Rocky Mountain Maple in the Landscape
Stephen L. Love, University of Idaho
Scientfic Name: Acer glabrumCommon Name: Rocky Mountain maple, rock maple, Douglas’ maple
Description: Rocky Mountain maple can be grown as a large shrub or trained to a small, single- or multi-trunked tree. This species provides a long season of beauty. In the spring, bright red, new growth appears. In the summer, soft green leaves cloak the plants. The most dynamic season is fall when the leaves change color and provide a dazzling display that ranges from bright, clear yellow, to intense pink-gold. In the wild, Rocky Mountain maple prefers moderately damp sites and often grows in sites with partial to deep shade. However, in the landscape, it can thrive under a wide range of situations – moderately xeric to moist, full sun to part shade, and alkaline to acidic soil conditions. This species has outstanding characteristics to serve in a specimen role or to simply fill a small space in the landscape.
Native Habitat: Riparian areas, wooded sites, canyon bottoms, and upland slopes throughout the western United States, including the Intermountain West. Often grows as an understory feature among large trees.
Cultural Requirement
Soil: Prefers slightly acidic soils but tolerates moderate alkalinity. Displays the best health in a deep, loamy soil.
Moisture Tolerance: Surprisingly drought tolerant, although this species will require periodic supplemental irrigation. Thrives in moderately dry shady sites. Grows very well in situations wherein soil moisture is consistently high (but not constantly saturated).
Sun/Shade/Preference: Thrives in a range of sun exposures, from moderately deep shade to full sun. Performs well in the understory of large trees.
Transplanting: Potted plants handle transplanting into the garden without serious issues. Transplanted trees will need frequent irrigation until fully established.
Propagation:
Recalcitrance with regard to typical propagation techniques is the most limiting factor for use of Rocky Mountain maple in the nursery landscape industry. This species is difficult to grow from seeds due to low seed germination rates. Significant overseeding will be required to produce seedlings in adequate numbers. Seeds should be collected in the late summer just as the samaras begin turning from green to light brown. Seeds of Rocky Mountain maple should not be stored for extended periods of time prior to planting due to tendency to generate deep dormancy. Shortly after harvest, soak the seeds for up to a week in room-temperature water (with frequent changes or with use of a bubbler). Sow the seeds and hold them at room temperature for up to 4 months. Subsequently, cold-stratify the seeds for at least 12 weeks. Seeds may also respond to fluctuating day/night temperatures during cold stratification. After seedlings emerge, either plant into flats and then transplant into deep pots, or plant directly into deep pots. Depending on the desired product size, plants may need 2 to 3 years of growth and be up-potted one or two times prior to sale.
Vegetative propagation is purportedly feasible using stem cuttings, although very limited research information is available referencing this technique. Success using stem cuttings has been variable and usually at low rates. This species can be propagated successfully using layering methods.
The most successful propagation of Rocky Mountain maple has been accomplished using micropropagation. This method also allows retention of the best landscape characteristics of selected individual plants. Best success has been achieved using Driver and Kuniyaki Walnut media fortified with iron and boron, multiplied in the presence of exogenous zeatin or meta-topolin, and rooted ex-vitro using a talc-based IBA treatment.
Maintenance (pruning, fertilization, deadheading, division, irrigation, etc): Rocky Mountain maple requires minimal care once plants have been properly trained and pruned. Determine the desired tree form, then begin removing unwanted basal branches while the tree is small. Occasional removal of sprouts growing from the base of the tree may be required. Supplemental irrigation is required in most Intermountain regions to maintain healthy growth and create optimal appearance. Routine nominal fertilization may be required, especially if plants show poor vigor.
Insect, disease, or other problems: Pests and diseases are typically not a problem with Rocky Mountain maple.
Landscape Value
Use in the Landscape: Rocky Mountain maple can be used as a specimen plant for areas where a woody plant of limited size is required. This species can also be used to complement large-statured trees as an understory plant.
Weediness/Invasive Potential: This species shows no tendency for spread from either rhizomes or seeds. Rocky Mountain maple is not considered to be invasive nor aggressive.
Foliage: Stems of Rocky Mountain maple are smooth and red when young and turn to gray and take on a rough texture as the bark develops. Leaves are deeply lobed, relatively small for the genus, and have typical maple shape. In the fall, leaves take on bright shades ranging from bright yellow to dark pink-gold.
Flower: Flowers open in conjunction with emergence of new spring leaves. The flowers are small, yellow, and tend to be inconspicuous.
Timing: April-May.
Fruit: In early summer flowers develop into winged samaras which stay on the plant into fall.
Form: Small tree or upright multi-stemmed shrub.
Texture: Medium
Ultimate Size: Varies depending on provenance and botanical variety – typically within the range 15 to 30 feet tall and approximately half as wide.
Rate of Growth: Rocky Mountain maple grows at a moderate pace, can be ready for sale as large nursery stock within 2 to 3 years, and grows to mature size in the landscape within 5 to 6 years of planting into the landscape.
Suggested Plant Partners: Rocky Mountain maple is best planted alongside species with similar moisture requirements. Species partners for placement as an understory species may include bearberry (Arctostaphylos uva-ursi), oil shale columbine (Aquilegia barnebyi), river mallow (Iliamna rivularis), aspen daisy (Erigeron speciosus), Sandia Mountain alumroot (Heuchera sanquinea), and Oregon grape (Mahonia repens). When placed in a sunny site, surround Rocky Mountain maple with species that require some supplemental water, such as harebell (Campanula rotundifolia), small-leaf alumroot (Heuchera parvifolia), mountain penstemon (Penstemon montanus), Rocky Mountain iris (Iris missouriensis), firechalice (Epilobium canum), and alkali sacaton (Sporobolus airoides).
Availability: Due to difficulties with propagation, this species may be hard to find but is occasionally available at nurseries specializing in western native plants.
Cultivars: None.
References:
Hathaway NA, Love SL, Tripepi RR. 2020. Micropropagation methodology for Douglas maple (Acer glabrum var. douglasii). Native Plants Journal 21(3):353-358.
Meyer SE, Kjelgren RK, Morrison DG, Varga WA. 2009. Landscaping on the new frontier. Utah State University Press, Logan, UT.
Robson KA, Richter A, Filbert M. 2008. Encyclopedia of Northwest native plants for gardens and landscapes. Timber Press, Portland, OR.