Hazel catkins. Hazel (Corylus sp.) is monoecious, meaning it has separate male and female flowers on the same tree. The pendulous catkins are clusters of male flowers. In spring they release pollen, the male reproductive cells of the plant. The female flowers are less conspicuous. They are similar to buds, but sprout red stigmas (not visible) the female pollen receptors. Hazel flowers between January and March.