Root Depth

• Most tree roots, contrary to popular belief, don't delve deeply into the ground. River birch roots have a dense mat of lateral roots just under the soil's surface. Lateral roots usually remain in the first 12 to 18 inches of ground, spreading out around the trunk like a fibrous pancake. These roots feed the tree, search for water and provide support. They also spread out many times the diameter of the birch's drip line -- a healthy, mature birch's root system can cover the equivalent of one-third of a football field.
  
  

Causes

• Older trees often have exposed roots.Hemera Technologies/AbleStock.com/Getty Images
  
  Roots don't pop out of the lawn overnight. River birches have strong lateral root growth and a need for water; if they don't receive deep and regular water, their roots will go hunting. Additionally, as roots continue to grow in diameter, the older roots of mature trees eventually grow out of the ground. This is especially true on dense, clay or compacted soils that discourage deep root growth. Mature lateral roots may grow quite large, often as large as a tap root. In some cases, rain and wind erode unprotected soil around the root system.
  
  

Solutions

• Roots have fine hairs that require oxygen.Hemera Technologies/PhotoObjects.net/Getty Images
  
  If you must cover surface roots, don't add more than 1/2 inch to 1 inch of soil. Deeper soil smothers the roots, causing slow but steady damage to your birch's system. Covering the roots with 2 inches of mulch is a better option, but don't let the mulch rest against the trunk. Don't plant a birch in a constantly wet area, either -- although river birches thrive in moist conditions, wet soils suffocate lateral roots and the birch may respond with more surface roots. Amend dense soils, and give your newly planted birch deep, regular water, particularly during hot weather.
  
  

Concerns

• Birches are bleeders; if cut in early spring, they ooze startling amounts of sap. If your mower or shovel nicks a birch root at this time, don't seal the cut. The copious sap washes any sealant off, and sealants may trap pathogens. Allow the cut to seal naturally. Avoid pruning exposed roots at all costs. Not only does this amputation slowly starve the birch, but it reduces the birch's stability, and the next windstorm may uproot the tree.