Kalferberries are picked from a Kalferberry bush which can only be grown at the Herblore Habitat on Karamja. The bushes are grown by planting kalferberry seeds in the vine bush patch in the centre section of the activity. Each full harvest yields 4 berries and earns 52.5 Farming experience for each. Similar to other berry bushes, kalferberries may be individually picked, without waiting for a full harvest, and they are also renewable, ie: if the patch is not cleared, more berries will appear after growth.
If an ent familiar is summoned, you need at least two inventory spaces to harvest or else it will yield the message: "With an ent familiar there, you may receive double, so you need two free slots to take them". This is odd though, since they are stackable.