Manitoba Maple (Acer negundo)

Issue: 
September-October 2009

THE CANADIAN PRAIRIES is not noted for its diversity of trees but Manitoba maple (Acer negundo) is naturally a tree of the grasslands of Manitoba to the eastern half of Saskatchewan. There, it grows mostly along the banks of lakes, streams and rivers. In fact, this species is naturally the most widely distributed of all North American maples, ranging from coast to coast and south to Guatemala! In the US, it ranges naturally from New York to Florida and west into the middle and southern Rocky Mountains. It even occurs naturally in California. There are small native stands in Ontario, but its common and familiar occurrence in eastern Canada, including the Maritimes and into New England and the Pacific Northwest, has come about through its being planted and becoming naturalized.

Species Gossip
Manitoba maple is often characterized as being an unpopular tree. It is described by some as “dirty” or “trashy” because it sheds, suckers, often has multiples stems, an irregular crown form, is prone to wind and ice damage, and is short-lived. Its high propensity for seeding into flower beds sometimes makes it a weed. Driving home even further damage, its wood is described as not being especially useful. It is even considered to be an invasive by some; and that may be valid in parts of China, Australia and Europe where it has been introduced. No wonder it has been written about in such derogatory terms.

But, let’s not dismiss this tree so fast – Manitoba maple has many saving graces. With care and pruning in its early years, it becomes a handsome, round-topped tree, and long-lived. It grows fastest in its first 20 years or so. Upon further inspection, it is also not as prone to injury from wind and ice as many other tree species. There is no better shade tree for the prairies, and it is effective as a wind-break. In North America and Europe, horticultural selections are grown for their interesting and variegated leaves, as well as for their use as shade. Manitoba maple is easily propagated, likes bright sunlight, grows quickly early in its life, and thrives on many kinds of soils. It is hardy and tolerates cold, heat, flooding and drought. It is tough and resistant to the ravages of most insects and fungal pests. Moreover, its leaves and young stems are characteristically and to me, attractively coloured. In short, the attributes of this native tree for arboriculture seem to far outweigh its faults.

Biology Basics
The leaves of Manitoba maple are unlike those of any other maple. They are light green, large, sometimes over 30 cm long, and compound with 3-9 leaflets that may be shallowly to quite deeply toothed and somewhat irregular. They turn yellow in fall. Linnaeus named the tree Acer negundo. The latter species name may be attributed to its leaf form as negundo refers to the medicinally useful shrub, Vitex negundo, called “nirgundi” in Hindi and “negundo” in Sanskrit. The common names used widely in the US – box elder maple, or simply box elder – and others refer to the form of the leaves resembling those of elders (Sambucus spp.) or ashes (Fraxinus spp.).

The stems are also an interesting study, being both multi-coloured and multi-textured. Young stems and shoots are green, with a whitish to pinkish-violet waxy coating that is rubbed off easily. As the branches age, they retain their smooth, fresh green appearance but slowly the bark becomes pale gray to light brown and eventually deeply and broadly ridged.

What about the flowers? Once the trees become mature at about 8 to 10 years, flowers are produced every year early in spring as the trees are leafing out. Manitoba maple is dioecious, meaning that the trees come in two sexes, male and female. The flowers of both sexes are greenish, without petals and sepals. Only the female trees produce seeds after wind pollination has served the needs of fertilization of the ovaries. The female flowers are borne on a central stem that droops so that the pistils are exposed to wind and the pollen it brings. The male flowers, each borne on a slender stem, are produced in bundles that allow them to be shaken by the wind thus liberating the pollen. Some trees may produce bisexual flowers that may set fruit. I do not know what the general sex ratio of the species is, but it would be interesting to know. In most dioecious plants, males predominate.

The fruit, samaras, paired and in drooping clusters, mature over the summer and are dropped in the fall and winter as the wind shakes them loose and distributes them. Thus, they have a lot of moisture and temperature options for germination. That, coupled with the wide variety of conditions its seedlings can tolerate, may help explain how these trees are so prolific. The seeds are important food for squirrels and birds throughout the winter months.

Mature trees are described as small to medium sized and short lived. Although these descriptors may hold true in natural stands, those of us familiar with this tree would say that trees reaching 15 to 20 metres in height with trunk diameters exceeding one metre in old trees are quite large. Certainly they can live more than 60 years as those of us who have them in our gardens and fence-rows know.

Upon Closer Examination
Horticulturalists have not been so quick to dismiss Manitoba maples. Some interesting cultivars include ‘Auratum’ with yellowish leaves with smooth undersides; several others with variegated leaves such as ‘Variegatum’ with its creamy white leaf margins; ‘Aureomarginatum’ with creamy yellow margins; ‘Flamingo’ with pink and white variegation; ‘Elegans’ has distinctly convex leaves; and the hardy cultivar ‘Baron’ is seedless (for those worried about the propensity for seeding). Where can one obtain those cultivars? The trees can be propagated vegetatively by cuttings and grafting.

Apart from horticultural, shade tree and basic botanical interest, Manitoba maple has been maligned for having few uses. However, its fast growth and fibrous root system make it a prime candidate for erosion control. Its capacity for coppicing suggest it could be used for biofuel production. Its light, close-grained, soft wood has little resemblance, except in colour, to the fine and dense wood of true box trees (Buxus spp.) and is considered undesirable for most uses. Even so, burls and knots have sometimes been made into bowls, dishes and drums by First Nations’ peoples.

The wood is burned for heat, cooking, various ceremonies and the smoke is reported to have medicinal value. The charcoal has been used traditionally for fuel and tattoos. Twigs can be hollowed out (de-pithed) and used for pipe stems and bellows tubes. The trees can be tapped for sap, which can then be boiled down to make syrup. Some First Nations’ peoples use scraping of the sweet inner bark to make an emetic infusion, or the scrapings have been mixed with other scrapings from the inside surface of animal hides, dried, and made into candy.

In summary, Manitoba maple is sweet, tough, handsome, widespread, useful and versatile, vigorous maybe to a fault, but certainly not “trash” by my standards!

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