hi linda,

i usually don't scrape my rhizomes. (i've never heard of this, sounds interesting.) i just rest them on top of the soil and maybe lightly push some up against the sides. they usually, even if bare, sprout new leaves. for more delicate begonias, i keep them in terrariums of in an enclosed environment until they seem well established. for 'Black Coffee,' it really wouldn't hurt to do this, but i imagine you could get away with leaving it out, if you're careful. (bowerae cultivars are alwaays easy, and i never really pay too much mind to them). you can certainly cut down your rhizomes if you want to produce more plants. i always keep it above 2 inches. if the section of rhizome doesn't have roots, keep track of which side is "up."

i think you'll have good success!
-ming

Thanks, Ming. I have potted in a six-inch pot to accommodate the rhizome.

With necky AVs, you prune the roots and scrape along the neck. Pot roots and neck up to a crown and where you scraped, new roots will form. With the trailers, if you injure the neck (I make little, shallow slits), a new crown and/or leaves will form.

Of course, these aren't rhizomes so I didn't know if it would work for the begonias.

Thanks, again,
Linda