the orange gardener

3–5 minute read

Trees and evergreen shrubs are highly susceptible to storm damage. Conditions favoring storm damage, such as shallow soil, are not always correctable but should always be considered. Choosing appropriate species and caring for plants properly can eliminate damage potential in most situations. Try to avoid a combination of adverse conditions. For instance, trees that fall often meet all of these criteria: shallow-rooted, heavy or unbalanced crown, and sited where soil was saturated by the storm.

Site factors

Oxygen

Most site problems are due to lack of oxygen for tree roots. When planting a tree, choose one that is naturally adapted to your site conditions.

Soil grade

Changes in soil grade also are frequent causes of tree failure. Even a few inches of fill or soil removal can cause extensive root damage. If possible, never remove or add large amounts of soil within the drip line of a tree. The table below lists species with known sensitivity/resistance to changes in soil grade:

species sensitivity to changes in soil grade
Sensitive
Beech
Dogwood
most oaks
Pine
Sugar maple
Spruce
Tulip poplar
Resistant
Elm
Locust
Pin oak
Sycamore
Willow

When soil air is decreased by filling, certain gases and chemicals increase and become toxic to roots. Symptoms may appear within months or years after filling has occurred.

  • The extent of injury from filling varies with the species, age, and condition of the tree; the depth and type of fill; and drainage.
  • Clay soils cause the most damage because the fineness of the soil shuts out air and water more than a gravelly or coarse soil.
  • Three to four inches of soil can be added to small areas under the tree provided the soil texture is coarser than the native soil. Finer textured soils should not be used for filling.

Plant factors

Form

Good form is important at several levels:

Good branch angle is critical. A right angle from the trunk is the strongest. Crotches become progressively weaker as the angle decreases. A crotch angle should always be at least 45°. Poor crotch angles result in included bark in the crotch, which ultimately leads to splitting (the Bradford pear is notorious in this respect).

Branch organization is equally important. The tree should have a single leader and widely-spaced branches both vertically and radially. Pruning to reduce branch density can be beneficial for species like crabapples and crape myrtle. The form should be triangular with most weight at the bottom of the tree. Trees with poor form are highly susceptible to storm damage.

Poor form
European mountain ash
Green ash
Amur cherry
Hackberry
Littleleaf linden
Maples — box elder, red, silver
Bradford pear
Willows

Decay

For every 3 inches of branch or stem diameter, solid wood should comprise at least 1–1.5 inches. A branch or stem with less is more likely to fail during a storm.

Decay problems common
Aspen
Birch — gray
Hackberry
Lindens — basswood, littleleaf
Oaks — Northern pin, red, black
Maples — box elder, red, silver
Willows

Girdling roots

Stem-girdling roots lie at or below the soil surface and partially or completely encircle the trunk of the tree. They eventually compress the lower trunk, creating a weak point that is often the point of failure in high winds. Many stem-girdling root problems are due to poor planting:

  • planting pot-bound trees without first pruning off the girdling root (no matter how large) or spreading roots out if they are still flexible
  • planting too deep rather than with the first branch roots just below the soil surface
  • failing to remove synthetic burlap from a root ball. Synthetic burlap does not decay and roots will never penetrate into the native soil.

Trees that are native to floodplain areas (elms, maples, etc.) have the tendency to form ‘stem’ roots in sites where they are frequently buried by floodplain material. These trees commonly form encircling roots when planted in the urban environment, particularly when they are planted too deep. The species below are the most likely to develop stem girdling roots in urban situations.

Girdling roots likely
Green ash
Littleleaf linden
Maples — Norway, red, silver
Poplars

Next steps

For information on how to monitor and address storm damage, see: Storm Damaged Landscape Trees