Tsuga canadensis (Eastern Hemlock)
|Also known as:||Canda Hemlock|
|Life cycle:||perennial woody|
|Habitat:||part shade, shade, sun; average to moist soil; mixed forest, ravines, rocky ridges|
|Plant height:||60 to 100 feet|
|Wetland Indicator Status:||GP: none MW: FACU NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Male and female flowers are cone like structures called strobili, borne on separate branches of the same tree. Male strobili are yellowish, nearly round, ¼ to 1/3 inch long, single in the leaf (needle) axils of 1-year-old twigs. Female strobili are single at the tips of 1-year-old twigs, nodding to hanging, oval-elliptic, up to about ¾ inch long, green with fan-shaped scales.
Leaves and stems:
Leaves are needle-like but soft, single, flattened, ¼ to about ¾ inch long, shiny green on the upper surface, the lower surface whitish with a green midvein and appearing striped. Needles are short-stalked, attached to a peg-like extension on the twig, and arranged spirally around the twig but those on top and underneath the twig twist so that needles appear to be mostly lateral (2-ranked), giving the branch a flattened appearance. Needles persist up to 3 years. Twigs are mostly opposite, initially green to yellowish and short-hairy, becoming gray-brown and eventually hairless.
Older bark is gray to brownish, with furrows, ridges and flat plates. Trunks can reach up to 5 feet in diameter, but more likely (in Minnesota) to about 2 feet at breast height (dbh). The crown is typically cone-shaped, with widely spreading branches and the lower branches often droopy.
Fruit is a brown, elliptic to egg-shaped cone, ½ to about 1 inch long, hanging from the tips of twigs. Scales are rounded at the tip, with minute, irregular teeth along the tip edge, slightly wider than long. Cones mature in autumn of the first year and shed seeds through winter, the cone dropping off soon after.
Eastern Hemlock is one of the rarest trees in Minnesota, where it reaches the western fringe of its range. According to the DNR, the largest stand of nearly 5,000 trees was in southwestern St. Louis County. However, it succumbed to heavy logging, then the Moose Lake-Cloquet fire of 1918, before it could be preserved. What remained of that population died out and only around 50 mature trees remain in all of Minnesota. Listed as a Special Concern species in 1984, it was elevated to Endangered in 2013. While it is rare in the wild here, it is widely available in the nursery trade with about 300 cultivars available. Eastern Hemlock is distinguished from other evergreens by the soft, flat needles that appear 2-ranked, and are short-stalked and attached to a peg-like extension on the twig, the peg persisting even after the needle falls off. While Spruce (Picea) trees have the same kind of peg-like attachment, their needles are 4-sided (not flat), not stalked, and spread in all directions around the twig. Balsam Fir (Abies balsamea) may also have flat needles that appear 2-ranked, but the needles lack the peg-like attachment and are attached directly to the twig.
A grave new risk threatens Eastern Hemlock throughout its range. Hemlock Woolly Adlegid (HWA or Adelges tsugae), native to Japan, was accidentally introduced into eastern North America in the late 1940s or early 1950s. While it has not been found in Minnesota yet, by 2015 it is estimated to have expanded into over 90% of Eastern Hemlock's native range. It is unlikely there are good treatment options for Minnesota's few trees, but several biological controls have been released in the east and are having an effect of HWA's impacts.
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- Eastern Hemlock tree
- Eastern Hemlock tree
- Eastern Hemlock tree © Steven J. Baskauf
- leaves appear to be 2-ranked
- lower branches
- branches with spent male strobili
- Hemlock woolly adelgid infestation ©Michael Montgomery, USDA Forest Service
Photos courtesy Peter M. Dziuk taken in Wisconsin. Tsuga canadensis tree by Steven J. Baskauf via http://bioimages.vanderbilt.edu used under CC BY-NC-SA 4.0. Adelges tsugae by Michael Montgomery, USDA Forest Service / ? Bugwood.org / CC-BY-3.0-US.
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