Aspens, cottonwoods, poplars – all are names for certain species of trees in the genus Populus. These trees, as well as birch, hickory, oak and willow trees, produce their flowers on spikes called catkins. Telling the catkins of these trees from one another is challenging, to say the least, yet some of NC’s readers correctly identified the catkins in the photograph as those of Bigtooth Aspen, Populus grandidentata. This tree blooms for one to two weeks in the spring and its mature male catkins open and extend to two to four inches in length. The wind, as opposed to insects, disperses the light, fluffy yellow pollen as the catkins dangle in the breeze. Some of the pollen remains intact even after the tree has shed its spent catkins onto the ground.
Because Bigtooth Aspen, and most species of Populus, are dioecious (male and female flowers develop on separate trees), there are only male flowers in this photo and beneath this tree. After fertilization, female flowers remain on the tree and form capsules which contain several small seeds embedded in tufts of fine, white hair. They will fill the air in several weeks looking like bits of floating cotton.
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Even with sub-zero temperatures and feet of snow on the ground it is possible to find signs, such as pussy willows, that spring really is around the corner. What we call pussy willows are, in fact, the soft, silvery hairs that insulate the emerging spike of flowers, or catkin, within a willow flower bud. A willow catkin consists of all male or all female flowers. The first catkins to emerge in the spring are usually males. The hairs, or “pussies,” that emerge when willow buds first open trap the heat from the sun and help warm the center of the catkins, where the flowers’ reproductive parts are located. This trapped heat promotes the development of the pollen (or in female flowers, the ovules) of the flowers deep within the hairs. Eventually the reproductive parts of the willow flowers – the stamens and pistils – emerge, but until they do, we get to enjoy their silvery fur coats.
Even though it snowed in Vermont this weekend, there is something else white and fluffy that is also being blown about, and it doesn’t melt when it hits the ground. The tiny white bits of fluff that are floating in the air are the seeds of aspens (also referred to as poplars), primarily Bigtooth Aspen (Populus grandidentata), that are borne in capsules that develop along a 3 to 6-inch dangling stem. These former flower clusters, and the capsules and seeds they developed into, are referred to as catkins. The capsules split apart when the seeds are mature, releasing the cottony-tufted seeds that are well-designed for dispersal by the wind. Looking into the fluff-filled sky, it’s not hard to believe that a single Bigtooth Aspen tree can produce over a million seeds.