Have you been wondering why your street or garden is suddenly covered in white fluff?
It means that there is probably a Poplar tree or two in the nearby area, and this is the time of year when they generate huge amounts of seed.
Being a wind-germinated plant (as opposed to one germinated by flying insects) they don't bother with showy flowers, so you might not even be aware that they have flowers at all - and their seeds, like those of the common Dandelion, have a wispy parachute of fine fibres, which help them to float on the wind and spread the seed away from the parent plant. This is why many of the Poplars - particularly the Black Poplar, Populus nigra - are known in America as Cottonwood trees.
Luckily for us, most Poplars are dioecious, which means that one tree will have male flowers, and another will have female flowers: so not all Poplars produce seed, and if a tree with “female” flowers is not sufficiently close to a tree with “male” flowers to be pollinated, it won't produce seed. In some parts of America it is actually forbidden to plant female Poplars, presumably to stop the country being over-run with them: so they can plant male trees, and enjoy the benefits of the tree, without having to weed out millions of Poplar seedlings.
In case you are now wondering why we are not also surrounded by Poplar thickets, the seeds are so tiny that they don't contain much of a food reserve, and they rely on landing in moist swampy soil, where they can germinate immediately. In the UK, we don't have many swampy areas - but where we do, you can probably find large stands of Poplar!
If you want to locate the trees which are dropping this fluff all over your garden, look upwards for large, stately trees whose leaves appear to twinkle on the branches. Poplar leaves have long petioles (botany-speak for “stalk”) which are flattened, not round: so the leaves can wobble easily, which gives the twinkling effect. The most dramatic of these is the Aspen, Popular tremula, whose leaves flutter in the slightest breeze...but they all do it, and if you take hold of a leaf, you can feel for yourself how flattened the petiole is.
Another tree which produces large amounts of fluffy seed is the Willow, so it's no surprise to learn that both Poplar and Willow are in the same botanical family, Salicaceae. And both of them are known to be Hay Fever triggers, so if you suffer from allergies, and you see what looks like a thin layer of snow on the street in the middle of June: try to stay away from it!