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Prunus nigra Ait. Canada plum

Flowers fully open showing their five petals

Habitat

Canada plum is uncommon, but occurs as a scattered small tree, or sometimes in patches, in open areas in river valleys on alluvial soil, or over limestone. As such soils have often been developed for agriculture, the species may be found in pastures, in hedgerows, or along fence lines.

Small tree in understorey

Form

Canada plum can grow to heights of up to 9 m, and to stem diameters of up to 25 cm, but is usually smaller. It is a straggling tree, often developing in clumps, and commonly having upward grow-ing, crooked branches originating low down on the stem. The crown is irregularly rounded and usually flat topped.

Morphology

The thin, fragile, deciduous, simple leaves are borne alternately. Each leaf is 5–12 cm long, obovate to oval (broadest above or at the middle), rounded, indented, or broadly wedge shaped at the base, narrowing abruptly to a long, slender-pointed tip, and neatly double toothed around the margin, with each tooth rounded and often gland tipped. The upper surfaces are dull, dark green, and the lower surfaces paler with a prominent main vein that joins a short, stout petiole (leaf stalk) which carries a dark gland on each side near the base of the lamina (leaf blade).

Twigs and thorns

The twigs are slender, reddish dark brown, and marked with small, beige, horizontally extended lenticels. Many short, side shoots each end in a sharp thorn. The buds (which are all lateral) are greyish brown, ovoid, up to 8 mm long, often ap-pressed to the twig, with overlapping scales that tend to have thin, pale, frayed tips. Some buds on shoots below thorns are like tiny bulges.

The showy flowers are borne on stems (pedicels) 15–30 mm long, in clusters of two to four, from buds on the previous year’s twigs, including side twigs that end in thorns. Each flower is 15–25 mm across and has five white petals that change to pale pink as they age and begin to shrivel.

Black, shrivelled diseased fruits held on the tree over winter

Blooming occurs just before or as the new leaves start to extend on shoots from other buds. Fruits (plums) develop from some of the flowers. The ripe, reddish-yellow plums are 25–30 mm long. However, many fruits fail to develop because they become infected by a fungus, and shrivel to brownish, crinkled, oval masses hanging from their stalks. Many of these remain on the twigs over winter.

The young bark is shiny, reddish dark brown to black with horizontally elongated beige to grey lenticels. Older bark is first separated by vertical splits with edges curled back, but eventually becomes ruggedly scaly.

Bark of young stem, 2.5 cm in diameter

Notes

The plum fruit is somewhat sour to the taste but is excellent when cooked or in jam. Some cultivars (varieties cultivated by horticulturalists) have been developed and may be available for ornamental planting because of their attractive but short-lived flowers or for fruit production.

The shriveled fruit or “plum pockets” are caused by a fungal disease, Taphrina ssp. The fruit is distorted up to ten times its normal size and does not produce a seed. This hampers Canada plum regeneration and limits the use of the fruit for human consumption.

Bark of stem, 8 cm in diameter

Canada plum, like all members of the genus Prunus, also suffers from black knot, a fungal disease of twigs and branches caused by Apiosporina morbosa (sometimes Dibotryon morbosum in older books). It causes large, irregular, black swellings which can girdle and kill small branches in a few seasons, affecting overall vigor. If the main stem is infected, the entire tree may be killed.

Members of the genus Prunus can be distinguished by their fruit or by injuring the bark or foliage and smelling the pungent odor of almonds or cyanide. Children have been poisoned by eating seeds or chewing twigs. Tea made from the leaves of various cherries and plums is also poisonous.