A power auger is a mechanical drilling tool used in landscaping to rapidly create holes in the earth. While often used for fence posts, an auger can significantly reduce the labor and time required for planting trees, especially on projects involving multiple trees or compacted soil. Selecting the correct auger size directly impacts the health and long-term survival of the transplanted tree. The proper size ensures new roots have adequate space to grow out of the original root ball and into the surrounding native soil.
Determining the Required Planting Hole Dimensions
The choice of auger size begins with accurately measuring the tree’s root ball, which is the mass of roots and soil held together by a container or burlap. For container trees, the root ball diameter is the width of the pot. For balled-and-burlapped (B\&B) stock, the diameter is measured across the soil mass itself. This measurement determines the final dimensions of the planting hole.
Current arboriculture standards dictate that the finished hole must be significantly wider than the root ball to encourage root establishment. This extra width provides a volume of loosened soil that is easier for new roots to penetrate. Since the root system naturally grows outward, a wide hole facilitates this lateral expansion. A narrow hole can cause roots to circle, eventually leading to girdling that chokes the tree.
Selecting the Ideal Auger Diameter
The industry guideline for planting hole width is two to three times the diameter of the root ball. For example, a tree with a 15-inch root ball requires a hole between 30 and 45 inches wide. This requirement means the auger bit diameter selected will often be smaller than the final required hole width, especially for larger trees. Auger bits large enough to drill a 30-to-45-inch hole are typically reserved for heavy machinery like skid steers or excavators.
For most residential projects using hand-held or two-person augers, the maximum practical bit size is usually between 10 and 18 inches. When the required hole width exceeds this maximum diameter, the solution is to drill multiple, overlapping holes. For instance, to create a 45-inch hole for a 15-inch root ball, an 18-inch auger could drill three adjacent holes, which are then cleaned and shaped with a shovel. This technique leverages the auger’s power to break up the bulk of the soil while achieving the necessary wider dimension.
Using the Auger for Optimal Tree Planting Depth
While width is important for root growth, planting depth is a matter of survival; the hole should be no deeper than the height of the root ball. Planting a tree too deep is a common mistake, leading to suffocation due to lack of oxygen. The goal is to ensure the root flare—the point where the trunk widens at the base—is slightly above the final soil grade.
The auger should drill to a depth about one to two inches shallower than the root ball’s height. This slight under-depth accounts for settling after planting and ensures the root flare remains above the surrounding soil level. A concern with augers, especially in clay or heavy soils, is “glazing,” where the spinning action of the bit polishes and compacts the sides of the hole. This glazed layer can act like a barrier, preventing roots from growing outward.
To counteract this root restriction, the sides of the augered hole should be “roughed up” or scored with a shovel or spade before the tree is placed. This action breaks up the compacted layer, ensuring new roots can easily penetrate the native soil. If the auger drills too deep, the bottom of the hole must be firmly tamped with soil before placing the root ball to prevent sinking. Correct auger usage involves using its speed for bulk excavation and following up with manual techniques to optimize the hole’s structure for root health.