Wild bitter melons grow in the forests in India. Called kaatu kaipa ( കാട്ടു കയ്പ ) / kaatu paval ( കാട്ടു പാവൽ ) in Malayalam and करेला in Hindi, the little green fruits are delicious in stir-frys, curries, and fritters. They grow on climbing vines enfolding bushes and small trees. The vines look like those of the regular bitter gourd, but the leaves are more densely packed and disease resistant. They need no chemical fertilizer or pesticides, and can be grown organically. The fruits are oval or round and just an inch or so in size (2 cm to 4 cm).
Propagation in nature is carried out by rodents, birds, and bats who carry away the ripe fruits. The seeds germinate with the onset of the rains, and start yielding in a couple of months. They continue to bear fruit until the dry season, when the plants dry up. In our garden, the vines are grown on farmyard manure and are watered in summer. So we have these throughout the year. You can grow them in pots on balconies too, provided they get sunlight for at least 3 to 4 hours a day.