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Don’t Let Landscaping Projects Kill Your Trees

Landscaping projects in Atherton do more damage to trees than do construction projects.  Why is that?  A construction permit requires a tree protection plan and ongoing inspections by an arborist hired by the owner.  Construction requires periodic inspections by the town building department and inspections by the town arborist when indicated.  But most landscaping projects require neither a permit nor inspections. 

Not all landscape architects or contractors know the best ways to protect trees.  Those that do know may be pressured by owners to bypass the best practices.  Yet trees are often the assets that contribute the most to the overall beauty of the landscape.  They represent the biggest landscaping investment and take the longest to replace if they die.

While unprofessional pruning or slashing a tree trunk with a backhoe can do irreparable harm, landscapers who kill trees are usually damaging the tree’s roots.  Most tree roots are within 6-8” of the surface of the soil and spread out from the tree in a pancake form.  Mature trees are actually more vulnerable to root damage from landscaping because their roots can reach out two to even three times as far as the branches.  These roots bring the tree nutrients, water, and, very importantly, oxygen.  Landscaping can rob any tree of these critical requirements in a variety of ways:

  • Rototilling the around a tree to plant shrubs or lawn.  Rototilling can destroy the majority of the tree’s roots, starving and suffocating the tree to death.  Many shrubs won’t grow under trees but, when they do, they should be planted individually in separately dug holes, preferably over a period of years rather than all at once.  The distance that lawns should be kept away from tree trunks varies with the age and type of the tree. 
  • Compaction.  Foot and wheelbarrow traffic compact the soil with more pounds per square inch pressure than most heavy equipment.  Compacted soil prevents sufficient moisture and oxygen from reaching the tree roots.  The problem is more acute in clay soils (common in Atherton) than in sandy soils.  Keep fences up around trees as long as possible.  If foot and equipment traffic can’t be avoided, put 4”-5” inches of wood chips around the tree (the chips let air and water through) and add plywood on top of the chips. 
  • Trenching for irrigation.  When a trench cuts across the circle of roots around a tree it can sever almost half the roots the tree needs to survive.  The best way to irrigate under a tree, when necessary, is to bring separate irrigation lines straight in toward the trunk.  Always dig by hand or with an air spade near trees, never with a trenching machine or a backhoe.
  • Changing the ground level around the tree.  Raising the ground level decreases the oxygen and moisture that reaches the roots.  Lowering the ground level cuts roots.  Drastic grade reduction can kill large roots supporting the tree and increase the chances of the tree falling over.  Leveling the ground around a tree on a slope cuts off roots on the uphill side and suffocates the roots on the downhill side.  There are ways to save trees while changing the grade and an arborist can help. 
  • Too much watering under trees.  New shrubs need watering for few years to get established.  Some shrubs need watering every year.  But watering in the summer encourages fungus that kills many trees – especially oaks.  Drip irrigation is usually the best solution, delivering small amounts of water directly to each shrub rather than spraying water evenly over the ground.  Any spray heads should be aimed away from tree trunks.    

One reason landscapers and owners are not more sensitive to root damage is that it takes months or even a few years to kill the tree.  But stressed and weakened by the root damage, the tree will eventually die from disease, insects, or something else.  Unfortunately, once the root damage is done it is difficult or impossible to reverse.  The tree looks worse and worse until it dies or falls over. 

The best way to prevent killing trees during landscaping is to have an arborist meet with you and your landscape contractor before work begins.  Some arborists say, “Saving trees happens before the project starts.”  Have the arborist put recommendations in writing and make them part of your contract with the landscaper.  Then have the arborist visit the job periodically to check that your valuable trees are being protected.

Live Oak The Atherton Tree Committee is a volunteer, non-profit, community based organization dedicated to the preservation of Atherton's heritage trees. The committee participates in a variety of programs designed to educate residents about the value of trees in our environment and to encourage appreciation and protection of our urban forest resources. For more information about town ordinances, policies, inspections, and plan reviews contact the Atherton Town Arborist, Sally Bentz (650-752-0526, or link to the Town of Atherton Web Site. For more information about the Tree Committee contact Rachel Croft, President (650 323-4714,