Rocky Mountain Maple

A small deciduous tree or large shrub native from southeastern Alaska south through the Rocky Mountains to New Mexico, found along mountain streams, on rocks, and in coniferous woods at 1,500 to 2,000 metres. Upright, multi-stemmed habit, often shrublike in dense settings but reaching 30 feet in the open. Leaves are opposite, 7 to 14 cm, with 3 to 5 lobes (occasionally divided into 3 leaflets), coarsely double-toothed, green above and blue-green below on long, often reddish petioles. Flowers are tiny, yellowish-green, 5 mm across. Samaras have nearly parallel wings, 18 to 22 mm. Fall color is yellow with a reddish tinge. Six recognized varieties span the western mountains.

Hardy to Zone 4a. Full sun to part shade, moderate moisture, adaptable to both acidic and alkaline soils (pH 4.5 to 8.5). Slow growth rate. Moderate drought tolerance once established. No cultivars in the trade. Nineteen diseases and fifteen pests documented at genus level, including Verticillium wilt, leaf spots, powdery mildew, and anthracnose. The slow growth and variable multi-stem form suit naturalizing in dry, rocky areas. Prune in summer to avoid excessive sap bleeding.

Rocky Mountain maple is a native species but primarily an east-slope and montane plant. In the Puget Sound lowlands, it is marginal: not as well-adapted to our poorly drained lowland soils and wet winters as vine maple, which fills the same ecological niche more reliably. Nineteen diseases and 15 pests are documented, mirroring the standard maple disease complex. If you want a small native maple for a lowland landscape, vine maple (Acer circinatum) is the better choice in almost every situation. Rocky Mountain maple has value in foothills gardens and east-side plantings where vine maple struggles with cold.

— Chris Welch, ISA Certified Arborist