A little more checking reveals that purple loosestrife is indeed sometimes known as purple willow-herb.
See here:
Salicaria is a reference to the willow-like leaves of the
loosestrife. While we can't know if this is what VN (or CK)
intended, a reference to purple loosestrife would be fitting,
since it is originally from Eurasia, then came to America
(as ship ballast) and has become a huge problem, particularly in the northeast. 
I'm not sure of the relative attractiveness of loosestrife as
opposed to other kinds of willow-herb/fireweed. That might be the key to figuring out which plant CK was noting.
Matthew Roth is sorry for posting twice

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