Eastern Hemlock

Size, growth and what to expect

Eastern hemlock grows slowly, about a foot a year in our growth estimator. It matures at 40 to 100 feet with a crown 25 to 35 feet wide. Trees can live more than 800 years, and the oldest dated by its rings reached 555. Only a tiny share of trees is ever dated this way.

Trees begin to bear cones at 20 to 30 years and still bear large crops past 450. Pollen is shed from late April to early June. Its cones, the smallest of any hemlock at half an inch to three quarters of an inch long, ripen in September and October. Red crossbills eat the seed, ruffed grouse eat the buds, and Columbia silkmoth caterpillars feed on the needles.

It grows wild from Nova Scotia west to northern Michigan, Wisconsin and Minnesota, and south through New England, New York and Pennsylvania. It follows the Appalachians to northern Georgia and Alabama. The hemlock woolly adelgid has killed most of the old hemlocks in Great Smoky Mountains National Park and about 95 percent of those in Shenandoah National Park.

How to grow Eastern Hemlock

Eastern hemlock does best in part shade in a cool, sheltered spot, and it grows in deep shade too. It takes full sun only where summers are cool, since hot sun and drying winter wind scorch the needles. It wants a moist, acid soil that drains well, and it stands neither drought, soggy ground nor salt spray. Plant it 20 feet from a wall, and set trees 15 feet apart for a screen. Keep it more than 50 feet from overhead wires.

Saplings ship bare-root or potted. Plant in early spring or early fall, in a hole wider than the roots and no deeper. Set the root flare at the soil line, put the native soil back in, and mulch a wide ring clear of the trunk. Seed needs two to four months of cold, damp storage in the refrigerator. Sow it in spring, barely covered, in a bed with light shade. Hemlock seedlings are hard to raise, since usually fewer than a quarter of the seeds sprout and young seedlings die if the bed dries.

Water weekly through the first two summers and in every dry spell after that, since drought is the worst danger to a young hemlock. Keep a mulch of leaves or bark over the shallow roots. Deer browse it heavily in winter, and rabbits and hares nibble young trees, so cage saplings until they are well above browse height. It takes shearing well as a hedge, and a specimen needs pruning only of dead wood.

Eastern hemlock carries male and female cones on the same tree and readily sets seed with its own wind-borne pollen, so one tree bears alone. The hemlock woolly adelgid, a tiny sap-sucking insect from Asia that hides under white wool on the twigs, is its gravest threat. An infested tree that is not treated usually drops its needles and dies within several years. Imidacloprid or dinotefuran applied to the soil gives good control, and these systemic treatments suit trees away from water. The elongate hemlock scale, an armored scale insect, often infests the same trees and yellows and thins the needles. The soil treatments for the adelgid do not control it, and nitrogen fertilizer helps it multiply.