Officially known by its cultivar name "Smaragd," many nurseries sell this upright evergreen under the more recognisable and pronounceable name of "Emerald" or "Emerald Green." The compact and dense green needle sprays remain green even in the cold of winter. Although a dwarf selection of the eastern arbor vitae, Emerald will still mature 12 to 15 feet tall and 4 feet wide in a sunny to partial shade location. Such a large mature size makes it difficult to grow in a container, but when it is smaller and young, container culture isn't difficult.
An Emerald arbor vitae grown in a container is subject to more severe growing conditions than if in the ground. First, the soil temperatures fluctuate more radically based on exposure to air temperatures and sun rays. Soil drainage is faster and more complete, causing more drought conditions to occur any time of year. While this arbor vitae is tolerant to cold down to minus 4.44 degrees C in the ground, growing it in a container allows it survive winters that don't get colder than minus -6.67 degrees C. Also, it grows better with less water stress in regions where there are no more than 60 days of summertime temperatures above 30 degrees C.
The larger the container, the better. Large containers that are at least 10 inches deep and 24 inches wide will house enough soil for the arborvitae's roots to flourish. Limiting the container size and amount of soil affects how quickly the soil dries out or warms or cools based on air temperatures. Based on the size container, select an Emerald arbor vitae appropriate for container culture -- ideally a young plant that is about 2 to 4 feet tall. The smaller the plant is, the longer time is has to grow and live in the container before it gets too large.
Using topsoil in a container is problematic, especially since pathogens and other organisms exist in it. After a few cycles of wet to dry soil after watering, topsoil compacts and often becomes rock hard. Instead, use a soilless peat-based potting mix formulated for containers to plant the Emerald arbor vitae in the container. Sprinkle balanced formula slow-release fertiliser granules -- such as 10-10-10 with micronutrients -- on the soil surface to provide trace nutrients. An acid-forming liquid fertiliser product may be used according to label directions from spring to fall to supplement the granular fertiliser or any compost placed on the soil surface.
Always keep the soil evenly moist. A moist soil diminishes the chances of needles dying or yellowing as the Emerald arbor vitae grows. Even in winter the soil needs to remain moist. Water as needed; as the plant grows and the roots fill up the soil matrix, watering twice a day may be needed in summer's heat to keep the plant from experiencing dieback. As long as the container soil is not frozen, water the soil to keep the plant hydrated. Do not, however, create a soggy, flooded soil by overwatering. The container must have at least one drainage hole.