EXTENSION REPORT

Alabama Cooperative Extension System/                                                                                                                        Beau Brodbeck                                                             

Baldwin County Office                                                                                                                                            Regional Extension Agent

302A Byrne Street                                                                                                                                          Forestry, Wildlife & Natural Resources                

Bay Minette, AL  36507                                                                                                                                                  March 11, 2008                                         

Understanding and Managing the Below-Ground Portions of Trees

The hidden below ground portion of trees we don’t see is often the ‘root’ of many problems in the urban forest.   Roots are as important as the trunk, branches or leaves on a tree.  However, because they are not visible it is difficult to understand the structure and characteristics of tree roots.  Failure to plan or manage for roots during new plantings or during construction around existing trees can lead to premature tree mortality or structural failure (i.e. property damage). As the late Dr. Alex Shigo wrote “If someday we are to know everything there is to know about the above ground portion of the tree, we will only know…half the story.”

Tree roots become even more important when the trees are growing in a city or suburban environment.  These areas have generally less soil and more barriers for tree roots to cope with.  This situation translates to problems for tree growth and the need for more planning.

Tree roots serve three primary functions; absorbing water, absorbing nutrients and minerals, and providing structural stability.  The absorption of water and nutrients are common knowledge.  Most know that trees need water and minerals from the soil for the production of sugars during the process of photosynthesis.  Without these vital elements trees die from drought or from insufficient nutrition, which is why trees need water and fertilizer. 

What is not always realized about tree roots is that they provide structural stability.  Ever wonder what keeps a large live oak standing during high winds in thunderstorms?  Trees have massive root systems to not only feed themselves, but to also keep trees upright.  Stability in trees comes from a radially distributed root system that anchors trees to the ground. 

Root systems are often as large as the above ground portion of a tree.  However, their distribution is far different.  “Much of what we commonly know about root systems is based on convenience or hearsay” said Jack Rowe, Urban Forester for the Alabama Tombigbee RC&D and Extension.  A quick examination of drawings, logos, or even some textbooks show tree roots as mirroring the above-ground tree canopy.  The idea of roots penetrating deep into the soil or extending only as far as the canopy or drip-line are common notions.  These notions, however, are incorrect.

Eighty to ninety percent of a tree’s root system is located in the top twelve to eighteen inches of the soil.  Roots need water, nutrients and air to survive.  Fresh air, especially, is limited to the upper layers of the soil. Thus, roots are generally found near the soil surface where organic matter as well as moisture and air are abundant. 

Additionally, roots regularly reach far beyond the drip-line of a tree.  A study conducted by the Morton Arboretum on a field-grown white oak illustrated that roots extended out radially as far as the tree was tall.  Laying a tree over on its side and measuring the distance from the top of the tree to its base will provide a rough sense of how far tree roots may extend.

Applying this basic knowledge of roots to designing a new planting or preserving existing trees during a construction project will greatly increase the health, safety, and longevity of trees.  When planting trees it is important to not only consider the amount of space the tree will need above ground, but also below ground.  Remember while the tree may be small today you need to plan enough space both above and below ground for the size you hope the tree to achieve. 

Research conducted by James Urban, a leading Landscape Architect and Arborist, provides a rough ratio of tree diameter to cubic feet of soil needed for healthy roots and trees.  His research roughly translates into a ratio of 50 cubic feet of soil for every inch in diameter at breast height (4.5 feet).  So, if you hope to have a newly planted tree reach 16 inches in diameter you will need approximately 800 cubic feet of soil available for the roots to colonize.  Remember, roots normally only penetrate between 18 to 24 inches, so this area needs to be based on no more than 24 to 36 inches in depth.

Roots of trees are common casualties when new homes, sidewalks, or sprinkler systems are installed.  Removal of tree roots can be far more devastating than cutting or damaging above ground portions of trees.  Root removal due to grading (remember roots are near the surface) or trenching are a major concern in many construction sites near trees.  Additionally, soil is often compacted by heavy machinery reducing the soil’s ability to absorb water and air needed for healthy roots. 

Research conducted by Dr. Kim Coder at the University of Georgia developed the concept of the “Critical Root Zone”.  This zone is the absolute root area trees needs to survive.  His formula for deriving the “Critical Root Zone” is for every inch in diameter at breast height multiply by 2.5 feet to derive the radius.  So if you have a 10 inch tree at breast height the critical root zone will be a radius of 12.5 feet or diameter of 25 feet.

Damaged or removed roots can translate into dead trees.  The amount of root system lost to a project near trees should be a major consideration of how the construction is planned, or a decision about keeping or removing the tree affected.

Trees grow from the roots up, and when roots become compromised the above ground portion of the tree will shortly reflect this damage.  Root damage can often be identified by dying twigs and branches in the upper or outer portions of the tree.  As roots are removed the tree can no longer support (supply) the outer-most branches in tree.  Trees decline from the outside edge towards their center.

Of greater concern is the possibility of structural failure due to compromised root systems.  Removing roots decreases tree stability.  Trenching along one side of a tree can remove 50 percent of a tree’s root system, increasing the chances of the tree blowing over during storms. Often, new homes are built next to or near large pre-existing trees which lose large amounts of their root systems to the construction.  Living in a hurricane prone region, like Baldwin or Mobile Counties, it is important to ensure trees have healthy radially distributed root systems for maximum heath, stability, and safety. 

So what does all this mean to the average citizen?  Understanding that a tree has a vital support system buried under ground can truly help a person make good decisions:

‘Do I have enough lawn to support a large tree?’

‘Will the new water line damage my shade tree if it goes past it?’

‘Should we keep the large tree or not if the new house sits next to it?’

Answering questions like these can help prevent costly arborist bills  or property damage and  help insure that valuable shade trees or ornamentals thrive, instead of failing to ever meet their promise of shade or beauty.

When you next look at a tree, remember, you only see “half the story.”

Upcoming Programs

There will be an Estate Planning workshop at the Loxley Civic Center on March 25 & 26 from 9:00-11:00 a.m.

            On March 28 a “Nutrition-It’s a Matter of Fact” program will be from 9:00-10:30 a.m. at the Thomas Medical Center in Daphne. The presenter will be Brenda Hale, MS, RD, LD, Diabetes Educator at Thomas Hospital.

            A Food Safety for Child Care Providers program will be held from 6:00-8:00 p.m. on April 1 at the Baldwin County Extension Office.

            Call the Baldwin County Extension Office at 937-7176 or 943-5611/928-0860, ext. 2222 to register for any of these programs.

Email address: brodbam@auburn.edu

Phone: 937-7176 or 943-5611, ext. 2222

Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.