While on a walk around my neighborhood this past week, I noticed some lovely large Catalpa trees in bloom. Likely the trees growing in my neighborhood were the Northern Catalpa (Catalpa speciosa), but the other Catalpa species in Illinois is the Southern Catalpa (Catalpa bignonioides). Northern is hardy to zones 4-8 and Southern is hardy to zones 5-8.
I still remember learning in my Woody Landscape Plants class that the easiest way to tell the difference is to open up the fruit and look at the seeds. Northern has many fringed hairs on the seeds and the tips are more blunt, while the hair on the seeds of Southern Catalpa is much less and has a more pointed tip to the seed. Not extreme differences, but some subtle difference that can help you identify which Catalpa you have.
NorthernSouthern
Photos by Gary Kling from the UI Plants Database
Illinois Wildflowers also lists these characteristics about the Northern Catalpa:
1) its crushed leaves do not have an unpleasant scent
2) its flowers are slightly larger in size with fewer purple spots
3) it tends to have fewer flowers per panicle
4) it has slightly longer and wider seed pod
5) for mature trees, its trunk bark is more furrowed
6) both tips of its seeds are blunt, rather than pointed on one side.
I've always loved Catalpas for their large leaves and dramatic white flower display in the late spring, but I know of a few gardeners who are not as much of a fan as I. You see, catalpas can be a bit messy. The fruits of catalpas are a long capsule that is 10-24" long. They look like green pencils or cigars and turn brown, persisting through the winter. I remember my dad hating this tree in our yard because of the abundance of capsules that fell to the ground. That tree is now removed from our yard of course. Catalpas are also a large tree reaching 90' tall. Select a site that can afford this large of tree and select an area away from sidewalks or driveways.
Overall Catalpa is a pretty great tree. It's adaptable to many soil conditions, withstanding wet or dry soils and alkaline or acidic pH. It is a pretty tough plant that can be grown in either sun or part shade. Definitely worth considering if you have the space to compensate it's size and messy pods.