Maturing into a narrow, upright cone 10 to 15 feet tall and only 3 to 5 feet wide, the Emerald arborvitae (Thuja occidentalis 'Smaragd') works well as a garden hedge or tall screen. Cultivated in Denmark and prized for its dense scaly foliage and retention of green coloration even across winter, the Emerald arborvitae is also sold under the cultivar name Emerald Green. Grow this evergreen arborvitae in U.S. Department of Agriculture hardiness zones 2 through 7, where there is a pronounced cool to cold winter.
Measure the size of the root ball of the Emerald Green arborvitae. Note the height (depth) and the diameter. These determine the size of the planting hole.
Locate an ideal planting spot in your landscape for the arborvitae. This evergreen tolerates any fertile soil that does not become soggy or flooded after rains or irrigation. For best growth and uniform shape, the plant needs to grow where it receives at least eight hours of direct sunlight daily. Although cold hardy, Emerald Green arborvitae may grow and look better if not planted where cold, dry winter winds bombard it.
Dig the planting hole with a shovel so it is equally as deep as the plant's root ball but two to three times as wide. Thus, a root ball that is 12 inches in diameter needs a planting hole that is at least 24 inches wide, although up to 36 inches wide is better.
Remove the rigid plastic container from the plant and lower it into the center of the planting hole. Avoid heavy handling that causes the soil around the roots to crumble away. If planting a ball-and-burlapped (B&B) arborvitae, cut off all wire or nylon rope ties from the root ball before planting. Although burlap decays, it does so slowly underground; remove as much as possible with a utility knife so the root ball comes in direct contact with soil.
Backfill unamended soil around the root ball of the shrub, tamping it down gently to stabilize the plant and remove soil air pockets. Place enough soil into the hole to fill it completely, but do not mound excess soil atop the root ball. The top of the root ball must match the top of the planting hole.
Water the soil and root ball of the newly planted Emerald Green arborvitae with a garden hose. Trickle water out of the hose to prevent erosion or use a shower head attachment. Provide enough water to wet the top 12 to 18 inches of soil. Add more soil after the water soaks away if compaction leaves the root ball top above the soil line in the hole.
Use leftover soil to create a 3- to 4-inch berm around the tree to create a catchment basin of irrigation water. You need to water the root ball and soil as needed to keep the soil evenly moist for the first 12 months after planting. More frequent irrigation is needed in hot summertime weather. Once the soil freezes, do not water.
Place a 3-inch layer of organic matter like coarse compost or bark nuggets atop the soil around the shrub. This mulch looks attractive and deters weeds, moderates soil temperature, conserves moisture and decomposes to provide nutrients to the arborvitae's roots. Replenish this mulch annually to keep it 3 inches thick. Extend the mulch 2 feet beyond the reach of the outermost branches and needles of the arborvitae.
Sprinkle well-balanced all-purpose fertilizer granules around the planting area no sooner than 30 days after planting, ideally in early spring or early fall. Do not place fertilizer into the planting hole or in piles atop the root ball. Scatter it evenly across the mulch.