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Populus balsamifera ssp. trichocarpa
black cottonwood
Western North America from southern Alaska through British Columbia, Washington, Oregon, and California, east to western Montana and Idaho; the dominant riparian tree of Pacific Northwest lowlands, growing along rivers, streams, floodplains, and moist bottomlands from sea level to 7,500 feet (2,300 m)
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Overview
Populus balsamifera ssp. trichocarpa is the largest native broadleaf tree in western North America, a fast-growing deciduous tree in the family Salicaceae reaching 80-150 feet (24-46 m) tall and 30-60 feet (9-18 m) wide in favorable riparian conditions. The crown is narrowly conical in youth and becomes broadly columnar to open and irregular with age. The trunk is straight and reaches 3-5 feet (0.9-1.5 m) in diameter on mature specimens, with smooth yellowish-gray bark on young trees that becomes thick, deeply furrowed, and dark gray-brown with age. Leaves are alternate, simple, broadly ovate to deltoid, 3-6 inches (8-15 cm) long, glossy dark green above and silvery-white beneath — the two-toned coloring creates a shimmering effect as wind turns the leaves and is the trait that carries the tree's identification at distance across a river valley. Leaf buds and emerging foliage are heavily resinous and release a strong balsam fragrance noticeable from considerable distance, particularly on warm spring days when the resin volatilizes; the fragrance is the source of the species epithet balsamifera. The subspecies is dioecious with male and female flowers on separate trees, and pendulous catkins open in March and April before leaf emergence. Female trees produce cottony seeds in May and June that disperse widely by wind and water over a 2-3 week period and travel considerable distances; male clones can be selected for cultivation to avoid the cottonseed litter. Growth is very fast at 36-60 inches (91-152 cm) per year in moist conditions, and the subspecies is planted in riparian restoration for rapid canopy establishment. Individual trunks are relatively short-lived at 100-200 years, but root systems continue to resprout and regenerate the stand over time. The wood is soft, weak, and brittle, and limb breakage in storms is frequent. Root systems are extensive, aggressive, and shallow, and they invade sewer lines, drainage systems, and building foundations in positions within several hundred feet of a planted tree. The subspecies is not suited to residential landscapes or confined urban spaces because of the combination of very large size, aggressive roots, brittle wood, and cottonseed litter from female trees.
Native Range
Populus balsamifera ssp. trichocarpa is native to western North America from southern Alaska south through British Columbia, Washington, Oregon, and California, and east to western Montana and Idaho. The subspecies grows predominantly along rivers, streams, floodplains, and moist bottomlands from sea level to 7,500 feet (2,300 m) elevation and is the dominant riparian tree throughout the Pacific Northwest lowlands. In its native range the subspecies anchors cottonwood-dominated floodplain forests that support beaver, salmon-rearing habitat through shade and large-wood recruitment, and a range of cavity-nesting birds and mammals.Suggested Uses
Used primarily in riparian restoration, streambank stabilization, salmon habitat enhancement, and large-scale ecological plantings at 15-30 foot (4.5-9 m) spacing in zones 3-9. The subspecies establishes rapidly from hardwood cuttings or live stakes inserted directly into moist soil — a practical technique for large-scale planting projects along rivers and streams where nursery-grown stock would be impractical. The fast canopy establishment makes the subspecies a nurse crop for slower-growing conifers in Pacific Northwest reforestation projects, where cottonwood shade protects seedling Thuja plicata, Picea sitchensis, and Tsuga heterophylla through their first decade. Not suited to residential landscapes, small parks, or urban forestry positions because of very large mature size, aggressive roots that invade sewer lines and foundations, brittle wood prone to storm breakage, and cottonseed litter from female trees during the May-June dispersal window. Male clones are substituted in positions where cotton litter is a concern.How to Identify
Appearance
Size & Dimensions
Height80' - 150'
Width/Spread30' - 60'
Reaches mature size in approximately 20 years
Colors
Bloom Information
Pendulous catkins open in March and April before leaf emergence, lasting approximately 2 weeks. Male catkins are reddish and 2-3 inches (5-8 cm) long; female catkins are green and elongate to 4-8 inches (10-20 cm) as seed capsules develop. The subspecies is dioecious with male and female flowers on separate trees and is wind-pollinated. Female trees release cottony seeds in May and June over a 2-3 week dispersal period, and these seeds travel considerable distances by wind and water. Male clones can be selected for cultivation to avoid cottonseed litter in positions where the fluff is a concern.Detailed Descriptions
Flower Description
reddish male catkins 2-3 inches (5-8 cm) long and green female catkins elongating to 4-8 inches (10-20 cm); the species is dioecious with male and female flowers on separate trees; wind-pollinatedFoliage Description
glossy dark green above and silvery-white beneath; alternate simple broadly ovate to deltoid leaves 3-6 inches (8-15 cm) long with finely crenate-serrate margins; the two-toned coloring creates a shimmering effect as wind turns the leavesGrowing Conditions
Sun Requirements
Requires 6-12 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight
Care & Maintenance
Care Guide
Plant in moist to wet soil with a pH of 5.0-8.0 in full sun; tolerated soil types include loam, clay, sand, and silt. The subspecies thrives in seasonally flooded sites and along active floodplains and is intolerant of shade and dry conditions — cultivation outside of moist bottomlands or actively irrigated ground produces stunted growth and early decline. Water is not a limiting factor in riparian restoration plantings established from live stakes in moist soil, and supplemental irrigation is usually not needed after the first growing season in appropriate sites. The subspecies is not suited to residential landscapes, parks, or urban sites because aggressive shallow root systems invade sewer lines, drains, and foundations within several hundred feet of the tree, brittle wood breaks in storms, and female trees release abundant cottonseed litter during the May-June dispersal window. Susceptibility to poplar borer, tent caterpillars, and various leaf rust and canker diseases is recurring, though established trees in appropriate riparian sites generally tolerate these without intervention. The primary cultivation use of the subspecies is riparian restoration, streambank stabilization, and large-scale ecological plantings rather than landscape ornamental.Pruning
Pruning is not a routine practice in ecological restoration plantings and is not needed on healthy trees in their native riparian habitat. In managed landscape positions where the subspecies has been planted for shade or erosion control, hazardous dead and storm-broken limbs are removed during the dormant season (January through February), and spring or summer pruning is restricted because sap bleeding is heavy at those times. Topping is not practiced because the resulting weak fast-growing water sprouts are more prone to failure than the original branches they replaced. Brittle wood and fast growth together produce frequent storm-damaged branches even on otherwise healthy trees, and routine cleanup after winter storms is part of maintenance for any planted specimen.Pruning Schedule
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