Planting a new tree successfully depends heavily on setting it at the correct depth, which is the most important factor for its long-term health. Correct depth ensures the tree can properly exchange gases, access oxygen for its roots, and establish a stable structure. Planting a tree too deep or too shallow creates significant physiological challenges that can lead to decline or premature death. Proper planting begins not with digging the hole, but with correctly identifying a specific part of the tree that serves as the ultimate guide.
Locating the Critical Root Flare
The root flare is the transition point where the tree trunk widens out just before the roots spread laterally. This natural swelling indicates where the trunk tissue, which is not adapted to constant soil moisture, meets the root system. The top of this flare must be at or slightly above the finished soil grade. This positioning ensures the bark remains dry and exposed to the air.
Nursery trees, especially those grown in containers or balled-and-burlapped, often have soil piled up over this flare. Before digging the hole, carefully remove the excess soil or potting medium from the top of the root ball until the trunk begins to flare out. If the widening is not visible, continue to excavate until the uppermost structural roots are exposed. The final depth of the planting hole is determined by the location of this exposed root flare, not by the height of the container or the existing soil line.
Digging the Hole to the Correct Depth
The planting hole should be dug based on the depth of the root ball once the root flare is exposed. Dig a hole that is no deeper than the height of the root ball, ensuring the base of the trunk is not buried below the final grade. It is safer to plant a tree slightly high—an inch or two above the surrounding soil—rather than too deep, as the tree will settle over time.
While the depth is precisely matched to the root ball, the width of the hole should be significantly greater. Dig the hole two to three times the diameter of the root ball to encourage rapid lateral root growth into the surrounding soil. This wide, shallow excavation provides a large area of loosened soil that allows new roots to extend quickly, which is crucial for stability and nutrient uptake.
If the existing soil is heavily compacted, especially clay, the sides of the hole can become smooth or “glazed” during digging. This smooth surface can act as a barrier, preventing new roots from penetrating into the native soil. Gently roughening or scoring the sides of the hole with a shovel or fork breaks up this glaze and promotes root expansion beyond the planting area. Ensuring the tree is placed on firm, undisturbed soil at the bottom of the hole prevents it from sinking deeper after planting.
Understanding the Dangers of Improper Depth
Planting a tree too deeply is the most frequent and damaging mistake, often leading to a slow decline in health. When the trunk’s bark remains buried, the tissue is constantly moist, making it susceptible to decay, fungal infections, and insect damage. The lack of oxygen in the deeper soil also suffocates the roots, causing them to expend energy growing upward toward the surface.
This upward-growing response frequently results in the formation of girdling roots, which circle the trunk or other roots just below the soil surface. As the trunk and roots increase in diameter, these circling roots compress the vascular system, restricting the flow of water and nutrients. Trees planted too deep often show initial stress symptoms like reduced growth, smaller leaves, and increased vulnerability to disease.
Planting too shallowly, while less common, presents its own set of problems for the tree. When the root ball is significantly exposed above the grade, the roots are at a higher risk of drying out quickly, especially in hot or windy conditions. Exposed roots can also suffer from temperature extremes, leading to desiccation and reduced ability to absorb water and nutrients. A tree planted too high may also lack the necessary anchoring, making it unstable and prone to blow-over in strong winds.
Backfilling and Mulching for Long-Term Success
Once the tree is positioned at the correct depth, use the original soil removed from the hole for backfilling. Amending the soil heavily with outside organic matter is discouraged, as it can create a distinct soil boundary that prevents roots from growing out into the native landscape soil. Instead, simply use the unamended native soil, gently breaking up any large clumps as you return it to the hole around the root ball.
Backfill the hole in layers, gently tamping the soil with your hands or the back of a shovel to remove large air pockets without compacting the soil. Water the soil thoroughly during and after backfilling to help settle it naturally around the root ball and eliminate remaining voids. This careful process ensures good root-to-soil contact and maintains the established depth.
The final step is to apply a 2- to 4-inch layer of organic mulch, such as wood chips, over the planting area. Mulch helps regulate soil temperature, conserves moisture, and minimizes competition from turfgrass. Keep the mulch pulled back several inches from the trunk, ensuring the root flare remains completely exposed to the air. This practice avoids “volcano mulching,” which traps moisture against the trunk and replicates the dangers of planting too deep.