The long-term survival and healthy establishment of a newly planted tree depend on getting the initial planting hole dimensions correct. Proper planting involves a precise approach to depth and width, not simply burying the root ball. Correctly sizing the hole is the defining factor for a tree’s success, affecting its oxygen supply, water drainage, and ability to anchor itself against strong winds.
Determining the Correct Hole Depth
The most important rule in tree planting is ensuring the tree is not planted too deeply. The planting hole should be dug only as deep as the height of the root ball, measured from the base to the root flare. The root flare is the point where the trunk widens before transitioning into the root system. This flare must sit at or slightly above the final soil grade to prevent long-term health issues.
Planting a tree too deep has severe consequences because the trunk tissue is not adapted to being buried. Buried bark remains moist, encouraging fungal infections and rot at the base. Deep planting also restricts the vital exchange of gases, leading to oxygen deprivation for the fine roots, which are primarily located in the upper 12 to 18 inches of soil. When deprived of oxygen, the tree struggles, exhibiting symptoms like stunted growth and increased susceptibility to pests and disease.
A shallow hole is necessary to prevent the tree from sinking after planting. If the soil at the bottom is loosened, the weight of the tree and watering will cause the root ball to settle, burying the root flare. To avoid this, the tree should be placed on a firm, undisturbed base of soil. The top of the root ball should be slightly elevated, about one to two inches higher than the surrounding ground level, to account for the natural settling of the backfill.
The Importance of Hole Width
While depth is important for the trunk’s health, the hole’s width is paramount for the tree’s long-term stability and growth. The planting hole should be two to three times the root ball’s diameter. This creates a large area of loosened soil where new roots can easily extend and establish themselves quickly. Since tree roots spread horizontally and are shallow, a wide, shallow hole mimics their natural growth pattern.
Fine roots struggle to penetrate compacted, undisturbed native soil beyond the planting area. Digging a broad hole breaks up the soil structure, providing a less dense medium for rapid root extension. This aerated zone allows the tree to anchor securely and access a wider volume of water and nutrients during establishment. If the hole is only slightly wider, roots may circle within the disturbed soil, failing to spread and potentially girdling the tree. The sides of the hole should be sloped rather than straight down to encourage outward root growth.
Preparing the Root Ball and the Planting Base
Before placing the tree in the ground, both the root ball and the base of the hole require preparation. The bottom of the planting hole must be left firm, not loosened, to prevent the tree from sinking and burying the root flare. If the soil is heavy clay or drains poorly, the hole should be dug shallower so a portion of the root ball sits above the surrounding grade, creating a slight mound to improve drainage.
Preparing the root ball is essential, especially for container-grown trees that often develop circling roots. Circling roots must be addressed by shaving the outer inch of the root ball or making vertical cuts along the sides. These cuts interrupt the circular pattern, encouraging new roots to grow outward into the loosened backfill soil. For balled-and-burlapped trees, any wire basket, twine, or synthetic burlap must be removed from the top and sides once the tree is positioned, as these materials can constrict the trunk and impede root growth.
Backfilling and Final Positioning
Once the tree is correctly positioned with the root flare exposed, the hole can be backfilled using the excavated native soil. Avoid adding soil amendments like compost or peat moss to the backfill, as this can create an interface that discourages roots from growing into the surrounding native soil. The backfill soil should be broken up to eliminate large clumps and returned to the hole in layers around the root ball.
Lightly tamp the soil with your foot as you fill the hole to eliminate large air pockets without overly compacting the soil. Ensure that no soil is placed directly over the top of the root ball, which would bury the root flare and inhibit water penetration. The final step involves forming a temporary watering berm—a small ring of soil—around the outer edge of the planting hole. This berm acts as a basin to hold water, directing it downward to thoroughly soak the root ball and surrounding backfill immediately after planting.