Rocky Mountain Maple (Acer glabrum)

The rocky mountain maple (Acer glabrum) is a small deciduous tree native to western North America and common in the Rocky Mountains. Its broadleaf foliage turns crimson in autumn. The tree grows deep roots, and so can be used to help with erosion control. Commonly found along streams and in similar damp habitats.

I. Appearance and Characteristics 

Acer glabrum is a species of maple native to western North America, from southeastern Alaska, British Columbia and western Alberta, east to western Nebraska, and south through Washington, Oregon, Idaho, Montana and Colorado to California, Arizona, Utah, and New Mexico.

Acer glabrum is a small tree growing to 6–9 meters (20–30 feet) tall, exceptionally 12 m (39 ft), with a trunk around 13 centimeters (5 inches) in diameter, exceptionally around 25 cm (10 in). The leaves are 2–13 cm (3⁄4–5 in) broad, three-lobed (rarely five-lobed), variable in the depth of lobing, occasionally so deeply lobed as to be divided into three leaflets; the lobes have an acute apex and a coarsely serrated margin. The flowers are produced in combs of five to ten, yellowish-green, at the same time as the new leaves in spring. The fruit is a samara or winged seed, which develops in fused pairs at an angle of less than 45° when mature, though some varieties spread out to 90°.

Varieties

There are four to six varieties, some of them treated by some authors at the higher rank of subspecies:

• Acer glabrum var. glabrum (syn. subsp. glabrum; Rocky Mountain maple)– Rocky Mountains, Montana to New Mexico
• Acer glabrum var. diffusum (Greene) Smiley (syn. subsp. diffusum (Greene) A.E.Murray; Rocky Mountain maple) – eastern California, Nevada, Utah
• Acer glabrum var. douglasii (Hook.) Dippel (syn. subsp. douglasii (Hook.) Wesm.; Douglas maple, also incorrectly vine maple) – Alaska south to Washington and Idaho
• Acer glabrum var. greenei Keller (Greene’s maple) – endemic-central California
• Acer glabrum var. neomexicanum (Greene) Kearney & Peebles (syn. subsp. neomexicanum (Greene) A.E.Murray; New Mexico maple) – New Mexico
• Acer glabrum var. torreyi (Greene) Smiley (syn. subsp. torreyi (Greene) A.E.Murray; Torrey maple) – endemic-Northern California

It tends to be found in brush fields arising from fire-disturbed sites. Conifers tend to replace it in well-forested areas. The foliage is browsed by game animals (especially deer and elk in winter), cattle, and sheep.

II. How to Grow and Care

Sunlight

Rocky mountain maple thrives in full sunlight conditions which promotes its ideal growth and health. It can tolerate partial sun, but excessive shade may impede its vitality by reducing photosynthetic activity, potentially leading to etiolation or less robust growth. Adaptively, rocky mountain maple can modulate leaf orientation to maximize light absorption under varying light conditions. For outdoor cultivation, rocky mountain maple should be planted in a spot where they receive ample sunlight for the majority of the day, as this will ensure optimal development and overall well-being.

Temperature

The rocky mountain maple has good cold resistance and is able to tolerate low temperatures. The rocky mountain maple is native to the margins of forests in China, Japan, and North Korea, so these trees generally prefer a humid environment. Most rocky mountain maple plants have shallow root systems, so they should be watered frequently during the summer. For some varieties, a sprayed mist should be provided during the summer to increase the humidity of the air around them.

Watering

Rocky mountain maple thrives in its native environment where it has adapted to withstand longer periods between waterings. This species is known for its drought tolerance, efficiently conserving water to survive the variable moisture conditions of its habitat. In cultivation, rocky mountain maple should be watered once every three weeks, aligning with its natural rhythm of scarce water availability. As a deciduous plant commonly grown outdoors, rocky mountain maple’s watering needs are reduced during dormancy in the colder months, reflecting an adaptation to seasonal changes in precipitation.

Soil

The rocky mountain maple does not have many specific requirements for soil. It is a forest native, so it prefers well-drained soil that is rich in humus and slightly acidic. Its main root will extend deep into the soil, but its tiny fibrous roots will be concentrated near the soil surface, so accumulated water in the soil tends to cause root rot.

Fertilizing

For rocky mountain maple, balanced nutrition fertilizers enhance vigor while high-nitrogen options promote lush foliage. Fertilize in spring and summer, when growth is active, following a monthly schedule. Use sparingly; a light hand avoids over-fertilization damage. Gradually decrease feeding as autumn approaches to avoid stimulating sensitive new growth before frost. For novices, dilute a general-purpose fertilizer to half strength, while experienced gardeners can adjust based on soil tests. Always water rocky mountain maple well after fertilizing, ensuring nutrients reach the root zone.

Pruning

Pruning is mainly used to enhance internal ventilation, improve shoot formation, promote growth, and for shaping. The common approach to pruning is to remove any old or weak branches, remove any branches that disrupt the shape of the tree, and remove any branches that cross other branches. Pruning is used to control the plant according to the required height and shape. Generally, this follows the “Y-type” pruning method.

It is possible to prune a rocky mountain maple throughout the year, but the coldest winter months should be avoided to prevent damage to the plant. Major pruning to shape a maple can be performed following defoliation in the fall, or prior to leaf growth in the spring. It is recommended to prune in the fall because withered and full buds can be easily distinguished.

According to the shaping plan, you can prune any withered buds and keep the full ones, thus making your tree healthier. For pruning in the summer, diseased, weak, or dead branches can be removed, mainly to enhance the tree’s ventilation.

If grown as a potted plant, the roots can easily occupy the entire flowerpot due to the limited space. This can make a rocky mountain maple prone to aging if an old flowerpot is not changed. It is recommended that old roots are pruned during the dormancy period, that is, cut off any old, coarse, diseased, and weak roots.

Propagation

Rocky mountain maple is a resilient deciduous shrub well-suited to cultivation through sowing. Achieving successful propagation involves selecting healthy, mature seeds, which should be stratified to overcome dormancy and stimulate germination. Planting should be in well-draining, fertile soil, ensuring adequate light for growth but avoiding direct, harsh sunlight to protect young seedlings. Consistent moisture is essential, but care should be taken to prevent waterlogged conditions that could lead to rot. Patience is key, as germination may take time, but with proper care, rocky mountain maple seedlings will develop into vigorous plants.

III. Uses and Benefits 

Native Americans utilized the strong stems for snowshoe frames, bows, and other applications. Some Plateau Indian tribes drink an infusion of Douglas maple as a treatment for diarrhea. Ramah Navajo uses an infusion of the glabrum variety for swellings, and also as a “life medicine”, or panacea.