Oaks have a reputation for huge root systems, and they deserve it in one sense: a mature oak’s roots spread far beyond its branches. But “invasive” usually means roots that seek out and break into pipes and foundations, like willows or silver maples. Oaks are generally not in that category. In most yards the bigger risk runs the other way: people damage oak roots far more often than oak roots damage homes.
The short answer: oak roots are not considered invasive in the way willow or silver maple roots are. Most of an oak’s roots are in the top foot or so of soil and can spread two to three times the width of the crown. They can lift sidewalks and patios and will enter sewer pipes that are already cracked, but they rarely damage sound foundations. Oak roots are very sensitive to compaction and digging, so protect them during construction.
How oak roots grow
Many people picture an oak held up by a deep taproot. Young oaks do develop a strong taproot; the Morton Arboretum notes white oak is difficult to transplant because of it. As the tree matures, though, most of its root system grows sideways. The International Society of Arboriculture notes that the majority of a tree’s roots develop in the top 12 inches of soil.
Those shallow roots extend well beyond the dripline, the edge of the branches. For a large oak, that can mean roots reaching well over 50 feet from the trunk.
Mature oak sizes
| Oak | Mature height | Mature width |
|---|---|---|
| White oak | 50 to 80 ft | up to 100 ft |
| Bur oak | 70 to 80 ft | 70 to 80 ft |
| Pin oak | 60 to 70 ft | 40 to 50 ft |
Sizes from the Morton Arboretum. Roots typically extend well past these crown widths.
What oak roots can damage
Sidewalks, patios and driveways
This is the most common problem. As surface roots thicken, they lift and crack paving close to the trunk. Leaving a generous unpaved area around an oak helps.
Sewer and water pipes
Roots of any tree will grow into pipes that already have cracks or loose joints, where water and nutrients leak out. Oaks are not known as pipe seekers the way willows are, but an old clay sewer line near a big oak can still fill with roots. Intact, modern pipes are rarely a problem.
Foundations
Tree roots rarely crack a sound foundation on their own. In shrink-swell clay soils, though, a large tree near a shallow foundation can dry the soil during drought and contribute to settling. Keeping large oaks a good distance from the house avoids most of this risk. See how close to a house you can plant an oak.
The bigger risk: harming oak roots
The Morton Arboretum notes that white oak roots are sensitive to soil disturbances such as compaction and construction. Damage often doesn’t show for several years, and then the tree declines.
Common causes of root damage:
- Trenching for utilities, irrigation or footings within the root zone.
- Driving or parking heavy equipment under the tree, which compacts the soil.
- Adding or removing soil over the roots (grade changes). Even a few inches of fill can suffocate roots.
- Paving over large parts of the root zone.
If you are building near an oak, fence off as much of the root zone as possible before work begins, ideally out to the dripline or beyond. A certified arborist can help plan a tree protection zone.
Root grafts and oak wilt
Oaks of the same species growing near each other often fuse their roots underground. The Morton Arboretum explains that oak wilt can spread through these root grafts from an infected tree to healthy neighbors, and that root graft transmission is responsible for most new infections in groups of oaks. If an oak on your property dies of oak wilt, ask an arborist about breaking root grafts before removal.
Should you cut oak roots?
Avoid it whenever possible. Cutting large roots can destabilize the tree and open wounds to decay. If roots must be cut for a repair:
- Have a certified arborist assess the tree first.
- Cut as far from the trunk as possible, cleanly, with a saw rather than a backhoe.
- Never remove more than a small portion of the root system.
- Water the tree well afterward, especially in dry weather.
- Avoid cutting roots in spring and early summer, when oak wilt risk is highest.
Planting oaks wisely
Before you plant, the ISA advises you to look up for power lines and look down for underground utilities; call 811 before digging. Leave room for the roots as well as the crown, away from sewer lines, septic fields and large paved areas.
If you need help, the ISA Find an Arborist directory lists certified professionals. For oak problems, see why oak trees turn brown.
Sources
- White Oak, The Morton Arboretum
- Bur Oak, The Morton Arboretum
- Pin Oak, The Morton Arboretum
- Planting a Tree, International Society of Arboriculture, TreesAreGood.org
- Oak Wilt, The Morton Arboretum
Top photo: Kolforn, CC BY-SA 4.0, via Wikimedia Commons