Common willow catkins on twigs. Four catkins from a common willow tree (Salix alba). A catkin is a cluster of flowers forming a spike-like structure. The small flowers (the green buds lining a catkin) do not have petals and will be either all male or all female. They go through various stages in the flowering season. The male catkins release pollen which fertilizes the female catkins to form seeds. The seeds are hairy, giving the catkins a silvery- grey, bushy appearance, and are scattered by the wind to form new willow trees. These catkins are at an early stage of development.