River Birch (Betula nigra), the southernmost birch of the United States, makes its best growth alongside bodies of water or in occasionally flooded bottomlands. It is native to the Atlantic coastal states, southern states, the lower Midwest, eastern Great Plains, and lower Mississippi River valley. In Ohio, it is native mostly in the south-central counties, and sparsely along Lake Erie. However, it is widely planted throughout Ohio and the eastern United States as an ornamental shade tree, prized for its flaky, orange, ornamental bark and rippling foliage in the breeze.
Its rapid growth rate (even in drier soils) allows for quick shade, and it is often propagated and sold in multitrunked form. When found in the open, River Birch may reach 70 feet tall and 40 feet wide as a single trunked tree, and about 50 feet tall and 40 feet wide as a multitrunked tree. As a member of the Birch Family, it is related to the Alders, Hornbeams, Filberts, and Hophornbeams, in addition to other Birches.
River Birch prefers moist to wet, rich, deep, acidic soils. It tolerates drier soils but with subsequent leaf drop from the interior of the canopy in summer, and somewhat tolerates soils of alkaline or neutral pH, but often with resulting chlorosis of the foliage. It grows in full sun to partial sun, and is found in zones 3 to 9.
Aside from drought-induced leaf drop and yellowing foliage due to high pH soil-induced chlorosis, River Birch may have aphids on its new stem and foliage growth, and leaf spot in wet springs (which also leads to leaf drop). However, it should be noted that River Birch is resistant to the bronze birch borer, which plagues the birches of colder climates when they are planted too far south of their natural range (the warmer winters do not kill off the larvae), and is very heat tolerant in summer.
River Birch has leaves that are more triangular in shape than Yellow Birch or Black Birch, due to its relatively wide leaf bases. The alternate, doubly serrated, ovate leaves of River Birch easily flutter in the slightest breeze.
In autumn, they turn chartreuse in normal years, but golden-yellow in above-average years.
Like other Birches, the dormant catkins (male flowers) of River Birch hang on the bare twigs in winter, then begin to swell in late winter and early spring with the advent of warmer weather. When female flowers emerge from the leaf axils, pollination may occur between the flowers of this monoecious species (upper left, with the green female flower below the yellow-brown male flowers).
The result of the tree's flower is a fruit (specifically called a strobile for members of the Birch Family) that contains many small nutlets that shed in late spring.
River Birch trees that occur in the wild usually have excessively flaky, dark gray to black outer bark with hints of an orange and cream inner bark.
Selections have been made by humans that have brilliant orange, cream, and near-white peeling bark on both the young and mature trunks and branches, making this Birch the easiest of all to identify, especially in winter.