Proper Planting and Irrigation Can Prevent Wind Throw of Trees

The past few weeks have brought the expected monsoon storms with strong winds and thick, blowing dust.  Sadly, there have been many trees that have been damaged or lost because of wind throw. 

Wind throw is a term that is used to describe the toppling of trees, large cacti, and some thick- branched shrubs by heavy winds.  It usually occurs when top heavy, poorly rooted, thick-canopied plants are pushed by strong winds, such as those found during our severe desert storms. Toppled trees fall over onto their sides leaving their roots upright and exposed to the air. 

The good news is that well rooted trees should be able to withstand even the most vicious of storms.  Wind damage is not unavoidable.  If sound horticultural practices are applied, most trees will be safe from even the most devastating of winds.  Proper planting and irrigating of trees and shrubs are the best protection against wind throw.

Most trees, especially the desert-adapted trees, are best planted following the instructions in the University of Arizona Cooperative Extension Bulletin AZ 1022 entitled “Planting Guidelines: Container Trees and Shrubs”.   Proper planting requires the excavation of a planting area three to five times the width of the root ball or box, but no deeper than the depth of the container.  The old adage of digging a hole “four feet by four feet by four feet” is no longer held in good favor because it tends to constrict the growth of roots into that specific, narrow area.  Large trees with small root systems are particularly susceptible to wind throw.

A caliche layer or other type of hard soil layer underneath the tree can also restrict the development of a strong root system.  If either of these are present, it is a good idea to drill or dig a drainage chimney to one side of the hole, so that excess water and dissolved salts can drain out of the root zone.  Make sure that the chimney is to one side of the root ball, not directly underneath.  With desert species, amending the soil that will go back into the hole is not recommended.

Timely and effective irrigation is another key to the development of a strong, dispersed root system.  Irrigation systems that are designed, installed and managed to provide the long and short-term water demands of the plant will help ensure vigorous root exploration and growth.

Please remember that roots only grow into moist, not dry, soils.  This is one of the basic rules of plant growth and we violate it only at the risk of causing landscape disappointments later on.   Irrigation wells or drip system emitters must reach out to the edge of the tree canopy in order to encourage the growth of a root system capable of withstanding the strong winds of desert monsoons.  Trees that receive water from small water basins next to the trunk of the tree will have small root systems that cannot provide good support during high winds.

Sometimes we want to cut back on irrigation expense.  If there is a large tree present, it will have developed an extensive root system wherever irrigation water has been applied.  If water is suddenly cut off to the tree under the guise of cutting back on the water bill, there is a good chance that the root system will also begin to die back, thus weakening the tree’s support.  As support weakens, the tree will be more susceptible to wind throw. 

A drip irrigation system is a good way to cut back on the water bill and still protect the health of trees and shrubs.  Installation of twelve drip emitters in an “X” pattern underneath trees is the best way to ensure good distribution of water with a mechanical irrigation system.  The first three emitters should be near the trunk with the last two in each arm of the “X” three to four feet out from the previous emitter.  This arrangement will allow root growth out to about twelve feet in all directions.  The resulting root system will be well dispersed and able to balance the top growth of the tree.  The more extensive the root system, the less likely a tree will blow over in the wind.

Root growth occurs throughout the year, not just during the active growing periods of the above ground parts.  With appropriate watering in the summer months, roots of a newly planted tree can grow beyond the leaf canopy in thirty to sixty days.  These newly planted trees and shrubs need the emitters placed near the trunk to help settle and compact the soil above the fresh root ball, but as the tree matures, however, emitters near the trunk should be closed in favor of emitters near the outer edge of the expanding leaf canopy.  Additional emitters might be needed if the tree canopy continues to expand past the last emitters on the line.

If stakes are used with young trees, they should not be left indefinitely.  Good root growth will encourage good trunk growth and allow the resulting tree able to stand on its own within a short period of time.  If a tree is dependent upon stakes and wires to hold itself up, there probably is a problem with the root system.

Tree form or structure also plays a role in wind throw.  Properly selected and shaped, low-branching, upright trees will have a natural strength that will stand up to strong winds. Some pruning might be helpful to lighten the wind resistance of the canopy but heavy pruning may do more damage than good.  A rule of thumb to follow when pruning trees and shrubs, especially the desert adapted mesquites, is to lightly and frequently remove about 20 percent of wood at any one time.  Most of the wood that is removed would come from the outer 20 percent of the canopy and promote natural branch tapering while opening up the canopy to light and wind.  This process also helps retain the natural beauty and appearance of the tree.

Wind throw is all too common a problem for trees and some shrubs during the high wind episodes of our monsoon storms, but, good planning and care of susceptible plants can help avoid the frustration of having to replant expensive trees every three to five years.  In Arizona, every tree is worth its weight in gold.

If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande.  The telephone is (520) 836-5221.

The University of Arizona is an equal opportunity, affirmative action institution.  The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice:    (520) 836-5221, 227
Fax:    (520) 836-1750
email:    gibsonrd@ag.arizona.edu