Keep the flowerheads on the plant - eventually those little bobbles on the end of the heads will turn dark and dry and you can then harvest the seed.

Do the bulbs multiply Bamboo?

Well they're supposed to, but here in my part of London, they just seem to disappear after a couple of years. Might be the clay soil.

Oh thats a shame Bamboo as I am on clay sounds as if I will need to plant more.