A catkin, also known as an ament, is a dense, elongated, and drooping cluster of flowers without petals. Most commonly, the flowers found on a catkin are of one sex. In some trees, such as the poplar, both male and female flowers are present. Catkins are typically found on trees which are wind pollinated, such as oak, birch, willow, and hickory. A few herbaceous plants, like stinging nettle, also produce catkins.
Most catkins form prior to the foliage leaves, to allow for wind pollination. The wind carries pollen from a male or female catkin to a flower of the opposite sex, usually in a different form, such as a flower spike. In rare cases, catkins may be insect pollinated, but they most often rely on the wind to complete this process.
Depending on the specific plant, catkins will form in the late winter to late summer, and will produce flower clusters several weeks after forming. These flowers form into seeds near the end of the growing season, which is usually in the late fall for most trees. The seeds fall from the catkin, and if they survive, the growing process begins to form a new tree.
Each catkin contains small, modified leaves, known as bracts, which differ from the leaves found on the branches of the trees. They are a different color than the foliage leaves, a different texture, or sometimes both. Some catkin bracts act as attractors for insects, in the instance of insect pollination.
Catkins are typically found hanging down from the branches of the tree or shrub, but may grow erect in some trees, such as the white birch. Some catkins, such as the pussy willow catkin, will be covered in fine hairs before the flowers bloom. These hairs often obscure the small bracts of the catkin, and they are further diminished after blooming.
Manual pollination is also possible using a catkin. This is a popular practice for American flowering chestnut trees. A male catkin is harvested from one tree, and then carried by hand to a tree bearing a female catkin. The male catkin is rubbed against the female, until the pollen is depleted, and then the process is repeated as necessary.
Homeowners with catkin bearing trees on their property often find them a nuisance. They will emit large clouds of pollen when disturbed, and can clutter a yard once they’ve dried and fallen off the tree. However, their appearance is necessary to the reproduction of these plants.