Warning Signs of Leaning Oak Trees

Oak Facts

• Wherever you live, odds are there’s an oak tree nearby. From the rugged northern bur oaks (Quercus macrocarpa), hardy from U.S. Department of Agriculture plant hardiness zone 2 through zone 8, to the storied southern live oak (Quercus virginiana), hardy from USDA zone 7b through 10, there’s an oak for almost every place. The long-lived trees grow from 60 to 80 feet tall and their broad crowns and dense trunks together weigh up to 4 tons. Some trees lean their whole lives, but every tree has its tipping point -- the point when roots begin mounding or pulling up on the side opposite from the direction of the lean -- and you must decide whether the tree poses a hazard.

Roots

• Oak trees depend on a massive, shallow root system to keep them standing, despite natural and human intervention. Their roots grow within the top 16 to 18 inches of soil and can extend up to twice the diameter of the crown beyond the drip line, the circle beneath the outer edges of the crown. Most of these outer roots are collectors that gather water and nutrients, but the roots directly under the crown -- the tree's “plate” roots -- are critical to its support.

Environmental Causes

• Oaks are sun lovers and young trees might lean toward light from a shady corner. Rain-soaked loamy or sandy soil, flooded clay soils or erosion by a shifting river channel might cause the collapse of a fully developed root system if accompanied by severe storms or hurricane-force winds. Once the plate roots are compromised by wind and water, your oak begins to lean. If the tree’s roots dangle above ground or form mounds around the trunk, it will likely continue to lean -- or fall. If roots appear to be securely in place, an arborist can tell you if the tree has reached its tipping point. Use staking or tying to correct a young tree.

Human Error

• Anyone who decides that an oak has plenty of roots and can spare a few to excavation for a driveway or patio is committing assault on the tree -- likewise for the city’s new sidewalk or street widening project. Each set of roots supports a specific group of branches. Dig a few roots up and you may deny water and nutrients to a branch or two, causing spotty to major die-off. Excavate more, however, and you’ve weakened the plate roots that depend on the collector roots to keep them strong. Once the plate roots start scrambling to regain their strength, all your tree needs is a strong wind on a rainy day to start leaning.