Cottonwood
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- This article is about the poplar plant. For other uses of the term, see Cottonwood (disambiguation).
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| Missing image Populus_deltoides.jpg Plains Cottonwood Populus deltoides subsp. molinifera | ||||||||||||
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Populus deltoides L. |
The cottonwoods are three species of poplars in the section Aegiros of the genus Populus, native to North America, Europe and western Asia.
The name is also commonly used for any species of poplar in the United States, including some poplars not in the section Aegiros, notably the Swamp cottonwood Populus heterophylla (in the section Leucoides) and the Black and Narrowleaf Cottonwoods P. trichocarpa and P. angustifolia (balsam poplars in the section Tacamahaca).
Those in section Aegiros are large deciduous trees 20-45 m tall, distinguished by thick, deeply fissured bark, and triangular-based to diamond-shaped leaves, green on both sides (without the whitish wax on the undersides of balsam poplar leaves), and without any obvious balsam scent in spring. An important feature of the leaves is the petiole which is flattened sideways, so that the leaves have a particular type of movement in the wind. The aspens (Populus section Populus) share this characteristic, but not the balsam poplars.
Male and female flowers are in separate catkins, appearing before the leaves in spring. The seeds are borne on cottony structures which allow them to be blown long distances in the air before settling to ground.
The cottonwoods are exceptionally tolerant of flooding, erosion and flood deposits filling around the trunk.
In the past up to five or six species were accepted, but recent trends have been to accept just three species, treating the others as subspecies of P. deltoides.
The Eastern Cottonwood Populus deltoides is one of the largest North American hardwood trees, although the wood is rather soft. It is a riparian zone tree. It occurs throughout the eastern United States and just into southernmost Canada. The leaves are alternate and simple, with coarsely-toothed (crenate/serrate) edges, and subcordate at the base. The leaf shape is roughly triangular, hence the species name, deltoides.
In the typical subspecies deltoides (Vermont south to northern Florida and west to about Michigan), the leaves are broad triangular, 7-15 cm across at the base. Further west (Minnesota south to eastern Texas), the subspecies molinifera (Plains Cottonwood; syn. P. sargentii) has somewhat narrower leaves 5-10 cm wide at the base. This is also the state tree of Wyoming. In western Texas, New Mexico and Colorado the subspecies wislizenii (Rio Grande Cottonwood; syn. P. wislizenii) occurs.
The Fremont Cottonwood Populus fremontii occurs in California east to Utah and Arizona and south into northwest Mexico; it is similar to Eastern Cottonwood, differing mainly in the leaves having fewer, larger serrations on the edge, and small differences in the flower and seed pod structure.
The third species, Black Poplar Populus nigra, native of Europe and western Asia, is distinct in its much smaller leaves, 5-11 cm across, with a more rhombic (diamond) shape; see the link for further details.
Cultivation and uses
Cottonwoods are widely grown for timber production along wet river banks, where their exceptional growth rate provides a large crop of wood within just 10-30 years. The wood is coarse and of fairly low value, used for pallet boxes, shipping crates and similar, where a coarse but cheap and strong wood is suitable. They are also widely grown as screens and shelterbelts. Many of the cottonwoods grown commercially are the hybrid between Eastern Cottonwood and Black Poplar, Populus x canadensis (Hybrid Black Poplar or Carolina Poplar).
There is a story of a traveling salesman that once planted a cottonwood seedling in the middle of one of the upper plains states, many miles from the next nearest tree, and watered it on his route every week. The tree survived and, once its roots were able to penetrate the water table, needed no further watering and lived for many years.