This past week I was walking along the edge of a woodland, when I spotted a beautiful branch of sassafras flowers. It was such a pretty sight with the new leaves in the center, that at first I thought the new leaves were the flower. But, no, it was a circle of flowers around the leaves. It was still chilly so there was not a flying pollinator anywhere. How would these little flowers be pollinated?
When I returned home, my neighbor’s maple tree was full of little flowers that preceded the leaves opening. They were out in the open, blowing in the wind.
Then I remembered. These little flowers are wind pollinated. How can the pollen from these little flowers on the end of a branch possibly find another flower and attach itself to the pistil of the flower to pollinate it?
Because this is not an easy target, these flowers must produce a much bigger batch of pollen than an ordinary garden flower. In order for some pollen to hit the target flower, wind pollinated flowers often produce millions of small lightweight pollen grains that can be swept by the winds to their designated flower pistils.
If you are allergic to pollen, you have probably suffered from wind pollinated pollen blowing around in the air currents. I have brushed huge deposits of pollen off my car windshield, and seen pollen spread across a parking lot surface. Too bad that all those little pollen grains will be washed away in the next rain and will not propagate a flower.
Trees are among the most common plants to use wind as their pollinator. Many of our most common trees such as maples and pines create pollen to be blown by early spring gusts. Their pollen can be spread before the leaves come out on the trees and before the busy insects are warm enough to be helpful. It also allows the trees to pollinate their flowers before they produce leaves that will block access from one flower to another.
As we all know, spring is a very windy time. Single catkins of a birch tree may produce as many as 3 to 5 million pollen grains. A single stalk of wheat may produce 40,000-70,000 grains of pollen (Willmer, Pat, Pollination and Floral Ecology, Princeton University Press, 2011). Plants pollinated by insects rarely approach these numbers, because insects usually travel from one species of flower to another of the same species, assuring a much higher likelihood of completed pollination.
Wind pollination works in the early spring, when it is too cool for insects, but it is also common in the summer and autumn for grasses that produce grain to be wind pollinated.
Goldenrod, which is often blamed for allergies, has heavy sticky pollen which cannot be carried by the wind. But its neighbor, ragweed, has pollen that is light and dry. Ragweed is very dull colored, so is often not recognized in a field of glowing yellow goldenrod.
Common ragweed, Ambrosia artemisiafolia, is native to the Americas. Its seeds are spread by birds. There are over 650 species of the Ambrosia genus worldwide. Almost all of them create allergies, because the lightweight pollen is easily lifted by the wind.
We don’t have to worry about ragweed until much later in the summer. For the time being, we can watch the new leaves unfurl in the trees along the boulevards and be grateful that their growth marks the end of the tree pollen season.
