Willow trees are members of the Salix genus, which contains several hundred species of trees and shrubs. These trees are primarily native to the Northern hemisphere, and are commonly grown as ornamentals, especially near water. Common willow types include the weeping willow, white willow and pussy willow. All willow trees produce elongated catkins of flowers, which may be very small or more showy, depending on the species and sex of the tree.
Most willow species produce yellow flowers in elongated clusters called catkins. The male flowers are usually more conspicuous than the females, but do not provide much visual interest. Willow flower catkins can be as small as 1 to 2 inches long or several times that size. Some species, such as the pussy willow, produce soft furry flowers, which are used to add interest to floral arrangements, or dried and displayed on their own.
Willows are dioecious trees; they produce male and female flowers, with each plant producing only one type of flower. Male flowers have no petals. Instead, they produce only stamens and nectar glands. Male willow flowers are usually larger and showier than female flowers. Female flowers also have nectar glands and no petals. Insects visit the male flowers for nectar and pick up pollen, then visit the female flowers and deposit their pollen, fertilising the willow tree. Once pollinated, flowers form small fruits full of tiny seeds, each of which has a tuft of silky white hair.
Willow trees tend to bloom in spring, producing pollen that can add to allergy problems. Some species, including the pussy willow, bloom very early, producing flowers while most other plants are still dormant. Others, such as the goat willow, don't bloom until later in the season. Willow trees are extremely cross-fertile, and may pollinate one another, resulting in new hybrid varieties.