Confusing Cottonwoods

Issue: 
September-October 2011

Cottonwoods are poplars. Balsam poplar (Populus balsamifera), black in BC and western Alberta and narrow-leaved in southern Alberta (P. trichocarpa and P. angustifolia), form one group, section Tacamahaca. But on the prairies, trees of the section Aigeiros grow, commonly called the plains cottonwoods, and then to the south and east as far as the St. Lawrence valley in Quebec, the eastern cottonwood is resident. The characteristically triangular leaves provide the scientific name P. deltoides for both the plains and eastern cottonwoods, though the plains is sometimes given its own species name, P. sargentii. Even though now regarded as the same species, the eastern and plains cottonwoods are differentiated with scientifically distinct sub-specific names, P. d. deltoides, and P. d. monilifera

And to make things even more confusing, eastern and plains cottonwoods hybridize freely between themselves resulting in a large part of their ranges containing integrations of the two types. Then, they also hybridize with several native species such as balsam poplar, trembling aspen and large tooth aspen, as well as with the introduced Lombardy poplar (P. nigra). For those who like the taxonomic specifics, Populus x acuminata is a hybrid of P. angustifolia and P. deltoides. Populus x bernardii is a hybrid of P. deltoides and P. tremuloides, Populus x jackii is a hybrid of P. balsamifera and P. deltoides, and Populus x polygonifolia is a triple parent hybrid of P. balsamifera, P. deltoides and P. tremuloides. Tree breeders have created many hybrids between eastern cottonwood and other poplars for the arboricultural trade.

Habitat, Growth & Lifespan
Back to the eastern or plains cottonwood, P. deltoides. Growing along watercourses, these sometimes huge trees shade the water and stabilize the banks. They grow where the water table is close to the soil surface and are intolerant of drought. This species is one of the largest, fastest growing North American hardwood trees and can reach 20-40 m tall with a trunk up to 1.8 m in diameter. When they grow in the open, the trunk is often short, massive and divided into a few large, spreading limbs that support a large, irregularly shaped and open crown. Forest growing trees tend to have single, straight trunks that support a rounded crown in the forest canopy. Typically they live for up to about 100 years, but much older trees are known. 

Biology Basics
The bark of the trunks of old trees is dark grey and deeply furrowed, but younger trees and branches have smooth, yellowish grey bark. Growing twigs are yellowish brown and quite stout. They seem angular, with narrow ridges that extend from either side of the buds. The winter buds are characteristically very sticky, long and pointed and yellowy-brown. The terminal buds, up to 2 cm long, are longer than any of the several lateral ones below. The leaf scars on the twigs are large and triangular, often with conspicuous traces of the basal venation of the previous year’s fallen leaf. 

The leaves flush late in spring after flowering has taken place. They are large, about as long as they are wide, from 4 to 10 cm. Although roughly triangular, they have a broad, straight base that then curves to form the characteristic toothed leaf margins that taper to a point. They are dark, shining green in the summer and appear to have a translucent margin. The leaves turn yellow in fall, but not so spectacularly as in the trembling aspen. Trees growing in dry places drop their leaves early, but those growing along watercourses can retain their dead, brown leaves well into winter. Plains cottonwood has fewer and larger teeth (5 to 15 per side) than does the eastern cottonwood with 20 to 25 per side. The leaf stalk (petiole) tends to be a bit longer than the leaf and is somewhat flat which allows for the leaves to shake in even a slight breeze. 

Like other poplars, eastern and plains cottonwoods are either male or female (the species is dioecious), with a sex ratio of about 1:1. The catkins’ buds form in the summer, but it is not until the next year that they mature. Maturation takes place before the trees’ leaves flush in early spring. Even so, actual timing of catkin production varies greatly from tree to tree and stand to stand. The male (pollen) catkins are reddish-purple, 8-10 cm long and the smaller female catkins are green, 7-13 cm long at pollination. During pollination by wind, pollen is shed from the 40 to 60 anthers on each male catkin in huge amounts and a day or so ahead of the female catkins maturing. Only a small proportion of the pollen shed is caught on the stigmas of the female catkins. After sexual fertilization, female catkins grow to 15-20 cm long as the 30-60 capsules each expand to 6-15 mm long. Once ripe, early in summer, about 1.5-2 months after flowering, the capsules split three or four ways and release the fluffy cotton and attached small seeds. The amounts produced are huge and sometimes thought of as a nuisance.

Seed production starts when the trees are as young as 5 to 10 years old and increases rapidly with age and growth. Cottonwoods usually produce good seed yields. It has been estimated that the annual seed production of a single open-grown tree can be as high as 48 million seeds. About 35 litres of fresh mature catkins yields about 1 kg of seeds for about 770,000 cleaned seeds.

Dispersal of the seeds via wind and water results in abundant seed banks along watercourses, especially as spring high waters recede. To germinate, the seeds need bare soil and full sun such as are found along muddy riverbanks and bars. The seeds germinate the same year they are shed and after only a few days. Germination rates may be very high (90% or more) but the seedlings are tender and grow slowly so many die of heat, exposure and damping-off. 

Although propagation from seed is easy, seedlings do require attention and water. Vegetative propagation is also simple. Healthy wands (about 2 m long) can be cut in fall and winter from sprouting stumps and coppiced trees. The wands can be cut into segments about 20 cm long, treated against fungi, stored in cool moist conditions, and planted in sandy soil in propagation houses for later transplanting. The segments from the basal end of the wand’s root establish more quickly than do segments from higher on the wand. Wands can be planted directly into moist soil (sometimes called the live stick means of propagation) and often take root. 

Once a seedling has become a sapling, or the wand has rooted, growth is stupendously fast. The young trees add height at rates of over 2 m per year, rapidly gain girth, and produce a huge spreading and deep root system. Growth slows after the first 25 to 30 years.

Multiple Uses
Lumber for cottonwood is light and the wood is rather soft. Nevertheless, it is stiff and thus has served almost every imaginable use. During the times that pioneers settled the prairies, it was used green for building houses, barns, fences, churches, schools, coffins and furniture. It does not split when nailed. However, it does tend to warp and check as it seasons, and rots quickly when in contact with the ground. Nowadays it is used primarily in manufacturing padded furniture frames, for making wooden pallets, boxes and crates (stenciled printing looks nice on the pale, clean wood) and for pulpwood. Eastern cottonwood is one of the few hardwood species that is planted and grown specifically for these purposes, and as such is one of the fastest growing commercial forest species in North America. The use of the trees for production of fibre, reconstituted wood products (chip and fibre board) and biomass production are derived from short-rotation operations (2 to 8 years). 

Cottonwood can also be used for firewood. It burns quickly but does not leave a bed of coals. Amerindians used cottonwood to start fires by friction.

Recently, cottonwoods have been produced and used for roughage food for livestock. The feed produced is quite rich in protein and minerals and free of tannins, so palatable to livestock. This palatability is also appreciated by browsing deer, rabbits, hares, rodents and beaver, which also use the sticks they cut for dam and lodge construction.

More specific to arboriculture, cottonwoods are utilized in landscaping. The male trees are preferred because they do not produce the fluffy cotton that some people find a nuisance. They are used for windbreak plantings on the prairies. Their fast growth and canopy makes them an excellent shade tree for lawns and parks when large spaces are available. The bark of both old and young trees is attractive, as are the leaves. Beware though, this is not a tree to plant near irrigation tile, sewers, septic tanks, drains or sidewalks. The roots seek out water and can invade pipes. The fast growth of both the aboveground and belowground parts of the tree can cause heaving of sidewalks and landscape installations. They sometimes tend to produce suckers and occasionally, the trees suffer from canker and breakage.

Susceptibility to Pests & Disease
Cottonwoods are host to quite an array of destructive insect herbivores, ten species are well known, and some can reach population densities of pest proportions. The most damaging are defoliators and wood borers, each exhibiting characteristic damage on particular parts of the trees. The former cause loss of vigor and the latter reduce lumber quality. Important defoliating insects are the cottonwood leaf beetle (Chrysomela scripta), cottonwood dagger moth (Acronicta lepusculina), forest tent caterpillar (Malacosoma disstria), poplar leaffolding sawfly (Phyllocolpa bozemani), fall cankerworm (Alsophila pometaria), tentmaker (Ichthyura maimbachiana) and the fall webworm (Hyphantria cunea). 

Borers include the poplar borer (Saperda calcarata), cottonwood borer (Plectrodera scalator), flatheaded wood borer (Dicerca divaricata), carpenterworm (Prionoxystus robiniae), poplar-and-willow borer (Cryptorhynchus lapathi), clearwing borers (Paranthrene spp.) and the bronze poplar borer (Agrilus liragus). Several mites and aphids infest cottonwood, but not often with fatal effects.

And if the insect pests are not enough, cottonwoods are also susceptible to a variety of diseases. Twelve are well known. For example, Septoria musiva causes small cankers that allow other canker-forming fungi to invade the trees. Fusarium solani enters such wounds creating larger cankers. Two other canker-producers are Phomopsis macrospora and Botryodiplodia theobromae. On vigorous trees, cankers often callus over. The leaves of cottonwoods are sometimes afflicted by the rust Melampsora medusa and leaf-spot fungus Marssonina brunnea that cause premature leaf-fall and reduce the trees’ growth. Septoria musiva causes both leaf spots and canker. Then, there are diseases that affect the roots and butts, such as those caused by Ganoderma lucidum, Armillaria tabescens and Scytinostroma galactinium.

The Right Tree For The Right Place
Despite the perceived wide range of problems facing cottonwoods, from hazards in their physical environment to those presented by their natural animal and fungal enemies, they thrive where and when conditions are right. Their value to riparian ecosystems, especially in the prairies as stabilizers of river and stream banks, shaders of those waterways, food for wildlife, and habitat for nesting birds, is huge. Their value to human beings for lumber, fibre and horticulture is growing, particularly as the gene pool of cottonwoods is being incorporated in hybridization and selectional arboriculture. Confusing – maybe – but beautiful, resilient, tough and valuable from both an environmental and economic standpoint. 

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