Muscle-wood is a small native tree with smaller specimens looking like a large shrub, that can grow 25 to 40 feet (rare) in height and with an often crooked trunk of a foot in diameter. Branches are spreading, producing a broad rounded crown. Typical height is 12 to 15 feet.
The bark and trunk is distinctive as the trunk is usually crooked and the bark is thin and irregularly fluted, with a blue-gray color cast. This gives a muscular appearance and hence the common name.
Twigs are gray to brown, slender and a bit of zigzag and with brown angled buds whose scales have a darker bottom and a top edge that is lighter, with a silky appearance.
Leaves are alternate, simple, oval to elliptic, 2 to 4-1/2 inches long, with double saw-toothed edges (or at minimum - two coarse sizes) that taper to a pointed tip. Leaf bases are rounded to heart-shaped. There is one mid-vein and the laterals are parallel to each other. The underside has fine hair on the veins. Fall color can be a brilliant orange to red.
Flowers: The tree is monoecious, that is with separate male and female flowers. The male flowers (staminate) appear in a slender drooping stalkless catkin, up to 1-1/2 inches long, back from the new branch tip on the prior years wood, yellow-green in color, with several stamens, but partially obscured by a small reddish-green bractlet. Female flowers (pistillate) also appear in catkins, from the branch tips of the prior years wood. They are paired, fuzzy, yellow-green to reddish-green in color and shorter than the male catkins. The stigmas are reddish-green and they too are partially obscured by a bractlet. Both appear with the leaves. Catkins are also referred to as 'aments'.
Fruit: Flowers mature to small 1/4 inch paired nutlets that are egg-shaped, hairy and greenish. The nutlet is attached to a 3-lobed, 1 inch long leaf-like scale (or bract) that has toothed margins. These hang in clusters on the stalk of the old female catkin and mature in late summer and are dispersed by wind and birds. Trees begin good seed production around 15 years of age.
Habitat: Muscle-wood grows best in moist areas and along stream banks and frequently as an understory tree of bottomland hardwood forests in part shade as it is shade tolerant. It does well as a landscape plant in full sun if given adequate moisture and fertilizer. It is slow growing, but short-lived.
Names: The genus, Carpinus, is, first, the Latin name for the Hornbeam tree which is said to be derived from the Latin carpentum, referring to a wooden wheeled vehicle. The species name, caroliniana, means of, or from, the Carolinas. The various common names are misleading as it is not in the Beech family and Hornbeam is more appropriately applied to other trees that use the word, such as the Hophornbeam (or Ironwood), Ostrya virginiana. Hornbeam, however has been applied because of the density of the wood. Muscle-wood is a more distinctive name of the tree and has the advantage of the name not used elsewhere. The author name for the plant classification - ‘Walter’ is for Thomas Walter (1740-1789) British born American botanist, best known for his catalogue of plants of South Carolina, Flora Caroliniana.
Subspecies comparisons: There are two recognized subspecies that have somewhat separate geographical areas but hybridize in areas where they meet. Subspecies caroliniana has a more narrow leaf blade with an acute to obtuse tip. Secondary teeth are small and blunt and the upper leaf surface lacks glands. In earlier times this subspecies was classified as C. americana Michaux. Subspecies virginiana has a more ovate to elliptic leaf, abruptly narrowing, but sometimes long and tapering, secondary teeth are sharp-tipped and the upper surface is tiny dark brown glands. This is the subspecies considered native to Minnesota.
Above: 1st photo - The distinctive fluted and smooth bark from which the name "muscle-wood" is derived. 2nd photo - A spring twig showing the slight zigzag shape, and the brown, angled buds have scales with darker bases and lighter edges. 3rd photo - Young branches have the bluish-gray color cast but do not show the fluting of larger limbs.
Below: 1st photo - The shape of a mature specimen growing in full sun as a tall shrub, is usually as wide as tall as seen below. Specimens growing in low light environments will be taller than wide and more leggy. 2nd photo - Fall color.
Below - Flower and seed development: 1st photo - Male catkins. Note the reddish-green bract covering the flowers. 2nd photo - the stamens have extended. 3rd photo - Female flowers are in a shorter catkin.
Below: 1st photo - Female catkins are frequently opposite each other on the twig. 2nd photo - green seeds developing under the overlying green bracts.
Below: Seeds develop in hanging clusters of paired nutlets that are egg-shaped, hairy and greenish and are attached to a 3-lobed, 1 inch long leaf-like scale (or bract) that has toothed margins. 1st photo - green seeds developing. 2nd photo - Mature nutlets in fall color. 3rd photo - Leaves have a double saw-tooth edge, a pointed tip and parallel lateral veins.
Below: Fall leaf color with a nutlet cluster.
Notes: This species is considered indigenous to the Garden area in that Eloise Butler transplanted a specimen on June 3, 1907, that she obtained from "the vicinity of the Lake Street Bridge" (in Minneapolis). This becomes somewhat confusing as she used the name Carpinus americana, which is a species native to the SE U.S. not Minnesota and, unless planted by someone, not native to the area. That scientific name, Carpinus americana, is now classified as Carpinus caroliniana Walter subsp. caroliniana. The only Minnesota native of this genus is ssp. virginiana - so Eloise may have slightly mis-identified it. However, on Oct. 5, 1913 she brought in two plants from Kelsey's Nursery in North Carolina, in which area both subspecies grow. Martha Crone reported planting the tree in 1934 using the name C. caroliniana. It was most recently planted by Susan Wilkins in 2006 and 2012. The species native to Minnesota, subsp. virginiana, is found in over half the counties, including the metro area. In North America it is found in the eastern half of the continent, except the gulf coast and the Canadian Maritime Provinces.
Uses: The wood of the tree is very hard, white, heavy, with close grain, but rots rapidly and must be kept away from moisture. It works with difficulty and use is restricted to small things that need to take a beating, like tool handles and mallets.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"