THE GARDEN SPOT: Saturated soil and wind — a recipe for toppling trees

Alabama has seen some pretty exceptional rainfall in February. In the last week, North Alabama experienced a generational rain event.

Some areas received more than 12 inches of rain, and final amounts won’t be tallied for days to come. With such a large amount of rain, especially when combined with wind, trees toppling can become a concern.

Tree anatomy

Trees are an engineering wonder suspending tens of thousands of pounds of wood, branches and leaves. Normally, this weight is translated down the trunk of the tree and into the large structural roots, called the root plate, which supports the tree from both vertical (gravity) and horizontal forces (wind).

The best illustration for understanding the load-baring structure of trees, is to imagine a wine glass sitting on a dinner plate. In this analogy, the top of the wine glass is the tree’s canopy, the stem is the trunk and the base is the structural root plate providing stability. The dinner plate, the rest of the tree’s roots, are there to absorb water and minerals to “feed” the tree.

Trees are anchored to the soil by their roots. When the wind blows, the canopy moves and the trunk acts as a lever translating the forces into the roots. The roots push and pull against the soil and the friction between the roots and soil keep trees upright.

In extreme weather events, when the soil is completely saturated and the wind is blowing, the movement of the tree causes the wet soil to take on a gravy-like consistency. Once this begins to happen, the tree’s roots are no longer held by the soil, but are rather slipping and sliding through it. At this point, if the wind is strong enough, the whole tree might topple.

Mismanagement

The mismanagement of landscape trees may aggravate and increase the risk of whole tree failures. Trees are more likely to fall over if they’ve had roots cut or damaged.

Root removal within 8 to 10 feet of the trunk has a major impact on tree stability. If trenching of any kind (home foundations, utilities, driveways, sidewalks, irrigation, etc.) has occurred near trunks, roots were severed and the risk is greater.

Remember the wine glass analogy: If half or even one-third of the base is cut, how stable will the wine glass be? Less roots equals less holding power for the tree.

Root decay

Root decay is another factor that aggravates tree stability. If roots are cut or damaged, they become susceptible to rot and decay, further eroding stability.

Root decay can progress over a number of years, working its way from damaged areas towards the base of the tree. Often the only signs of decay are dead branches along the edges of the canopy or mushrooms on the lower trunk or exposed roots.

Compacted soil

Large trees growing on heavily compacted soils are also good candidates for toppling in wet windy weather. Tree roots can only grow in soils that have space for oxygen. The more compacted a soil is, the shallower the root system. Shallow roots plates do not provide strong anchoring, and more susceptible to saturated, soupy surface soils.

No room to grow

Similar to compacted soils, trees growing in tight spaces are also primed for toppling. Roots need ample space to spread and provide a radially distributed root system.

Trees in tight spaces can develop uneven or one-sided root systems (remember wine glass base). Examples include large trees planted in narrow street medians, parking lot islands or between sidewalks and streets. Soils in these confined areas also tend to be heavily compacted, further aggravating the problem.

Most of these root defects are preventable. Remember, don’t cut large roots and regularly look for signs of rot and decay. If planting trees in compacted soils or in confined spaces, select small to medium size trees more suited to those environments.

Finally, evaluate trees for any new leans. Prior to toppling, some trees could exhibit a lean as the adhesion between soil and roots fatigues. These trees should be considered dangerous and professional assistance should be sought immediately.

It is not uncommon to see trees topple in the days following heavy rains. If concerned about existing trees contact an International Society of Arboriculture-certified arborist. Many of these professional arborists are tree risk assessment-qualified and can help evaluate tree risk.

— Brodbeck and Rowe are regional Extension agents for forestry, wildlife and natural resources. For information on topics related to the home and garden, contact any office of the Alabama Cooperative Extension System. The Limestone County Office is located at 1109 West Market St. in Athens. Office hours are 8 a.m.-4:30 p.m. Monday through Friday. For more information, call 256-232-5510 or visit www.aces.edu.

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