The flowers are wind-pollinated pendulous catkins, produced in spring. In spring, the catkin cup "Ciboria amentacea" grows on fallen alder catkins. The alder usually grows in wet ground, with small, pendulous catkins. Bigtooth aspens produce seeds from wind-pollinated flower clusters, known as catkins. The wind-pollinated fruiting catkins are erect, long and broad. The flowers are inconspicuous unisexual catkins that occur in the spring. The flowers are wind-pollinated catkins. By the summer when the catkins have fallen, the caterpillars discreetly mimic oak twigs. They are cylindrical and drooping, somewhat resembling catkins. The flowers are small wind-pollinated catkins, produced in spring. The species is monoecious with male and female catkins found on the same tree. It feeds on beech buds, birch catkins, and holly leaves. The flowers are catkins, with male and female catkins usually on separate plants (dioecious). The flowers are wind-pollinated catkins, produced in early spring before the leaves. The flowers are catkins, with male and female catkins on separate plants (dioecious).