White Poplar is a large much branched tree, introduced to North America from Europe as early as colonial days, reported in New England as early as 1785. It typically grows to 80 feet in height (130 feet known) with a trunk that usually leans and narrow irregular crown. Largest diameter is usually to 2 feet (but 7 feet known). Trunks can easily be crooked if the tree is growing in confined spaces. The crown can be rounded if the tree is unobstructed.
Bark: The young bark is greenish-white, smooth, developing small lenticles which are blackish and diamond shape, which then enlarge with the bark becoming rough, dark, and furrowed near the base. Twigs are gray to reddish-brown and are usually covered with dense white hair. Buds are ovate and pointed, reddish brown, and with some white hairs. Twigs chewed have an aspirin taste.
The leaves are 2-1/2 to 5 inches long, almost as wide, ovate in shape with some irregular teeth. Later leaves develop 3 or 5 lobes resembling maple leaves. Young leaves on older twigs will not have the well-defined lobes (see next paragraph). The base is truncate to slightly heart-shaped. The upper surface is a bluish-green, the underside dense with white hair, as is the leaf stalk. The stalk is flattened as with most Populus species, causing a trembling in the wind. Autumn color in some areas may be reddish but normally is undistinguished.
P. alba is strongly heterophyllous, that is, it has two types of leaves - those that are formed in the winter bud for early spring growth, known as preformed or early leaves and leaves produced later in the season known as neoformed leaves. The later leaves will be those that resemble small maple leaves and are the last to fall from the tree in Autumn. The shape difference in P. alba between the preformed and the neoformed leaves is unique in the Populus genus.
Flowers: P. alba is dioecious, that is male and female flowers occur on separate trees. Female flowers (pistillate) are in dense 1 to 3 inch long greenish-gray drooping catkins. Female individual small flowers have a basal disk, a 2-chambered ovary, pistil with a pair of stigmata at the tip. Male (staminate) catkins are slightly longer, a stalked basal disk with 6 to 12 reddish stamens. Both flowers have a small brown bract obscuring them, whose upper edge is deeply cut. Flowers appear before the leaves and are wind pollinated.
Fruit: The U.S. Forest Service reports that male trees are rather rare and most seed production is restricted to White Poplar hybrids, which are difficult to distinguish from each other and from the real thing. Fertilized female flowers mature on the catkin to a 5 mm long egg shaped seed capsule that contains two small seeds with white fluffy pappus. Seeds are mature by late Spring or early Summer; germination rate is high.
Habitat: White Poplar prefers moist well-drained soils and is found along roads and fields. It can tolerate some drought and occasionally water-logged soils. The root system suckers a great deal creating many sprouts and can become very weedy. It is very tolerant of urban pollutants and can be a measure of soil toxicity (see notes below). White Poplar can hybridize with Quaking Aspen and Bigtooth Aspen resulting in Populus × heimburgeri B. Boivin (Heimburger's poplar), a cross between P. alba × P. tremuloides (Quaking Aspen) and Populus × rouleauiana B. Boivin (Roulwau's poplar), a cross between P. alba × P. grandidentata (Bigtooth Aspen)
Names: The genus Populus is the Latin name for the Poplar. The species name, alba is Latin for 'white'. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: White Poplar is identified by the greenish-white bark of young branches with numerous small black lenticels and the 3 to 5 lobed leaves with white hairy undersides - the only species of the genus to have such.
Above: White Poplar in Summer dress and in Spring with buds swelling. Trees in more confined places will have a narrow conical crown.
Below: 1st photo - Many times a trunk will be leaning when growing in habitat with other trees. 2nd photo - The lower bark on older trees becomes furrowed and scaly on the ridges. 3rd photo - bark on the upper trunk and on young trees is greenish-white with many black lenticles.
Below: Twigs are reddish brown but covered with dense whitish hair. The conical buds are also reddish brown with some hair.
Below: 1st photo - the neoformed leaves develop lobes that resemble maple leaves while preformed leaves -2nd photo - have small lobes of just rounded teeth or are just ovate in shape. Both can be on the same tree. 3rd photo - detail of the white hairs on the leaf underside.
Below: The crown of the tree has several main branches and many smaller ones.
Notes: White Poplar is found throughout the United States except Arizona and in the lower Canadian Provinces of British Columbia and those from Manitoba eastward except for New Brunswick and Labrador. Its spread can be attributed to seeding in local areas, clonal root spreading and planting for ornamental purposes. In Minnesota the DNR reports populations in nine widely scattered counties (excluding landscape specimens). This short list includes Anoka, Hennepin and Ramsey in the metro area. It is not known to have ever been planted at Eloise Butler.
There are only four species of Populus that are native and commonly found in Minnesota: P. balsamifera, Balsam Poplar; P. deltoides subsp. monilifera, Plains Cottonwood; P. grandidentata, Bigtooth Aspen; and P. tremuloides, Quaking Aspen. Two others are reported: One is a native cross - P. X jackii, Jack's Cottonwood, which is usually sterile [The DNR does not track county populations of it]; and the other is the introduced P. alba, White Poplar.
Uses: White Poplar is planted as a landscape specimen and for windbreaks. The wood which is soft and white does not have much commercial use today. Studies in Spain have indicated that the leaves can be used as a biomonitor of the level of pollution of trace elements in the soil and atmosphere, especially cadmium, lead, zinc, copper and arsenic. [White Poplar (Populus alba) as a biomonitor of trace elements in contaminated riparian forests - Madejon P., Maranon T., Murillo JM, Robinson B.; Environ Pollut., Nov. 2004.]
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"