Carpinus caroliniana aka Ironwood

Carpinus caroliniana, the American hornbeam, is a deciduous tree in the birch family. Gardeners also know it as musclewood, ironwood, blue beech and water beech. It grows wild across eastern North America, from eastern Canada south to Florida and Texas. In nature it favours moist woods, stream banks, river edges and ravine bottoms, where it forms part of the forest understory.

This is a slow-growing tree with a rounded to oval crown. Most garden trees reach 20 to 30 feet (6 to 9 metres) tall and spread as wide or wider. Some sources give heights of up to 35 or 40 feet (11 to 12 metres). It often grows with several trunks and branches that droop at the tips. The bark is the main feature. It is thin, smooth and bluish gray, stretched over ridged trunks that look like flexed muscles.

The leaves are simple and alternate, with an ovate-oblong shape and doubly serrate edges. They measure about 2½ to 5 inches long. In autumn they turn yellow, orange or red, and some brown leaves may hang on into winter. The tree is monoecious, so male and female catkins grow on the same plant in early spring. The flowers are small and not showy. Female catkins develop into hanging clusters of ribbed nutlets. Each nutlet sits on a three-lobed, leaf-like bract that helps the wind carry it. The fruit ripens from late summer into autumn and turns from green to brown.

American hornbeam thrives in part to full shade but will also take sun. In deep shade it grows open, while in full sun it becomes dense. It prefers moist, fertile, organically rich soil that is slightly acidic. It tolerates clay, sand and loam, and copes with periodic flooding. However, it dislikes drought, heat and compacted soil. It is hardy in USDA zones 3 to 9 and rated H7 in the UK. Field-grown trees transplant poorly, so container plants or spring moves work best.

The tree suits naturalised areas, woodland edges, pond margins and rain gardens. It also works as a lawn tree, a street tree, a hedge or a screen, since it accepts pruning. Its bark gives good winter interest. Wildlife value is high. Eastern tiger swallowtail and red-spotted purple butterflies use it as a larval host. Songbirds, grouse, turkeys and squirrels eat the seeds and buds. The very hard wood once served for bowls, tool handles, mallets and levers.

Problems are usually minor. Leaf spots, cankers and twig blight occur from time to time. Canker can kill branches, and a girdled trunk can kill the whole tree. Other possible problems include powdery mildew, coral spot and honey fungus. Caterpillars, aphids and maple phenacoccus may also feed on it. No hazards are known. The cooked seed is edible but serves only as an emergency food. Some Native American tribes used an astringent inner-bark infusion for diarrhoea.