Evan,
I put A. plumbea in quotes because I do not believe it to be a true species. I cannot state that botanical academia agrees with my assessment; however, my assessment is based on actual field studies involving cross pollination. If sterile plants can be considered true species, then the species problem, which is a fundamental weakness in the foundation of the theory of evolution, is worse than I thought!
The convention for varieties of a species is that the varietal name should be in single quotes, not double, and the variety, if valid in terms of the ICNCP (aka Cultivated Plant Code) should start with a capital letter ('NIgra', correct; "nigra", incorrect). However, since I do not know if nigra is a valid variety according to the ICNCP, the distinction may be spurious at this point.
In my experience, A. plumbea is no more or less vigorous than the A. macrorrhizos types I've grown. I'm a little more inclined to believe that the plant is a sterile sport rather than a natural or artificial hybrid.
LariAnn
Aroidia Research
