Gack! What a PITA of a tree. They have a habit much like Andy's Blue Gum; they're basically a weed with bark. Unlike most trees, they come in genders. The male spits sticky pollen pods in early spring, about the size of a kernel of corn (but narrower). They stick to everything, and if you run them over with an electric lawnmower (and if that's that kind of lawnmower you have, you cannot keep from running them over; they're everyfuckingwhere), they burst open, and the pollen shorts out the motor. Walk on them, and you'll track them everywhere (including Puerto Rico...). The females turn that pollen into seeds...and a cottonwood seed pod is about the size of a sweet pea, and when it dries, it splits open to dump out a bunch of wispy fibrous cottony things in which is buried the actual seed. And there are literraly bajillions of these fuzz balls per tree.

When they're not having sexual relations, both male and female spend their time dropping leaves basically all year long, and just for fun, will periodically divest themselves of various sized twigs, branches, and even whole limbs, for no apparent reason. (But a stiff breeze is plenty enough of an excuse!).

They're also a very wet tree, which means that they make a good lightning rod.

Finally, the damn things are fucking lazy, and won't send roots down very far; they like to spread out. Which means they like to burrow their roots just underneath such ammenities as driveways and sidewalks. (They don't seem to have an affinity for drainage pipes, but I don't know how they like septic fields; you can be sure I'm much happer not knowing....)

And they're butt-ugly trees to boot. But they grow very quickly. The one we took down in our front yard (a female) was 25 years old, and about 100 feet tall. They're reputed to be the most populous tree in Illinois. (And this surprises you how?)