Trees in the landscape are there for important reasons, imparting beauty and balance within the landscape, and proving shade.
At the entrance into a bank building, there was a large raised bed in which a young pin oak tree had been planted. It was unique in its shape and gave beautiful fall color. Wanting to add additional color to the raised bed, a local garden club was given the responsibility for periodically placing annual flowering plants in the raised bed surrounding the tree, starting in the early spring, and then during the summer and fall months, making for a colorful display during the entire growing season. Over the next several years, no one paid any attention to the pin oak tree whose growth had slowed. Dead branches began to appear within its canopy each spring, but no one took notice. Finally, after several years, the pin oak tree failed to leaf out in the spring as it had died during the winter. Why? The constant stirring of the soil around the tree adversely affected tree root function, while the added fertilizer elements accompanying the annual flowering plant’s rooting medium added nutrient elements that were detrimental to the tree.
It should be remembered that feeder tree roots grow up toward the soil surface, not down into the soil. The roots that grow down into the soil provide a firm brace for the tree, but are not the major avenue for the uptake of water and nutrient elements. The most active area of root function is at what is called the “drip line,” that circle around the edge of the tree where rain water falls as the tree foliage acts as an umbrella. If the tree is to be irrigated and fertilized, it is at the drip line that such additions are most effective. Compaction of soil under the tree can also adversely affect root function, restricting the movement of air and water into the soil. Therefore, foot and vehicular traffic under a tree can reduce growth, and may even result in death.
For many trees, the absorption and movement of water and nutrient elements is in a direct pathway from the root up the truck of the tree and out to the branches and leaves as there is no distribution into the whole plant canopy. Therefore, any damage or root absorption restriction will be seen as a slow or impaired development on that side of the tree.
A homeowner had 2 pecan trees in his front yard, just at that age when they begin producing sizeable quantities of nuts. At this time, the homeowner was required to tie into a newly installed sewer line that went down his street in the front of the house. This required cutting a trench from the street to his house which was cut between the 2 pecan trees. Over the following years after the trenching for the sewer line installation, the sides of the pecan trees facing the trench developed more slowly than the other portions of the tree, looking as if that side of both trees were being manually pruned yearly.
Since the vascular vessels are just under the tree bark, any restriction around the trunk, or scaring of the bark, can interfere with the upward movement of water and nutrient elements from the roots into the plant canopy of branches and leaves. Support wires placed around the trunk and/or the hitting of the trunk of the tree with mowing machines can affect tree growth if the vascular vessels are being pinched or damaged. In an orange grove, the trees at the end of the double rows were being periodically hit by the edge of mowers when rounding the corners, sufficiently hard to cut through the vascular tissue. The result was that those trees were growing slower and produced fewer orange fruits than the trees next to them.
Most trees have fairly low nutrient element requirements and are sensitive to excessive levels of applied essential plant nutrient elements, particularly the 3 fertilizer elements, nitrogen (N), phosphorus (P) and potassium (K). Trees in the landscape that are surrounded by managed turf (lawns), or are in flower beds, can be adversely affected by fertilizer that is being applied to meet the requirements of the turf or flowering plants. Trees located on golf courses will be adversely affected by fertilizer and chemicals being applied to tee areas, fairways and greens unless steps are taken to minimize runoff that would carry these substances into the rooting zone of the trees.
I accompanied a County Agent who was making a visit to a home owner who was concerned about the status of several large pine trees in her front yard. They were dropping their needles and had a number of dead limbs in their canopy. There were fairly large flowering beds around these trees. In addition, the turf (lawn) under these trees was being well managed, frequently irrigated and fertilized to keep the warm-season grass growing in order to remain good “green” color during the hot summer months. The irrigation water and fertilizer nutrient elements being applied were adversely affecting pine tree growth. The solution did not set well with the home owner who was advised to move the flowering beds elsewhere and to be more judicious in irrigating and fertilizing the lawn under these trees.
Maintaining a tree or trees in the landscape in a healthy state can be a challenge. Understanding how trees grow and knowing their sensitivities can assist in making the correct decisions necessary to keep landscape tree or trees healthy.
Dr. J.B. Jones, Jr. is the author of several books including. Instructions for Growing Tomatoes in the Home Garden and Greenhouse. GroSystems, Inc., Anderson, SC (available in soft cover and e-book format for Kindle at Amazon.com).