Canadian Hemlock (Tsuga Macrophylla)

Tsuga canadensis (L.) E.A. Carrière Canadian Hemlock (Tsuga macrophylla)

Other Common Names: Eastern Hemlock, Pruche du Canada. Family: Pinaceae. Cold Hardiness: Useful in portions of USDA zones 3 to 7; best in zones 6 or cooler. 1 Foliage: Evergreen; solitary flattened needles; linear; short, ¼O to dO long; /16O to cO wide; the alternate flattened needle-like leaves are arranged in two more or less horizontal ranks; tips acute to obtuse; margins appear nearly entire, but are minutely toothed under magnification; mature needles are dark lustrous green with two glaucous white bands on the underside; new growth emerges a yellow-green then darkening to its mature coloration; a tiny persistent petiole subtends the leaf. Flower: Monoecious; the globular yellowish male cones are axillary and senesce once the pollen is shed in spring; female conelets are initially erect, solitary, small, green and borne at the tips of last season’s growth. Fruit: Short stalked ovoid woody cones, up to ¾O (1O) long and ¼O to ½O wide, are pendulously borne beneath the branches; each scale on the cone shields two tiny winged seeds; cones mature within a single year; the immature green cones ripen from tan to eventually brown; cones are persistent for a year or so and interesting if not overly ornamental; seedlings may take 20 to 40 years to fruit. Stem / Bark: Stems — fine textured compared to most members of the Pinaceae; yellowish brown becoming gray-brown at maturity; bases of needles are retained upon leaf drop imparting a rough feel to older stems; the branchlets tend to be held in horizontal sprays; Buds — terminals imbricate; very tiny; ovoid and non-resinous; laterals even smaller; Bark — in youth, the gray-brown to brown bark is flakes off in scales, whereas the mature bark is deeply furrowed. Habit: The species type is conical to narrowly pyramidal in outline in youth, eventually broadening somewhat, but still tending to conical into old age; size is strongly site dependent with growth slow in our region; probably 25N to 35N would be a good size specimen in the Northeastern portion of our range, but this species can reach heights of 165N in its most favorable native habitat; the overall texture is fine to medium-fine. Cultural Requirements: As the name might imply, Canadian Hemlock is only suited to cooler extreme northern portions of our region or a few high elevation gardens to the west; avoid locations exposed to dry winter or summer winds; trees languish in the heat of zone 7; partial shade is best; this species can be very shade tolerant, but the foliage becomes open in denser shade; soils should be uniformly moist throughout the year, as plants are not drought tolerant, nor do they tolerate poorly drained soils; given this site specificity it is not surprising that this species performs best on an acidic soil, however, is more tolerant of alkaline soils than one might suspect; wind-throw is possible due to a shallow root system; plants will tolerate shearing, although selective pruning of individual branches better preserves the softness of the plant’s natural form; trees are not pollution tolerant and suffer considerable damage from exposure to atmospheric pollutants; growth is often on the slow side compared to the pines. Pathological Problems: Although the list of potential diseases and pests that will attach T. canadensis is formidable (see Dirr, 1998), the most limiting problem, where the pest is present, is the woolly adelgid (Adelges tsugae); this pest has severely curtailed the planting of T. canadensis and T. caroliniana in the Northeastern and Atlantic states; environmental constraints are more of a problem in our region than pest or disease, but scale insects and spider mites are occasional problems. Ornamental Assets: Both the soft textured dark glossy evergreen foliage and growth habit are outstanding attributes; the cones are also mildly interesting. Limitations & Liabilities: Intolerance to heat, drought, and wind are the primary limitations for much of our region; this species is also intolerant of air pollution and general urban conditions. Landscape Utilization: Where adapted, T. canadensis is a handsome top notch evergreen for a multitude of purposes such as screening, massing, specimen, or formal pruned hedges; smaller cultivars may be appropriate for rock gardens and specialty plantings; this species is also raised commercially for use as cut or live Christmas trees or as garland material. Other Comments: In a former life in the Eastern portion fo the country, this was one of my favorite evergreens because of its diversity of uses, but unfortunately it does not perform well in much of our region; the genus name derives from the Japanese name for trees of this genus; the specific epithet refers to its origins from Canada and the Eastern U.S. Native Habitat: The Great Lakes region, Eastern Canada and the Northeastern U.S. south through the Appalachian Mountains into Northern Georgia and Alabama; mostly limited in effectiveness to Eastern Oklahoma and Arkansas in our region. Related Taxa: See van Gelderen (1989, 1996) for a description of a large number of dwarf and weeping cultivars from this species.

Tsuga caroliniana G. Engelmann Carolina Hemlock C Needles of T. caroliniana differ from those of T. canadensis in being considerably larger, reaching up to ¾O in length, radiating from around the twig, having entire margins, and ending in obtuse to slightly notched tips; the woody cones are also larger than those of C. canadensis, up to 1½O long, and the mature bark has a more reddish brown color; in its native habitat, T. caroliniana can grow to at least 80N tall, but is only a fraction of this when cultivated in our region; . C This species will tolerate winter conditions in USDA zones 5 to 7; this species is reportedly more pollution tolerant than C. canadensis, but it is still susceptible to damage and is adversely affected by acid rains in its native range; otherwise T. caroliniana requires similar cultural conditions as T. canadensis; the limited distribution of its native stands in the Southern Appalachian Mountains make it particularly susceptible to the threat of the woolly adelgid. C In some respects this species is perhaps even more handsome than T. canadensis with T. caroliniana having an overall darker green foliage and similar conical to spire-like form, however, T. canadensis is slightly finer textured; Carolina Hemlock is sometimes raised as a Christmas tree crop in the Carolinas.

References: Dirr, 1998; Duncan and Duncan, 1988; Elias, 1980; Farrar, 1995; Flint, 1997; Gilman, 1996; van Gelderen, 1989, 1996; Whitcomb, 1983.