A western red cedar’s roots look less like a carrot and more like a thick doormat: a broad, dense web of fine roots spreading out near the surface. For a homeowner, that means the roots are rarely a threat to the house, but the tree itself is easily hurt by whatever happens to the ground above them.
How Deep Western Red Cedar Roots Go
Western red cedar has a poorly formed central root, or none at all, to anchor it and reach down for water.110,000 Things of the Pacific Northwest. Thuja plicata (Western Red-Cedar) Instead the tree grows many roots that run outward.2Government of British Columbia. Western redcedar – Thuja plicata It cannot reach deep water when the surface soil dries out.3Nisqually Land Trust. Species Spotlight – Western Red Cedar
On wet soils the roots stay shallower and spread less than they do on deep, moderately dry soils.410,000 Things of the Pacific Northwest. Thuja plicata (Western Red-Cedar) Where a thick layer of fallen needles and decaying litter covers the ground, many roots grow in that layer rather than in the soil beneath it.510,000 Things of the Pacific Northwest. Thuja plicata (Western Red-Cedar) Still, red cedar, western hemlock, and Douglas-fir of similar size on similar soil root to similar depths.6USDA Forest Service (research.fs.usda.gov). Western Redcedar (Thuja plicata) When cedars seem shallower than firs, it is often because cedars tend to grow on wetter sites.
Roots also stop at hard layers. Cedar’s fine roots gather just below the leaf litter, on top of any hardpan, and in pockets of loose or organic soil.7Canadian Journal of Forest Research. Root System Morphology of Western Hemlock, Western Red Cedar, and Douglas-fir In stony or shallow soil, the root system becomes lopsided.8Canadian Journal of Forest Research. Root System Morphology of Western Hemlock, Western Red Cedar, and Douglas-fir
In a yard, “shallow” means you should expect fine, ropey roots within the first spade depths near the tree. A shovel pushed into mulch under a cedar will often hit a mat of them. On a compacted lawn over hardpan, the roots may run just under the turf. Shallow does not mean weak or few, though.
Because the roots stay near the surface, the tree depends on roughly the top foot of soil, and that layer is the first to dry out in a rainless Northwest summer. Wet ground keeps roots shallow because roots need oxygen, and waterlogged soil has little of it below the surface.
To learn about your own tree’s roots without hurting it, look at the ground first. Roots that bulge at the base of the trunk or ridge up through the lawn show where the main ones run. If you must dig, use a hand trowel or garden fork a few feet out and work carefully, since a shovel can cut the very roots you are trying to see. Stop if you meet a dense web of fine roots; the area is full of working roots and is no place for a hole. For a larger project, a tree service can use an air tool that blows soil away from roots without cutting them.
How Far the Roots Spread
Of three common Northwest conifers, cedar has the highest density of thin, rope-like roots and fine roots that take up water and nutrients.910,000 Things of the Pacific Northwest. Thuja plicata (Western Red-Cedar)10Canadian Journal of Forest Research. Root System Morphology of Western Hemlock, Western Red Cedar, and Douglas-fir Douglas-fir roots, by contrast, spread farther and are thicker than cedar’s.11Canadian Journal of Forest Research. Root System Morphology of Western Hemlock, Western Red Cedar, and Douglas-fir A Douglas-fir sends out fewer, thicker roots that travel far, while a cedar fills the nearer ground with a tighter mesh of thin ones.
For a yard, that means a cedar claims the soil close around it very thoroughly, leaving little water or nutrients for plants rooted in the same patch. A neighbor’s Douglas-fir may affect your yard from farther away, while a cedar’s effect is more concentrated near its own base.
Expect your tree’s roots to extend well past the outer edge of the branches. Roots curve around rocks and buildings, so they are rarely symmetrical, and lawn, beds, and the tree all compete for the same surface water.
Fine roots matter as much as big ones because they do the water and nutrient uptake. They are also easily torn by shallow digging, and losing them stresses the tree even if the big roots are untouched.
Setting changes the shape of the whole system. Trees that tower over their neighbors have proportionally larger and more even root systems than shaded trees.12Canadian Journal of Forest Research. Root System Morphology of Western Hemlock, Western Red Cedar, and Douglas-fir A cedar in a crowded stand has to share ground, so its roots end up lopsided and smaller. Buildings, pavement, and compacted ground block roots on some sides and push the system toward open, loose soil.
Will Cedar Roots Damage Foundations, Driveways, or Pipes
Arborists rate western red cedar as low risk for driveways and sewer lines because of its fibrous fine roots.13Blooma Tree Experts. Seattle Tree Root Damage Protect Your Property Now Even so, about 10 feet from paving, sidewalks, and patios is a common recommended distance.14Blooma Tree Experts. Seattle Tree Root Damage Protect Your Property Now
Low risk is not no risk. Any root can enter an already cracked sewer pipe that leaks water, and a tree in a narrow strip may lift thin paving as its roots thicken over decades. When a pipe is involved, the existing leak is the problem to fix, not the tree itself.
Roots grow where moisture and loose, aerated soil are, so a joint that leaks water and vapor can attract them, while a sealed pipe offers little reason to enter. Older clay or concrete sewer lines with many joints are more vulnerable than modern sealed plastic pipe. Once fine roots get inside a cracked joint, they can thicken into a mass that slows drainage. Repairing the leak or replacing the old joint is usually the lasting solution, rather than removing a healthy tree.
If you suspect trouble, slow drains near the tree call for a camera inspection of the pipe. Look for cracks that line up with the direction of the tree, and ask an arborist before cutting any large root.
How Far Apart to Plant Western Red Cedars
- Hedges: 8 to 10 feet apart.15Blooma Tree Experts. Western Red Cedar Trees: Complete Care & Growing Guide
- Windbreaks: 12 to 15 feet apart.16Blooma Tree Experts. Western Red Cedar Trees: Complete Care & Growing Guide
- Specimen (single feature) trees: 15 to 20 feet from other trees or structures.17Blooma Tree Experts. Western Red Cedar Trees: Complete Care & Growing Guide
The numbers differ because the jobs differ. A hedge is meant to grow together into one green wall, so plants sit closer than they would if each tree were meant to be seen alone. A windbreak needs a bit more room per tree because it is usually several rows or a longer line of trees meant to grow large and hold up against wind. A specimen tree gets the most room because it is meant to develop a full natural shape with its roots spreading freely.
Within a hedge, closer spacing fills in faster but means more competition for water. Wider spacing gives each plant more of the shallow soil to draw on, so in dry areas lean toward the wider end.
For the site itself, moist, well-drained soil suits cedar best, and spots that bake dry in summer are best avoided. Leave room beyond the spacing for the roots’ wide spread, and keep the tree away from future digging zones like utility trenches.
Watering and Drought Care for Shallow Roots
Because the roots cannot reach deep water, dry summers hit cedars hard.18Nisqually Land Trust. Species Spotlight – Western Red Cedar The tree tolerates a wide range of sites, from rich to very poor or wet soils,19Forest Ecology and Management. Ecology of western redcedar (Thuja plicata): Implications for management of a high-value multiple-use resource so soil moisture, rather than fertility, is usually what limits a yard cedar. Young cedars under moderate late-summer dryness do put extra growth into their roots,20Canadian Journal of Forest Research. Growth and morphology of western red cedar seedlings as affected by photoperiod and moisture stress but that is no reason to let a young tree go thirsty.
Water slowly and deeply under the outer edge of the branches, the area arborists call the drip line, not just at the trunk. Let it run long enough for water to soak several inches down rather than run off, which in many yards means an hour or more per spot. In hot dry weather, soaking about once a week beats a short sprinkle every day, because shallow sprinkling keeps only the top inch damp. Newly planted trees have a small root ball and need regular water through their first few summers while the roots spread into the surrounding soil.
To tell whether your cedar is short of water, look at the inner part of the crown, nearest the trunk. Browning or dropping foliage there is one common sign of drought stress, though it can also be normal needle shedding in fall. Thinning tips or a sparse crown after a dry summer can mean the same, and the damage may show up a season later. Check the soil by pushing a trowel or screwdriver in under the drip line; if it is dry several inches down, the tree needs a soak. Don’t judge by the lawn, since grass can look green from light sprinkling while the soil under the tree stays dry.
Mulch helps the surface roots. Cedar roots readily grow into a layer of organic litter,2110,000 Things of the Pacific Northwest. Thuja plicata (Western Red-Cedar) so a few inches of mulch out toward the ground under the outer edge of the branches keeps those roots moist and cool. Use wood chips or bark, and leave fallen needles in place. Keep the mulch a few inches away from the trunk, and don’t pile it into a volcano against the stem.
What Not to Do Around Cedar Roots
Adding soil near the trunk changes root depth, cuts the water reaching the roots, and lowers the oxygen they need.22Extension Ask. Creating a raised bed around a mature Western Red Cedar Raised beds built over a mature cedar’s roots can slowly starve the tree. If you want to change the ground level, keep the area under the branches at its original grade and build changes beyond it. Also avoid grade changes, soil compaction, and digging that severs roots.23For The Love Of Trees, LLC. Western Red Cedar Stress
Safer alternatives exist for most projects. Use containers instead of raised beds under the tree. Route trenches outside the drip line, or have them bored under the roots. Lay boards or plywood to spread the weight of machinery during construction, and plant small perennials in individual holes rather than tilling. Signs of root damage include dieback in part of the crown on the side where digging was done, which may appear one to several years later.
Do Cedars Blow Over or Get Root Rot
In pulling tests, red cedar resisted toppling about as well as western hemlock of the same trunk weight.24Canadian Journal of Forest Research. Overturning resistance of western redcedar and western hemlock in mixed-species stands in coastal British Columbia Some situations raise the risk. A tree that grew up sheltered among neighbors has not built up the wind-hardened trunk and roots of an open-grown tree, so it is more exposed if those neighbors are cut down suddenly. Saturated winter soil gives shallow roots a looser hold, so a windstorm after weeks of rain is riskier than a gale on dry ground. Soil heaving on one side of the trunk and cracks in the ground are signs that roots may be shifting. Mushrooms at the base can mean decay in the roots or lower trunk. Either one warrants a prompt visit from an arborist.
On disease, laminated root rot, a major killer of Douglas-fir, did not kill western red cedar in field tests.25Western Journal of Applied Forestry. Susceptibility of Western Conifers to Laminated Root Rot (Phellinus weirii) in Oregon and British Columbia Field Tests But when seedlings were deliberately infected, some root-rot fungi, including Armillaria, caused disease in 10% to 60% of them, depending on the species.26PubMed Central. Controlled inoculation provides insight into western redcedar resistance to multiple root- and butt-rot pathogens At the same time, the trees partner with a rich variety of root fungi that help them take up nutrients,27PubMed. Exploring the symbiont diversity of ancient western redcedars: arbuscular mycorrhizal fungi of long-lived hosts so a fungus on the ground is not automatically a sign of disease.
Disease fungi need a way in, and wounded roots, such as those cut by digging, offer easy entry. Good drainage and uncompacted soil keep roots healthy and make it harder for harmful fungi to take hold. If a tree has died of root rot, ask an arborist before planting cedar in the same spot, since some of the fungi can stay in the soil and infect new roots.
What Cedar Roots Do for the Soil
Soil under cedar is less acidic, with more calcium and a thicker organic layer, than soil under hemlock.28Soil Science Society of America Journal. The Influence of Western Hemlock and Western Redcedar on Soil Properties That is because cedars concentrate fine roots near the surface to take up calcium and nitrate.29Forest Ecology and Management. Adaptation to exploit nitrate in surface soils predisposes yellow-cedar to climate-induced decline while enhancing the survival of western redcedar: A new hypothesis
Leave the needle litter in place, since it keeps feeding that organic layer and raking it all away removes a free source of fertility. The tougher limit is competition for water, more than soil chemistry, so choose plants that tolerate dry shade. Newly planted perennials need extra water for their first season while they establish among the cedar’s roots.