Why is there so much white poplar fluff in Russian cities in summer?

In June, many large Russian cities find themselves at the mercy of poplar fluff, which literally blankets the streets. The problem arose when the USSR began rebuilding cities after the Great Patriotic War.

During the massive construction boom of the 1950s, trees were needed to make residential neighborhoods greener. The choice fell on the balsam poplar, a hardy and fast-growing tree native to North America. The plan was to grow only male trees – it was known that female trees produce fluffy catkins (long, narrow, hanging clusters of tiny, usually scentless flowers found on trees and shrubs like willows, birches and oaks). However, due to haste, trees of both sexes were planted. Additionally, trees sometimes changed sex after pruning.

Today, Moscow alone has over 300,000 poplars, half of which produce fluff. The fluff itself is not dangerous, but it carries pollen and dust that can cause allergies in sensitive people.

But, poplars also provide benefits: they clean the air and produce large amounts of oxygen. So, they won't be completely cut down – old trees are periodically removed and replaced with new ones that don’t produce fluff.