Every spring, evidence of wind-pollinated flowers can be seen in the feathered wisps of threadlike hairs with a small seed attached at one end that go drifting through the warm spring air. These seeds are the end product of wind pollination, which occurs in many of the hardwood trees of temperate North America, such as the willow, cottonwood, popular and alder.
Flowers of wind-pollinated plants do not have to attract insects for fertilization to take place, so there is no biological advantage to having a colorful and aromatic flower. As a result, most wind-pollinated flowers are green or dull-colored, and they usually lack the sepals and petal that most flowers have. One large group of plants where wind pollination are common are the grasses, especially the cattails and rushes that grow in wet areas. With these plants flowers are not very colorful or conspicuous, and they often form spikes of small flowers. Wind-pollinated grasses tend to produce large quantities of pollen, which can cause allergy problems in the human population.
Many of the flowers of wind pollinated trees form catkins, a type of flower that hangs downward from the branch with multiple small flowers that are arranged in a spike and have no odor. Catkins are usually formed in male flowers, so the pollen can be and easily dispersed and then travel through the air to a female flower. Some of the trees that bear wind-pollinated catkins are the alders, birch, cottonwood, hickories, oaks and poplar. The female flowers of these trees do not usually come in the shape of catkins but develop small, round and inconspicuous flowers.
The Dioecious Pussy Willow
The pussy willow is a shrub of the willow family(Salix discolor), which has plants that contain either all male flowers or all female flowers. The fuzzy growth that nature lovers enjoy picking as the first sign of spring are actually the male catkins, which are more showier than the catkins found on the female plant. Branches of the male pussy willow plant often end up in a vase of water at someone's home to celebrate the arrival of spring. They are unusual among wind-pollinated plants in that both the male and female flowers form catkins.