A few years ago, we contacted Michael Dana, Professor of Horticulture at Purdue University, regarding the growth of landscape plant roots. Specifically, we were looking to provide one of our clients with information related to several trees on their property and the impact, if any, their root systems would have on an underground pipe located six feet below the trees. The following is an excerpt from Professor Dana’s response:
There is a lot of misinformation floating around about landscape plant roots. To put it simply, tree roots grow near the soil surface. They do not grow, as is commonly thought, in a somewhat mirror-image of the above-ground portion of the plant.
A thorough review of the research literature on this subject published a few years ago stated “ninety-nine percent of the tree root system is located in the top 0.9 m (3 ft.) of soil” (Gilman, 1990, Journal of Environmental Horticulture). To be fair, Dr. Gilman does go on to qualify his assertion somewhat by recognizing that on very loose, deep soils deeper roots do occur, but that is infrequent. In the urban landscape associated with road, building and utility line construction, soil compaction rather than looseness is more common resulting in shallower rather than deeper root growth.
The reason woody plant roots grow in the top few feet of soil is primarily that the air required for cellular respiration is not available at greater soil depths. Roots are living plant organs that require oxygen. The deeper into the soil one goes, the less oxygen is present. Roots also have evolved to grow near the soil surface because in the natural world, plant nutrients from decomposing organic matter are also found near the soil surface.
An often misunderstood aspect of tree root growth is the taproot. Many people have the mental image of a tree with a large, deep vertical root similar to a carrot, only at the larger scale of a tree! Such a picture isn’t quite accurate. Most landscape-tree species are not strongly taprooted. Taprooted trees can be more difficult to transplant and thus are less common in the landscape industry.
It is true that some tree species (oak, hickory, walnut, and pine are usually cited) develop taproots that grow straight down. They do this from the earliest stages of seed germination, and in the wild those taproots may grow up to eight feet down and persist for the life of the tree. However, such taproots seldom extend more than a few feet into the ground. They naturally stop growing vertically for the same reason, lack of aeration, mentioned above. As taproot growth slows, secondary, horizontal roots take over, growing out radially from the tree trunk and developing at a depth (generally the upper 1 – 3 feet of soil) where air, water and nutrients are available.
It is also worth pointing out that a tree taproot, once cut, will almost certainly not commence vertical re-growth. Rather, the lateral roots take over. This is relevant to the issue of root depth of landscape plants because nursery-grown trees are all dug and re-planted at least once and usually several times during production. Any nursery-grown taprooted trees arriving at a landscape site have long-ago lost their taproots.
Another type of less common deeper tree root that is sometimes raised as a concern is a “sinker root.” Some trees growing on deep, loose, well-drained soils do develop sinker roots that grow down vertically from horizontally growing lateral roots. Gilman’s review indicates they may occasionally reach a depth of 3 – 6 feet, but they are small and usually unbranched. And they are also limited by the compaction and poor soil aeration found in the developed landscape.
To summarize, observations of root morphology and the experimental data support a focus for any discussion of tree root growth on the upper two or three feet of soil. In the built, or developed landscape, that focus might be more accurately aimed at the upper one to two feet of soil given the greater soil compaction that ordinarily accompanies construction.