The arborvitae plants known as Emerald Green and Smaragd are the same thing. More commonly called and marketed as the Emerald or Emerald Green arborvitae, its legitimate, registered cultivar name is "Smaragd". This tree is a selection of eastern arborvitae (Thuja occidentalis), native to eastern North America. It tolerates winter cold and summer heat very well, retaining a deep green color year round.
Emerald Green/Smaragd arborvitae is a semi-dwarf cultivar. It attains a narrow, pyramid-like silhouette with dense foliage. The glossy deep green needle scales grow in flattened sprays that look lacy. A common mature size of the plant measures 12 to 15 feet tall and 3 to 4 feet wide at its base. Very old, healthy plants can get up to 20 feet tall and 6 feet wide. In both hot summer and cold winter conditions the foliage remains deep green, not bronzing or dulling in sheen like other arborvitae.
Grow the Emerald Green/Smaragd arborvitae in regions with a defined cold winter dormancy period. This evergreen tolerates winter low temperatures between 15 and minus 45 degrees Fahrenheit, which correlates to U.S. Department of Agriculture Plant Hardiness Zones 2b through 8a. It does best in climates that do not have more than 100 days of temperatures above 86 F. All cultivars of eastern arborvitae look their best in humid climates.
Plant this arborvitae in sunny locations to promote the densest branching and foliage. Eight or more hours of direct sun a day suffices. "Smaragd" demonstrates some tolerance to drought, but the lushest growth occurs in deep, fertile soils that are always moist. Do not plant it in slow-draining soils that always flood or remain soggy after rain or irrigation.
Trim branch tips in early spring before new growth starts to shape the plant or maintain its size as a specimen or component of a hedgerow. Mulch over the root zone to conserve moisture, keep roots cool and to provide a supply of nutrients as the organic mulch materials slowly decompose.
Although not in USDA Zone 7 or 8, coastal California's cool and rainy winters are conducive for Emerald Green/Smaragd arborvitae culture. Irrigate plants in summer to ensure soil remains moist in the hot summer conditions. In windy regions, especially with hot air or cold dry air, plant the arborvitae in a protected landscape spot, as repeated bombardment by drying winds can cause some foliage to scald or die back. Deer graze on the leaves and insect issues that may arise include bagworms, spidermites and leaf miners.