Tree Sex Part 3

Black alder flowersSome trees are pollinated entirely by wind.  Many wind-pollinated hardwood trees make complex slender compound flower structures called catkins.

Later this spring, we will see catkins in all our oak trees, mulberries, walnuts and chestnuts, and further north on true poplars. Now, in early spring, you will notice catkins on alders, willows and birches.

Male catkins are large and long, getting longer as they expose their flowers to the wind. The female catkins are much smaller, often hardly noticed.  The female catkins are not open to receive pollen when the male flowers are releasing it – this ensures that the female flowers are not covered in pollen from the same tree.

This tree is European black alder, Alnus glutinosa.  Although not native to North America, it was introduced as an ornamental and has become naturalized throughout the east.

Click the video to see wind stirring the catkins, releasing pollen into the breeze. The catkins are very flexible. In even a modest wind, the catkins shake, releasing the light pollen into the wind.



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