Tree Root Systems

Tree roots serve a variety of functions for the tree. Roots absorb and transfer moisture and minerals as well as provide support for the above ground portion. There are two basic types of roots, woody and nonwoody.

Nonwoody roots are found mostly in the upper few inches of soil. The primary function of these roots is to absorb water and nutrients. These are often called feeder roots. In addition, some trees, particularly deciduous trees such as ash, have extensions called root hairs which increase root surface area and increase nutrient and water uptake. Evergreen trees such as pine may not have root hairs but possess mychorrhizae. Mychorrhizae are fungi which live on and in the feeder roots. This fungi do not cause any harm to the tree. In fact, for some species it is very beneficial for the tree to have this fungal association.

Woody roots are large lateral roots which form near the base of root and stem (the root collar). The primary purpose of these roots is support and anchorage for the tree. They also provide water and mineral transport as well as carbohydrate storage. These roots are distinct for each tree species and provide the framework for the tree's root system. The general direction for this framework is radial and horizontal. These roots are located 8 to 12 inches below the soil surface and can extend 4 to 7 times the drip line of the tree. These roots are perennial and show annual growth rings, which is why many tree roots eventually become exposed.

In drier soils, some tree species will form "striker roots" at intervals along the framework system. These roots grow vertically downward until they encounter an obstacle or soil with insufficient oxygen for growth. They will often branch and form a second layer of roots deeper in the soil. These roots function as water and food storage areas for the tree.

Another type of root is the adventitious root. These roots will often form spontaneously at the root collar from large woody roots. Although it is not known exactly what causes their formation, they usually develop as a result of injury.

There are many misconceptions about root growth in trees. Horizontal root spread is one of the more important. It is often said that the majority of feeder roots are concentrated at the dripline of the tree. Roots extend to that distance and much farther. Studies have shown root spread to be 4 to 7 times the dripline distance (radius) of the tree. This is an important fact to remember when applying herbicides, fertilizers, insecticides, and other soil treatments around trees. Careful consideration can prevent serious injury to your trees.

Another misconception is root depth. Roots will grow wherever the environment is favorable. They require water, oxygen, minerals, support, and warmth. These requirements are usually found in the upper few feet of soil. Roots rarely grow below four feet although there are numerous cases stating the opposite. The major portion of a tree's root system is in the top few inches of soil. This makes it easier to understand why trees can be easily uplifted during wind storms or other soil disturbances.

The main point to take home from this is that tree roots are extensive and are located in the upper few inches of soil. Broadcast fertilizers are very much available to the tree roots as are herbicides and other chemicals. Soil compaction is one of the biggest problems a tree root can have. Water and oxygen become unavailable when the pore spaces are closed. Avoid large grade changes during construction, both filling and removal. Avoid the use of plastic as a mulch or under mulches, use weed barriers that breathe. Many tree problems are accidental, by understanding more about the tree root system, these problems can be avoided.

This article originally appeared in the April 1, 1992 issue, pp. 1992 issue, pp. 43-44.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 1, 1992. The information contained within may not be the most current and accurate depending on when it is accessed.