Allegheny Spurge

Size, growth and what to expect

Allegheny spurge grows slowly, a few inches a year in our growth estimator, and spreads by underground stems into a carpet 6 to 12 inches high. Each plant reaches 1 to 2 feet across, and two Arnold Arboretum plantings untended for 36 years covered only 3 by 4.5 and 3 by 7 feet. Its broad leaves, 2 to 4 inches long and toothed near the tip, gather at the top of each stem, mottled silver and purple.

The leaves are evergreen in the South, though worn by midwinter. Farther north they turn bronze in fall and brown by spring, and new shoots replace them by May or June. Fragrant white to pinkish spikes, 2 to 4 inches long, rise from the base of the stems in March and April, often before the new leaves. The small capsules, ripe in July and August, seldom form in gardens.

It grows wild from Kentucky and western North Carolina south to Louisiana and northern Florida. Its home is rich, moist woods on limestone, often on slopes along streams. North of that, as in Pennsylvania, it is thought to have been brought in.

How to grow Allegheny Spurge

Allegheny spurge grows in part shade to deep shade, as under tall trees, and too much sun bleaches its leaves. It wants a loose, moist soil that drains well and is rich in organic matter, slightly acid to neutral. Established plants ride out dry spells. Shelter from north winds helps it keep its leaves in winter. Set plants 2 feet apart, or 6 to 12 inches apart for a quicker carpet, and keep them 2 feet from a wall. At its size it can go within 15 feet of overhead wires.

Saplings ship bare-root or potted. Plant them in spring or fall, in a hole wider than the roots and no deeper. Set the crown at the soil line, put the native soil back in, and mulch with leaves, clear of the stems. It seldom sets fruit in gardens, so it is raised from cuttings or by dividing clumps. Treat any sowing of its seed as a trial.

Water it every day for the first two weeks after planting, then every two to three days until week 12. Before each watering, push a finger a few inches into the soil, and wait if it is still wet. After that, soak the bed weekly through the first two summers, and water the soil rather than the leaves. Thin crowded plants now and then so air moves through them, and cut out yellowed or dead growth.

Each spike carries many male flowers above and a few female flowers at its base, so there are no male and female plants to pair. It is usually trouble free, though leaf blight can be serious, and root and stem rot, aphids, slugs, scale and mites can turn up. Boxwood blight, a fungus disease of boxwood, also infects it. It can poison animals that eat it, sometimes fatally.