the Alethi warcamps near the Shattered Plains, where Kaladin gathered it for his bridge crew.
Knobweed grew most heavily on the west side of rock formations and was ready to pull back into the shadow of a boulder if a highstorm approached. Their thin, rigid stalks were topped with delicate fronds that could retract into the stem. The stems themselves were immobile, but they were fairly safe growing behind boulders. Some would be pulled free in each storm perhaps to attach themselves in a new location once the winds abated.
Knobweed is used to produce knobweed sap or knobweed milk, a powerful antiseptic that prevents and counters infection. The sap can be removed from the stalk by squeezing it along the stem, which produces two drops of the creamy white substance.