Their feeding causes the leaves to become distorted. In the spring, spiny witch hazel gall aphids that either overwintered as eggs on witch hazel bark ( Hamamelis spp.) or as immature females beneath the birch bark feed on the sap of newly expanding river birch leaves. River birch is one of two hosts required by this aphid to complete its life cycle. River birch is resistant to bronze birch borer, a North American native insect that attacks white-barked birches, but is susceptible to the spiny witch hazel gall aphid. This typically occurs late in the season and usually is not severe enough to warrant fungicide applications. Fungal leaf spot diseases may result in early leaf drop during rainy summers. Choose a location that will accommodate its mature height and spread. In naturalized landscapes, river birch thrives along stream banks and ponds. Use river birch as a shade tree, as an attention-drawing specimen accent, or in a copse or group of trees. To avoid sap exudation, prune after the leaves have fully expanded in the spring to mid-summer or in early fall after the trees have shed their leaves. This leaking sap is not harmful-only unsightly. River birch tends to “bleed” or exude sap when healthy live branches are removed in late winter or early spring. Naturalized landscape planting of Cully river birch ( Betula nigra ‘Cully’ Heritage®). The wind-pollinated female catkins develop into brown, dangling structures that bear many small, brown, winged nutlets, which are dispersed by the wind. Birch trees are monoecious, meaning that male and female flowers are produced in separate structures on the same tree.
These upright female catkins resemble small pine cones. In the spring, when the drooping, dark brown pollen-bearing male catkins expand and bloom, separate, greenish, ¾- to 1¼-inch long non-showy female (pistillate) catkins emerge from shortened shoots called spurs. They appear in late summer and autumn and remain on the tree during the winter. In addition to the exfoliating bark that offers year-round appeal, river birch produces showy, reddish-green, 2- to 3-inch long male (staminate) flowers called catkins at the ends of branches. Growth RateĮxpect river birch to grow at a medium to rapid rate (1 ½ to 2 feet per year), particularly in moist, fertile, well-drained locations in full sun to moderate shade. Multi-trunked trees (with an odd number of trunks)-considered the most attractive form-develop an irregular crown at maturity. Young single-trunked trees have an oval to pyramidal habit that matures to a rounded crown. This deciduous tree typically grows 30 to 60 feet high and 20 to 50 feet wide. River birch produces distinctive ovate- or egg-shaped leaves with doubly serrated leaf margins.