What do you call the fully developed cottonwood seeds? We use тополиный пух in Russian

Aha! Where I live -- not far southwest of Washington DC -- we get small amounts of тополиный пух in May. But these trees don't grow in huge numbers here and are less common than, for instance, oaks or maples.

But since the cottonwood isn't widespread here, I didn't know the correct name until I saw your post, and I've always referred to the seeds as "tree fluff"!

Right across the street from me, there is a small section of forest with maybe three or four cottonwoods growing near the parking lot of the Home Depot. So, a few weeks ago, it looked as though it were snowing in the parking lot, because of all the "пух". But I don't recall seeing such "tree fluff" anywhere else in this area.

I suspect that people who live in regions where these trees grow very widely would call it "cottonwood fluff", by the tree's proper name. And in any case, "fluff" will usually work for "пух".