An ash tree’s roots run far more out than down, forming a broad, shallow mat rather than a deep spike. That means the soil you’re most likely to disturb with a shovel, a trench, or a new patio is exactly where most of the roots live. A healthy ash holds on remarkably well, and the real trouble for most ash roots comes from pests and disease that weaken them from inside the tree.
How Deep Ash Tree Roots Go
Like most trees, ash keeps its spreading side roots mostly in the top 12 inches (30 cm) of soil, with fine feeding roots growing near the surface.1Journal of Environmental Horticulture. Tree Root Growth and Development. I. Form, Spread, Depth and Periodicity Green ash, the most widely planted ash in North America, typically sends roots about 3 to 4 feet (0.9 to 1.2 m) down.2USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.) In one well-studied clay soil, green ash roots reached about 3.6 feet down and were spread fairly evenly through the top 3 feet.3USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.) Along the wet edges of ponds and sloughs, they have been found as deep as about 4.5 feet.4USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.)
In the southern part of its range, green ash has a saucer-shaped root system with no single deep main root.5USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.) Nurseries often cut the main downward root of young ash, and it does not grow back; several smaller downward roots form just below the cut instead.6COFORD. Poor quality planting of ash – getting to the root of the problem
Groundwater sets the depth limit. In mountain ash, which is not a true ash but forms a similar root pattern, anchor roots reached about 3 feet in drier ground but only about 2 feet or less where groundwater sat higher.7Journal of Forest Science. Mountain ash (Sorbus aucuparia L.) root system morphogenesis Roots cannot grow into waterlogged soil, so where the water table is high the tree grows only a shallow surface root system.8Journal of Forest Science. Mountain ash (Sorbus aucuparia L.) root system morphogenesis
This matters most when you’re about to dig. A fence post hole or garden bed cuts mostly into the main root zone, not below it, and a shallow utility trench of 1 to 2 feet runs right through the densest roots. Foundation footings often sit below most ash roots, though roots can still reach them, especially in shallow footings or along cracks.
A single post hole costs the tree little, but a long continuous trench cuts a whole line of roots and does far more harm. Before digging near an ash, mark the spot and probe for thick roots with a hand tool rather than a spade or machine, so you find them before you cut them. Buried gas, water, and electric lines run in the same shallow layer as the roots, so the call-before-you-dig service is worth using, and it also tells you where a trench can safely go.
How Far Ash Tree Roots Spread
The dripline is the circle on the ground under the outermost branch tips. In one comparison, about 54% of an ash’s roots grew outside it.9HortScience. Tree Root Spread in Relation to Branch Dripline and Harvestable Root Ball The farthest ash roots reached about 1.68 times the distance from trunk to canopy edge.10Arboricultural Journal. PREDICTING ROOT SPREAD FROM TRUNK DIAMETER AND BRANCH SPREAD So if your ash’s branches reach 15 feet from the trunk, roots may extend around 25 feet.
Mature trees spread very wide. A mature green ash in clay soil had side roots extending about 48 feet (15 m).11USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.) This wide network is what makes green ash fairly resistant to being blown over.12USDA Forest Service. Green Ash (Fraxinus pennsylvanica Marsh.)
To estimate your own tree’s root zone, stand at the trunk and pace out to where the outermost branches end. Multiplying that distance by about one and a half to two gives a cautious guess at how far roots may reach. It is an estimate, not a map. Roots grow toward loose, moist, aerated soil and tend to avoid compacted ground and pavement, so the zone is lopsided rather than a neat circle. During construction, fencing off only the area under the canopy isn’t enough: over half the roots can lie beyond it, so heavy machinery just outside still crushes roots.
Grass and shallow flowers compete with the ash’s fine roots for water, and raised beds piled over roots can smother them. Mulch helps the tree more than turf does.
Can Ash Roots Damage Pipes, Sidewalks, or Foundations
Tree roots growing into sewer pipes can cause blockages and pipe failures that are costly to repair.13MDPI / Sustainability. Review of Root Intrusions by Street Trees and Utilising Predictive Analytics to Improve Water Utility Maintenance Strategies Roots seek the water and nutrients leaking from cracked joints, which is why older clay or jointed pipes are most at risk. Catching root intrusion early through regular inspection limits damage.14MDPI / Sustainability. Review of Root Intrusions by Street Trees and Utilising Predictive Analytics to Improve Water Utility Maintenance Strategies
Signs that roots have reached your sewer line include slow drains in several fixtures at once, or toilets that gurgle when another fixture runs. A backup that clears and then returns within months suggests something is regrowing in the pipe, and roots are a common cause, though not the only one. A plumber’s camera inspection can confirm roots and show where they enter. Because most ash roots grow in the top foot of soil, thickening roots near paving can lift and crack slabs, especially where roots are squeezed into narrow planting strips.
Foundations are less at risk than people fear. Roots rarely break intact concrete; they exploit existing cracks. On shrinking clay soils, heavy water use by big trees can contribute to settling, but an ash several dozen feet from a house is rarely a foundation threat.
If roots are causing trouble, root barriers can steer them downward away from paving. For pipes, replacing cracked sections works better than only cutting roots out.
Will an Ash Tree Blow Over
Young ash trees withstood forces far beyond what 100 mph (160 km/h) winds would produce before breaking or tipping.15Arboriculture & Urban Forestry (AUF). Root Barriers and Windthrow Potential Young ash planted inside root barriers ended up even more resistant to tipping after three growing seasons.16Arboriculture & Urban Forestry (AUF). Root Barriers and Windthrow Potential The long-term effects of circling barriers on tree health, though, are still unknown.17Arboriculture & Urban Forestry (AUF). Tree Root Response to Circling Root Barriers Removing large roots on one side removes anchorage on that side, and the roots nearest the trunk matter most, so trenching close to the trunk is riskier than cutting near the root tips.
Some signs suggest a tree is in trouble. Soil that cracks or lifts on one side of the trunk after a storm suggests roots on that side are loosening or failing. A lean that is new, or growing, is more serious than one the tree has had for decades, since old leans are often balanced by growth. Mushrooms growing at the base of the trunk can indicate decaying roots underneath, although their absence does not prove the roots are sound. Having a certified arborist assess a tree showing these signs before the stormy season gives you time to act.
Emerald Ash Borer and Root Starvation
Emerald ash borer is an invasive beetle whose larvae tunnel under ash bark. In heavily infested trees that had lost more than 40% of their canopy, roots received less sugar from the leaves.18Arboriculture & Urban Forestry (AUF). Optical, Acoustical, and Fine Root Analyses of Emerald Ash Borer Infested Ash Trees Fine roots die off, leaving the tree less able to take in water, which can lead to decay in larger roots.19Arboriculture & Urban Forestry (AUF). Optical, Acoustical, and Fine Root Analyses of Emerald Ash Borer Infested Ash Trees
The canopy of an infested tree often thins from the top down, with bare upper branches while lower ones still leaf out. Small D-shaped exit holes in the bark are left by adult beetles leaving the tree. Woodpeckers stripping off patches of bark to reach the larvae leave pale, flaky areas, and the bark may split vertically over old tunnels. Shoots sprouting from the trunk base or lower trunk are a stress response, and they can appear when the upper tree is failing.
Treatment works best while most of the canopy is still alive, and an arborist can advise on systemic insecticide options. Trees with heavy canopy loss are often removed because dead ash becomes brittle. Near a home, a dead ash is a real hazard, and removal gets more dangerous and costly the longer it stands.
Ash Dieback and Root Rot
Ash dieback, a fungal disease widespread in Europe, causes dead patches at the trunk base and on roots.20Forest Ecology and Management. Synergistic negative effects of ash dieback and Armillaria root rot on health and stability of mature ash trees These wounds often let in honey fungus, a root-rot fungus.21Forest Ecology and Management. Synergistic negative effects of ash dieback and Armillaria root rot on health and stability of mature ash trees Rotted roots and trunk bases make ash prone to uprooting or snapping in storms.22Forest Ecology and Management. Synergistic negative effects of ash dieback and Armillaria root rot on health and stability of mature ash trees In one long-term monitoring of mature ash, about 75% developed trunk-base damage, nearly all colonised by honey fungus.23Forest Ecology and Management. Synergistic negative effects of ash dieback and Armillaria root rot on health and stability of mature ash trees
Clusters of honey-coloured mushrooms around the base in autumn can indicate honey fungus feeding on the roots or trunk base. Dark, dead bark near the soil line suggests the tissue there has been killed. White, fan-like fungal growth under loose bark at the base is a classic sign of honey fungus, and an expert usually confirms it. Dieback in the canopy, with twigs losing their leaves, can accompany these problems, although other causes also produce it.
A tree near paths or buildings with base rot needs an expert check. A healthy-looking crown can hide serious root decay.
Waterlogging and Drought Stress on Ash Roots
Narrow-leaved ash kept growing normally under prolonged root flooding, while common ash from drier sites was strongly harmed.24PubMed. Differences in C metabolism of ash species and provenances as a consequence of root oxygen deprivation by waterlogging Flood-sensitive ash build up compounds in oxygen-starved roots that may become toxic.25PubMed. Differences in C metabolism of ash species and provenances as a consequence of root oxygen deprivation by waterlogging
Drought works differently. Young green ash under drought grew less wood and responded mainly by reducing leaf area.26Canadian Journal of Botany. Effects of drought stress on hydraulic architecture of seedlings from five populations of green ash
For a stressed yard ash, water slowly and deeply so moisture soaks down into the top foot or two, and aim it over the outer part of the root zone instead of right at the trunk. A layer of mulch over the root zone keeps the shallow roots cooler and moist, but keep it off the trunk itself. Watering during a prolonged dry spell is worth doing, whereas a tree in soggy soil needs its drainage addressed instead of more water. Avoid adding fill or regrading soil around the tree, which can change drainage and suffocate roots in wet seasons.
Planting and Transplanting an Ash
Transplanted green ash survived and grew equally well whether moved bare-root, balled-and-burlapped, or in fabric bags.27Journal of Arboriculture. Harvest Method Has No Influence on Growth of Transplanted Green Ash Roots grown in fabric containers, though, tended to circle, like pot-grown roots.28Journal of Arboriculture. Field-Grow Fabric Containers Do Not Affect Transplant Survival or Establishment of Green Ash
Before planting, look over the root ball and loosen or cut any roots that wrap around it, since they can keep circling and fail to spread outward. Set the tree so the root flare, where the trunk widens, sits at soil level instead of buried. Expect a slow first year, as the tree is mostly rebuilding roots. Water consistently through that season, since the roots haven’t yet spread.
When choosing a spot, allow room for roots well beyond the future canopy, and keep the tree away from sewer lines and narrow paving strips. Consider borer risk in your region before planting ash at all.