Smoke trees

Sorry for the misleading title if there are any stoners out there, but in this instance "smoke" isn't a verb.  Our smoke trees (Cotinus obovatus) usually flower and then produce a hazy purple display for days before we notice.  Many of you are probably familiar with the smoke bush (C. coggygria), a fairly common ornamental shrub of Eurasian origin, but for reasons we have been unable to determine, our yard has 4 smoke trees, and they are native (N. Amer.) members of the anacard family (sumacs).  This time of year they put on quite a display that it tends to go unnoticed because it's up there in the crown of these 30-35 foot trees. Our smoke trees have clearly been there for some decades, and around here they are quite uncommon; ours are the only ones we know of in the area. So they are a puzzle as to how they came to be here. The "smoky" display is produced by the persistent, reddish-purple flower stalks of the much branched, poofty (technical term) inflorescences. Our neighbors have the best view, and that's how it goes. Several other ornamentals plants perform better for neighbors than for us.