Hemlock and cedar are both conifers that share overlapping ranges across North America, but they differ sharply in wood durability, structural strength, chemical makeup, and the roles they play in a forest ecosystem. Cedar heartwood is famously rot-resistant and aromatic, while hemlock is denser and stronger in structural applications but decays quickly without treatment. The differences go well beyond lumber, though, reaching into soil chemistry, understory microclimates, and even how each genus might fare as the climate shifts.
Which Trees Are We Actually Talking About
The comparison gets complicated immediately because “hemlock” and “cedar” each refer to multiple species. In North America, the two main hemlocks are eastern hemlock (Tsuga canadensis), a shade-tolerant giant of Appalachian and Great Lakes forests, and western hemlock (Tsuga heterophylla), a dominant timber species from Alaska to northern California. On the cedar side, western red cedar (Thuja plicata) is the species most people picture: massive trunks, fibrous reddish bark, and that distinctive smell. But yellow-cedar, also called Alaska cedar (Callitropsis nootkatensis), occupies a different niche in the Pacific Northwest and has its own distinct chemistry. Eastern white cedar (Thuja occidentalis) rounds out the group in the northeast.
None of these “cedars” are true cedars in the botanical sense. True cedars belong to the genus Cedrus and are native to the Mediterranean and Himalayas. North American cedars are spread across several genera. This matters less for practical purposes than it does for trivia, but it explains why western red cedar and yellow-cedar behave quite differently from each other despite sharing a common name.
Structural Strength and Density
If you need wood that can bear a load, hemlock has the advantage. A study comparing the radial variation in stiffness, strength, and density of planted western hemlock and western red cedar in Great Britain found that western red cedar consistently had worse properties for structural use.1Annals of Forest Science. Models to predict the radial variation of stiffness, strength, and density in planted noble fir, Norway spruce, western hemlock, and western red cedar in Great Britain Cedar is lighter, softer, and less stiff, which makes it easier to work with hand tools but a poor candidate for beams, joists, or framing lumber.
Western hemlock, by contrast, has higher modulus of elasticity (how much it resists bending) and modulus of rupture (how much force it takes to break). Its density runs roughly 25 to 40 percent higher than western red cedar’s, depending on the position within the tree. This density gap is why hemlock has long been a workhorse species in construction framing, while cedar gets steered toward applications where weight and weather exposure matter more than raw strength: siding, shingles, fencing, and decking.
The trade-off is straightforward. Hemlock gives you strength and stiffness at the cost of durability when exposed to moisture. Cedar gives you weather resistance and dimensional stability at the cost of structural capacity. For a deck frame hidden beneath boards, hemlock (pressure-treated) makes sense. For the boards themselves, exposed to rain and sun, cedar’s natural durability wins.
Decay Resistance and Natural Chemistry
Cedar’s reputation for lasting outdoors without treatment is not marketing. It is chemistry. The heartwood of cedar species is loaded with secondary metabolites that function as natural preservatives. Yellow-cedar heartwood, for instance, contains oxygenated monoterpenes, oxygenated sesquiterpenes, and tropolones, compounds that together give the wood fungicidal, bactericidal, and insecticidal properties.2SpringerLink / PubMed Central. Yellow-Cedar, Callitropsis (Chamaecyparis) nootkatensis, Secondary Metabolites, Biological Activities, and Chemical Ecology Western red cedar has a similar, though chemically distinct, suite of extractives, particularly thujaplicins, which are also tropolone-type compounds with strong antifungal activity.
These compounds are why cedar fence posts can last decades in the ground while untreated hemlock posts rot within a few years. They also explain why cedar is the traditional material for wooden shingles and why old-growth cedar logs found on forest floors can remain structurally sound long after the sapwood has disintegrated. The extractives are concentrated in the heartwood; sapwood from either species decays at roughly the same rate.
Hemlock has none of this chemical armor. Its heartwood is classified as non-durable, meaning it has little natural resistance to fungal decay or insect attack. For outdoor use, hemlock needs pressure treatment with preservatives, which adds cost and limits its appeal for visible applications where the chemical smell or greenish tint of treated wood is unwelcome.
Insect and Pest Repellency
Cedar’s chemical defenses extend beyond fungi to insects. The same study on yellow-cedar found that nootkatone, a sesquiterpene concentrated in the heartwood, shows enough activity against arthropods that researchers have investigated it as a potential commercial biopesticide and repellent for ticks and mosquitoes.3SpringerLink / PubMed Central. Yellow-Cedar, Callitropsis (Chamaecyparis) nootkatensis, Secondary Metabolites, Biological Activities, and Chemical Ecology Cedar closets and cedar chests trade on this chemistry: the volatile compounds deter clothes moths, carpet beetles, and other fabric pests, at least when the wood is fresh or recently sanded to re-expose the extractive-rich surface.
Hemlock, lacking these extractives, is far more vulnerable to pests. Eastern hemlock is currently under severe threat from the hemlock woolly adelgid (Adelges tsugae), an invasive insect that feeds on the base of hemlock needles and has killed vast stretches of eastern hemlock forest since its introduction to North America from East Asia. Western hemlock faces its own pest pressures, though none as catastrophic. The contrast is striking: cedar’s chemistry keeps most pests at bay, while hemlock’s lack of chemical defense has made it one of the most ecologically threatened conifers in eastern North America.
What Each Tree Does to the Soil Beneath It
Hemlock and cedar do not just grow in different conditions; they create different conditions. A classic soil study comparing forest floor chemistry under western hemlock and western red cedar found that soil pH, exchangeable calcium, cation-exchange capacity, and base saturation were all greater under cedar than under hemlock. Soil nitrogen, carbon, magnesium, and potassium were generally higher under cedar as well, though those differences were smaller. The total weight of organic horizons (the accumulated leaf litter and humus on the forest floor) was also greater under cedar.4Soil Science Society of America Journal. The Influence of Western Hemlock and Western Redcedar on Soil Properties
In practical terms, cedar litter breaks down more slowly (partly because of those same extractives that resist fungi) and builds up a thicker organic layer, but the soil underneath is less acidic than under hemlock. Hemlock needles decompose relatively quickly and release acids that lower soil pH over time. This acidification can influence which plants and fungi thrive in the understory and how available nutrients are to neighboring trees. If you are managing a mixed stand or choosing species for a planting site, the long-term soil trajectory is worth considering. Hemlock will acidify the soil more aggressively, while cedar will build organic matter and maintain a higher pH.
How Hemlock Shapes the Understory
Eastern hemlock is one of the most effective shade-casting conifers in North America. Its dense, layered canopy blocks sunlight so thoroughly that the forest floor beneath an old-growth hemlock stand is dramatically different from the understory beneath a cedar canopy. Research in an old-growth eastern hemlock forest found that photosynthetic light levels in the understory were typically about 1% of full sunlight, and the canopy suppressed summer air temperatures by 3 to 4°C compared to above the canopy.5ResearchGate / Ecoscience. Understory microclimate and photosynthetic response of saplings in an old-growth eastern hemlock (Tsuga canadensis L.) forest
That cooling effect has cascading consequences. Streams running through hemlock stands stay cooler, which benefits cold-water fish like trout. The deep shade suppresses most understory shrubs and herbs, creating a distinctive open, cathedral-like forest floor with sparse ground cover. When hemlock woolly adelgid kills the hemlocks, the canopy opens and temperatures spike, often leading to replacement by hardwoods or other conifers that create an entirely different microclimate. This is why the loss of eastern hemlock is an ecological event, not just a forestry concern: the species acts as a climate regulator for everything living beneath it.
Cedar canopies are dense too, but not to the same extreme. Western red cedar allows more dappled light through, and its litter layer creates different moisture conditions on the forest floor. The result is a wetter, more fungally active understory with somewhat more plant diversity than the deeply shaded hemlock understory.
Pulp, Paper, and Fiber Quality
When it comes to papermaking, hemlock has a clear edge. Western hemlock produces fibers that are well-suited to pulping, with moderate to long fiber length and consistent density that translates into strong paper. A genetics study on western hemlock found strong family-level effects on fiber properties, with heritability estimates for wood density, fiber length, and fiber coarseness ranging from 0.5 to 0.9, meaning these traits can be improved through selective breeding programs.6Canadian Journal of Forest Research. The potential of improving western hemlock pulp and paper quality. I. Genetic control and interrelationships of wood and fibre traits This is unusually high heritability for wood properties, and it suggests that future hemlock plantations could be bred specifically for superior paper production.
Cedar is rarely used for high-quality paper. Its lower density, shorter fibers, and the presence of extractives that interfere with the pulping process make it a poor choice for most paper grades. The extractives can cause pitch problems in paper mills, gumming up equipment and discoloring the final product. Cedar does get chipped for low-grade fiberboard and landscape mulch, but its value in the fiber-products world is a fraction of hemlock’s. If you see “hemlock-fir” on a lumber stamp at a building supply store, the hemlock likely originated in a forest managed partly for pulpwood production alongside saw timber.
Climate Change and Future Range
One area where hemlock and cedar find themselves in similar territory is climate resilience. A modeling study examining the vulnerability of fifteen tree species in the Pacific Northwest under climate change scenarios found that both western red cedar and western hemlock remained highly adapted under projected future conditions, with the potential for range expansion of up to about 50% relative to the baseline period.7Ecological Modelling. Estimating the vulnerability of fifteen tree species under changing climate in Northwest North America That is notable because many Pacific Northwest conifers, including some fir and spruce species, are expected to lose substantial portions of their current range as temperatures rise.
The reasons for their resilience differ somewhat. Western red cedar already tolerates a wide range of moisture conditions and can grow on both wet and dry sites, giving it flexibility as precipitation patterns shift. Western hemlock depends more on consistent moisture, but its ability to regenerate in deep shade means it can persist under denser canopies where other species struggle. Both trees are already established across broad elevational gradients, which gives them room to shift upslope as conditions warm without requiring long-distance seed dispersal.
Eastern hemlock’s outlook is more concerning, not because of climate directly but because hemlock woolly adelgid is expanding northward as winters become milder. Cold winters once limited the adelgid’s range, killing off populations before they could establish permanent footholds. Warmer winters remove that check, and the adelgid’s front line has moved steadily into New England and southern Canada over recent decades. For eastern hemlock, climate change is less about temperature tolerance and more about the indirect threat of a pest that climate previously held in check.
Cedar in Climate Science
Yellow-cedar has a surprising role in paleoclimate research. Its slow growth, long lifespan, and well-preserved wood make it useful for dendrochronology, the science of reading tree rings to reconstruct past conditions. A study generating blue intensity chronologies from yellow-cedar tree rings in Juneau, Alaska, found that the latewood signal contained a much stronger temperature record than ring width alone, correlating positively with January-through-August maximum temperatures for most of the twentieth century.8Canadian Journal of Forest Research. Yellow-cedar blue intensity tree-ring chronologies as records of climate in Juneau, Alaska, USA The chronology did lose sensitivity after the mid-1970s Pacific climate shift, a known complication in dendrochronology, but the pre-shift record was robust.
Hemlock is used in dendrochronology too, but its rings tend to be more responsive to moisture than to temperature, making it better suited for reconstructing precipitation histories than temperature histories. The two genera complement each other: researchers studying past climates in the Pacific Northwest can combine hemlock ring-width data (for moisture signals) with cedar blue-intensity data (for temperature signals) to build a more complete picture of historical climate variability. Old-growth cedar specimens, some over a thousand years old, provide especially long records. Few hemlock specimens reach comparable ages, partly because hemlock heartwood decays faster and old trees hollow out.
Choosing Between Them for Practical Projects
If you are picking between hemlock and cedar lumber for a project, the decision usually comes down to where the wood will end up. Cedar is the better choice for anything exposed to weather or ground contact without pressure treatment: fences, raised garden beds, outdoor furniture, siding, shingles, and decking. Its light weight also makes it easier to handle during installation. The main downside is cost; cedar typically runs two to three times the price of hemlock per board foot, with old-growth or clear grades commanding even higher premiums.
Hemlock works well for interior framing, flooring, trim, paneling, and anywhere the wood stays dry. It takes stain and paint well, machines cleanly, and its tight grain gives it a clean appearance. Some builders prefer hemlock for interior trim over cedar specifically because hemlock does not have the aromatic oils that can bleed through paint over time. Pressure-treated hemlock can be used outdoors, but at that point you are paying for treatment and losing the natural aesthetic appeal that typically drives people toward wood in the first place.
For landscaping mulch, cedar wins again. Cedar mulch resists decomposition and repels some insects, while hemlock bark mulch (sold in many garden centers) is valued for its deep reddish color and slow color fade but offers no pest resistance. Both work as ground cover, but cedar mulch lasts longer between applications. One caution with cedar mulch: the same extractives that resist decay can inhibit the germination of some small-seeded plants if the mulch is applied too thickly around seedlings, so keep it a few inches away from tender stems.
Scent, Allergies, and Indoor Air
Cedar’s distinctive aroma is one of its selling points for closets, saunas, and decorative paneling, but it can also be a drawback. The volatile organic compounds responsible for the smell, particularly plicatic acid in western red cedar, are a well-documented cause of occupational asthma in woodworkers. Sawing, sanding, or machining cedar generates fine dust that carries these compounds deep into the lungs, and repeated exposure can cause permanent sensitization. Hemlock dust causes the usual respiratory irritation common to all wood dusts, but it lacks the specific allergens that make cedar a particular hazard. If you work with cedar regularly, a respirator rated for organic vapors is a smart investment, not just a standard dust mask.
In a finished indoor application like a cedar-lined closet, the volatile compounds off-gas at low levels that most people tolerate without issue. Over time the surface oxidizes and the scent fades, which is why sanding a cedar closet periodically refreshes both the smell and the insect-deterrent effect. Hemlock paneling, by contrast, is essentially odorless once installed and does not off-gas anything of concern. For people sensitive to strong scents or with respiratory conditions, hemlock interior paneling is the more neutral choice.