Go seePlants
Hemlock groves in winter: less snow, less light, and deer shelter
When: November to March
An eastern hemlock grove is so dense that far less snow reaches the ground beneath it than in the woods next door, and the light dims noticeably the moment you step under the canopy. That shelter is why deer concentrate in hemlock stands all winter, and it is shelter under threat, because an introduced insect called the hemlock woolly adelgid is now in every county in Massachusetts.
Why this time of year
Winter is the only season you can feel the difference, because the difference is made of snow and low light. Hemlock woolly adelgid has been identified in all Massachusetts counties and in all communities with significant hemlock, and in 2024 the state treated hemlocks at Middlesex Fells Reservation for it.
How to notice it
- Find a stand of eastern hemlock (short, flat, blunt-tipped needles in two rows, cones under an inch long) that borders open hardwoods, and walk from one into the other.
- Notice how much the light drops as you pass under the canopy. Eastern hemlock is the most shade tolerant tree there is, and a full stand lets so little light through that an understory seldom develops at all.
- Compare the snow on the ground. It should be noticeably thinner under the hemlocks than in the hardwoods you just left, because the canopy intercepts it on the way down.
- Check the undersides of twigs where the needles attach for small white woolly tufts. That is hemlock woolly adelgid.
Try this with kids: Walk from the regular woods into a hemlock grove. Is it darker in here? Look down: is there more snow behind you, or under the trees?
hemlock woolly adelgid: a tiny introduced insect that kills hemlock trees by draining sap from the needles
Sources
- extension.unh.eduUNH Extension. Verbatim: 'Dense stands reduce snow depths and help to moderate temperatures', which helps wintering deer conserve energy; several species of owl are 'strongly associated with hemlock stands year-round' while songbird use is concentrated in breeding season; HWA is well established in southern New Hampshire and cold winters can crash its population, with 98 percent mortality from winter kill in 2018
- extension.unh.eduUNH Good Forestry. Hemlock, with northern white cedar, provides the best winter cover for deer due to superior ability to INTERCEPT SNOW; deer move into wintering areas once snow exceeds 10-12 inches and rely on core shelter once snow exceeds 16-20 inches
- research.fs.usda.govUSDA Forest Service Silvics. Eastern hemlock 'is the most shade tolerant of all tree species' and can survive on as little as 5 percent of full sunlight; 'Fully stocked stands of eastern hemlock form such a dense canopy that an understory seldom is able to develop'; it is 'considered important as cover for ruffed grouse, turkeys, and many other animals' (NO winter qualifier on that phrase) and is essential for shelter and bedding of white-tailed deer DURING THE WINTER
- fs.usda.govUSDA Forest Health Highlights, Massachusetts 2024. Verbatim: 'HWA has been identified in all Massachusetts counties and in all communities with significant hemlock resources.' Hemlocks at Middlesex Fells State Reservation were among those treated with dinotefuran in May 2024 (1,751 inches, 84 trees, 3.1 acres). The 2024 aerial survey mapped '2,379 acres of impact from hemlock woolly adelgid'. ALSO: HWA populations 'greatly fluctuated over the last three decades', crashing during cold winters and building in mild ones
- umass.eduUMass Extension. HWA feeds by piercing and draining sap from hemlock needles, causing needle drop and progressive dieback of limbs from the bottom up; previously stressed trees can die in 3-5 years
- massaudubon.orgMass Audubon. Grouse and rabbits eat hemlock buds and needles, red squirrels and mice feed on the seeds, and deer use hemlock groves; Massachusetts hemlocks are threatened by hemlock woolly adelgid, for which there is currently no cure
Checked against these sources on September 15, 2026.
In the guide for November, December, January, February, March