bignoni question

[Jerome V Ward]

I have a question about Catalpa (Indian cigar tree).  We have two species
in Alabama, C. bignonioides and C. speciosa which have somewhat
overlapping ranges.  In the former the corolla is 3 cm or less and the
latter ca 4 cm.  The bark is fissured in the latter but not the former.
 I would like to ask how easily these species can be
separated?  Wouldn't slight differences like this be a result of
phenotypic plasticity?
Since I don't know a lot about the genus I don't know if hybrids form.
My inclination would be to treat this as one species.
Sincerely,
Jerome Ward
Auburn University
Auburn, Ala.