Answer: I couldn't find a source for seed for this plant. As far as I can tell, it is only available as small transplants. There are a number of possible reasons for this--perhaps the plant doesn't breed true to type from seed, germination of the seed is poor, and/or growing the plant from seed takes a long time. It is likely propagated by cuttings.
If you want to increase the number of plants, you might buy some transplants, then take cuttings of your own and root them. The least expensive source I could find is Jungs, ph# 800-247-5864; they offer quantity discounts.
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