Wait a minute, Lupinus arboreus isn't black bamboo!

think im right in saying when bamboo go to seed then they are on there way out.
collect the seeds and grow some more to replace it.

From the Plants for a Future Database:

"Seed - surface sow as soon as it is ripe in a greenhouse at about 20°c. Do not allow the compost to dry out. Germination usually takes place fairly quickly so long as the seed is of good quality, though it can take 3 - 6 months. Grow on in a lightly shaded place in the greenhouse until large enough to plant out."

From what I hear, the seed doesn't store well at all, so be sure to plant it as soon as it starts dropping from the heads.