Birches are termed "monoecious," meaning that the trees bear both male and female flowers on the same branch. Closely related to oaks and beech trees, each birch produces sufficient pollen to fertilise its female flowers in the springtime.
Birch trees (genus Betula) have delicate leaves, papery bark and graceful silhouettes. Flowers appear in catkins. The male flowers -- erect staminate catkins producing pollen -- appear at summer's end at the tips of long shoots. They remain on the tree until spring pollination.
Female birch flowers -- solitary pistillate catkins -- pass the winter enclosed in buds. They appear about the same time as birch leaves, presenting as short catkins of less than 1 inch on short shoots. Each catkin contains three flowers.
In the spring, the male catkins grow and change colour. When they are several inches long and a purplish yellow hue, they shed pollen for three to five days. The female flowers become receptive just before the male flowers on the same tree shed pollen.