When Strong Winds Uproot Trees
Marianne C. Ophardt
Washington State University Extension
Area Horticulture Specialist
It’s not unusual in this region for strong winds to sometimes blow trees over and or at least cause them to lean a bit more than they had been before the windstorm. Why do trees fall over in the wind? It’s usually due to the failure of the root system to keep the tree upright, not the size of the tree. In addition to absorbing water and nutrients, roots have the task of job of anchoring the tree along.
There are many diagrams of trees that show large trees with a big tap root directly beneath the trunk. This concept has been proven wrong by many different studies of tree root systems. Most trees do not have tap roots, especially in urban situations where the soil is compacted or where the there is a high water table. You will not find tap roots on most of the trees in your yard.
Most tree roots are in the top 6 to 24 inches of soil. Tree root system consists of larger, woody perennial roots and smaller, fine feeder roots. The woody perennial roots horizontally, radiating outward from the trunk. Most of these roots will be found in the top six inches of soil, but vertical anchoring roots along this horizontal network may grow several feet deep or more. The function of the woody perennial roots is anchorage, water conduction, mineral transport, and food storage.
The fine feeder roots are much smaller than the perennial roots. Their function is to absorb water and nutrients. They grow outward and upward from the perennial roots and are predominantly located in the top several inches of soil. Feeder roots die and are replaced on a regular basis. These feeder roots are only about 1/16 of an inch in diameter or smaller and increase the root system's surface area. This is important because, the greater the surface area, the greater their capacity to provide water and nutrients to the tree.
Major perennial tree roots may not be as deep as we've imagined in the past but they are longer than most people think. The framework of roots often grows outward to a diameter one to two times the height of the tree. This is much further than the simple branch spread or "drip‑line", where they were once thought to terminate. On average, tree roots spread about three times the spread of the branches. Studies have indicated that over 50 per cent of the roots are outside the drip‑line.
The major perennial roots have the job of anchoring the tree. In order for a root system to support a large, long‑lived plant like a tree, it must be free of structural defects and the main structural roots should be distributed evenly around the trunk. Wind‑throw results most often from poorly distributed roots or poorly developed root systems.
Poorly distributed roots can result from kinks and circling roots; improper planting practices; physical damage or severing of roots; restriction of root growth; or death of roots from cultural or climatic factors. Poorly developed root systems can result from improper planting practices; compacted and shallow soils; and poor growing conditions.
Roots fail to hold a tree upright when there isn't a healthy, well‑distributed root system. When a tree uproots and falls over in the wind, we have to ask ourselves what caused its failure. This takes examining the roots and trying to detect the cause. It also involves reviewing the trees situation and history. Were major tree roots severed recently or years ago? Were there any girdling or kinked roots evident? Was there a poorly developed system? What might have been restricting root growth on one or more sides?
Can fallen trees be uprighted and saved? Large trees, over eight feet tall, can't and shouldn't be saved. Removal is the only option. Even if you can successfully upright the tree, it may become a hazard because of the damage that has occurred to woody roots anchoring the tree. Uprighted trees can pose a serious hazard, especially as they grow in size and girth.
For trees smaller than eight feet, it may be possible to upright the tree and save it. However, it will probably only be successful if one‑third to one‑half of the roots are still in the soil and not exposed and if the roots which are exposed are fairly compact and undisturbed.
The process of “uprighting” the tree may require some type of lift equipment to pull the tree upright. Before pulling it upright, you should remove some of the soil beneath the exposed root mass to allow the root mass to be situated at soil grade level. Once you have the tree back in place, fill in the soil around the roots as needed, being sure to keep the tree at the same soil level as it had been growing. Water the tree thoroughly to get rid of air pockets and settle the soil around the roots.
Since the tree has lost a portion of its anchoring roots when it blew over, you will need to provide some support in the form of staking. Drive two stakes into firm ground about six to eight inches away from the trunk. Then tie the trunk to the stake at the lowest height which stabilizes the tree in an acceptable upright position. The stakes should be removed when the tree can stand by itself. Check the tree every year to make sure it's still stable. If it's not, removal may be needed to maintain safe conditions.