University of Maryland Extension

Mechanical injury

broken stems on azalea

Scattered dieback throughout a tree or shrub may be a result of mechanical damage. When equipment or machinery bangs into a tree or shrub it can crush the cambium layer or tear off bark and break branches. Repeated bumping of the trunk or stems, especially in young trees and  shrubs, can severely damage or kill sections of the cambium layer just below the bark. Sap flow is interrupted between the roots and leaves, causing some twigs or branches to die. Mechanical damage also opens the tree or shrub to disease and insect organisms, which may further damage the plant. Lawn mowers, tractors and string trimmers are the primary cause of mechanical damage to plants. Trees and shrubs planted near parking lots, sidewalks, driveways or roads are also vulnerable to damage from cars or foot traffic bumping into them. Avoid mechanical injury by maintaining a mulched, weed free area around ornamentals.

mechanical damage
Broken branches on juniper

damage due to compacted soil
Dieback on shrub from compacted
soil due to foot 

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