Updated: Mar 18
The signs of spring are coming thick and fast now; with the pussy willow opening up. There are several species of willow that grow in the UK, but the one that has the furry male catkins that look like cats paws when developing is goat willow. If you remember, hazel and alder have both male and female catkins on the same tree (they are monecious). Willows do it differently, they are dioecious, so the male and female catkins grow on separate trees. On goat willow, he male ones are grey, stout and oval, becoming yellow when ripe with pollen (top image); and the female catkins longer and green (bottom image). You can see the anthers covered in pollen on the male catkin in the picture above. The pollen is blown by the wind onto the green female catkins on another trees and then woolly seeds develop, which are then dispersed by the wind.
Most willows can also propagate themselves by lowering their branches to the ground, where they then develop roots. This means all you have to do to grow a new willow tree, is to cut a young branch off a tree and stick it in the ground (best done around Jan/Feb before the catkins develop). In fact, it has been known for bits of willow stem as thick as a human thigh to take root if put in damp ground, but young branches are best. And don't worry, all this will do is cause the tree to grow even more branches the next season (like coppicing). One of the most amazing properties of willows is their incredible growth properties due to the naturally occurring plant rooting hormones that they contain. You can even make homemade rooting water by steeping chopped up willow in water. Here's how https://deepgreenpermaculture.com/diy-instructions/home-made-plant-rooting-hormone-willow-water/