Catkins are produced by many trees. The most noticeable catkins are those produced by the hazel in late winter and early spring but other trees like the oak also have catkins. Oak catkins grow after the tree has come into leaf and are difficult to see.

There are two kinds of catkin. One kind is made entirely of male flowers and is called a male catkin and the other kind is made entirely of female flowers and is called a female catkin. A tree may have both male and female catkins. Usually the male catkins grow first and release their pollen then the female catkins grow and open shortly afterwards to receive pollen. By releasing and preparing to receive pollen at different times the tree has less chance of receiving its own pollen and a greater chance of receiving pollen from other trees. The pollen from other trees can produce stronger offspring.