The Emerald Green Arborvitae (Thuja occidentalis ‘Emerald Green’ or ‘Smaragd’) is valued for its dense, bright green foliage and narrow, pyramidal shape, making it popular for privacy screens and hedges. Ensuring its long-term health hinges on planting it at the correct depth. Planting even a single inch too deep can lead to trunk decay and eventual death. Proper planting depth relates directly to a specific anatomical feature that must remain exposed to the air.
Locating the Critical Root Flare
The most important biological indicator for planting depth is the root flare, the point where the trunk widens before the structural roots spread outward. This transition zone must be situated at or slightly above the surrounding soil grade because the bark in this region is not adapted to constant moisture. If the root flare is buried, the bark remains perpetually damp, encouraging rot, girdling roots, and disease pathogens.
To identify the root flare, you may need to brush away excess soil or nursery material from the top of the root ball until the trunk visibly begins to flare out. Many nursery-grown trees, especially those in containers, have their root flares buried several inches deep under the potting medium. Exposing the flare is the first necessary step, as the depth of the planting hole will be determined by the height of the root ball once this flare is visible.
Digging the Planting Hole Dimensions
The correct depth of the planting hole is shallow, designed to ensure the root flare sits above the finished grade, while the width must be extensive to encourage lateral root growth. The hole should be dug to a depth that is slightly less than the height of the root ball, aiming for the top of the root ball to be 1 to 2 inches above the surrounding soil level. This slight elevation accounts for future settling of the soil and prevents the trunk from sinking below grade over time.
While the depth is shallow, the width of the hole should measure two to three times the diameter of the root ball. This wide excavation provides loosened soil, allowing the fibrous roots to penetrate and establish quickly. Before placing the tree, roughen the hole walls to prevent glazing, especially in heavy clay soils, which impedes root expansion. Use the excavated native soil for backfilling; amendments are not recommended as they can create a “container effect” that discourages roots from leaving the planting hole.
Setting the Tree and Initial Care
Once the hole is prepared, the tree can be carefully lowered into position, ensuring the visible root flare is sitting 1 to 2 inches above the final grade. If the arborvitae came wrapped in burlap or a wire basket, completely remove all twine, wire, and non-biodegradable burlap once the tree is stabilized. This prevents these materials from constricting root growth as the tree expands into the surrounding soil.
Begin backfilling the hole with native soil, tamping gently to eliminate large air pockets without compacting the earth, which hinders water percolation. Once filled, create a small soil berm around the outer edge of the planting hole to retain water during irrigation. Provide a deep, thorough watering to settle the soil and eliminate remaining air pockets. Finally, apply a 2- to 3-inch layer of organic mulch over the backfilled area, keeping it several inches away from the trunk to maintain the exposed root flare.