A New Look at Root Growth in City Spaces

By Carl E. Whitcomb PhD. Lacebark Research

Contrast the trees in the foreground that are dead or near dead with about 16 square feet each versus the trees in the background that have grown well and are contributing to the city environment with about 400 square feet each. During the time the background trees have been growing and contributing to the landscape, those in the foreground have been replaced – twice.

Along with tight spaces come big challenges when tree planting is on the to-do list. Planting in areas surrounded by concrete and unfriendly soil increases the chance that trees will fail.

To increase your odds, look to the roots. Planting a root-bound tree is asking for trouble. This is something nursery growers know and understand. A new method to grow healthy roots is now being used, and at LCN we thought it may be of interest to contractors who also grow tree stock.

Through your planting experience, you understand that tight spots are especially tough for trees.

Exaggerated heat and wind, generally poor soils only a few inches deep with limited drainage, plus pollution, shading, and other factors make it especially challenging. Trees in general do poorly in city planting spaces for two basic reasons: failure of nurseries to develop tree root systems better suited to these conditions and insufficient space/ soil volume for root growth to sustain the tree following initial establishment.

Persistent poor performance of conventionally grown trees in city spaces forced researchers to investigate new approaches. Through their testing they have found a way to grow trees in such a way that success is more likely immediately following planting, in all but the most deplorable locations. Further, they found that if trees are provided additional space, the appearance and functional lifetime could be increased substantially. Planting trees in spaces with less than 30 square feet of exposed soil surface typically provides only a brief glimpse of green in the plant top. Even for the toughest species this is then normally followed by a slow decline leading eventually to death. Because of this fact superintendents prefer to plant one tree with a shallow, fibrous root system with at least 50 square feet or more of real estate, rather than planting three or four trees with inadequate space.

Try to avoid situations like this whenever possible. These trees were planted into openings about four by five feet. But once planted, concrete units were positioned over most of the surface leaving an exposed soil area of only about 16 by 20 inches. Scale is provided by the ball pen on the concrete.

Conventional Production

Trees grown conventionally in field nurseries typically have no roots in the top four to eight inches of the soil ball at time of planting into the landscape. This is due to a combination of deep planting initially of bare root stock and soil moved up and around the stem during cultivation. Bare root liners typically have a very poorly branched root system to begin with. To establish the tree, production of new roots out into the surrounding soil must occur.

The least desirable situation results from trees harvested balled-in-burlap and held above ground surrounded by mulch. Many roots are typically produced at the cut face of old roots because the mulch is well aerated. However, roots produced at the cut face of old roots consume energy but extend out into mulch material containing few nutrients. Further, these roots are mostly or entirely lost during transplanting. Following planting, new roots must again be initiated only under less favorable conditions of aeration and with fewer energy reserves in root tissues.

Pictured are Shumard oak trees in 100 gallon RootBuilder® containers in the foreground and 100 gallon RootTrapper® containers (in white) in the background, ready for landscape installation. Roots in these newly designed RootTrapper® containers (Inset Left) grow out, contact the sidewall, and since the tips are trapped they are forced to branch. However, roots in a conventional smooth walled container (Inset Right) will grow out, contact the sidewall and grow downward.

Maximize Utilization of Soil Volume

Limited space for root growth in city landscapes functions similar to growing plants in containers. With experience and experimentation it has become clear that a tree will grow and remain healthy in a given volume of container growth medium for a limited period of time. With smooth walled conventional containers, roots become concentrated and congested against the inner sidewall with little root exploration of the central volume of the container. Once that volume is exploited, root exploration and nutrient mining is restricted and the tree begins to stagnate and decline. On the other hand, health and vigor can be retained far longer when a tree starts out with a fibrous root system and roots are stimulated to branch and never allowed to circle and become congested. This allows roots to fully utilize the limited volume available.

When you are shopping for new trees, in actuality you are shopping for nurseries. A rule of thumb when purchasing from a new nursery is to have your crewmembers take a few trees out of their boxes or burlap and inspect the root balls. This will give you clues to how the nursery operates.

Differences among Containers

In order to grow trees best suited to city spaces air-pruning of the taproot must occur within a few days following seed germination. When air-pruning occurs about four inches below the seed, secondary and more horizontal roots form along the vertical axis of the very short, young, taproot. The key is to maintain roots mostly in a horizontal position with continued branching.

Trees grown with fibrous root systems and active white root tips at time of planting have a much greater likelihood of success in restricted city spaces. The fibrous root system can immediately begin to exploit any favorable soil volume that exists. However, trees grown with fibrous shallow root systems have the capacity to function well in restricted city spaces much longer than conventional field grown trees.