A compact, upright evergreen with rich green foliage all year round, the Emerald Green Arborvitae is an excellent plant for small to medium-sized hedges or as a specimen plant, especially, but not exclusively, in colder growing regions. A selected form of the American arborvitae, discovered in Denmark, this plant has stood the test of time, and it is a reliable choice superior to the wild plant for its rich green foliage color in winter, and more compact form, that needs little trimming.
Emerald Green Arborvitae
Emerald Green Arborvitae is a coniferous tree with a slender, upright profile. It forms a narrow column, typically reaching 5 to 10 feet tall in ten years, with a spread of about 2 feet after ten years. Mature plants may be 12 to 15 feet tall, and 3 to 4 feet wide. Young plants will grow between 6 and 12 inches a year, depending on the growing environment and the level of care given. Growth continues throughout the life of the plant, but it reduces to only 1 or 2 inches a year in untrimmed plants. Mature trimmed plants produce more vigorous new growth after trimming, and they may grow shoots of 6 inches each year, indefinitely.
Individual plants can live for many years, although in garden situations the vast majority will succumb to storms or decay within a century. When trimmed into hedges their effective life is shorter, because even with tight trimming they will eventually outgrow their allotted space, and then they cannot be regenerated by cutting back to bare branches – these will not re-sprout. Consequently hedges have an effective life of 30 to 40 years in most garden situations.
Trunk and Branches
Young trees are often multi-stem, but older plants, untrimmed, will eventually develop one or a few main trunks, which can in time reach a diameter of 6 inches or more. The bark on younger branches is reddish-brown, and a medium to dark gray on older trunks and branches. Mature bark is ¼ to ½ inch thick, and fibrous with deep fissures which cause the bark to peel off in long, narrow strips.
The leaves are scale like and very small, no more than 1/6th of an inch long, and less than 1/10th of an inch wide. They are pointed, only slightly glossy, and mid-green in color. They are tightly attached to the young stems, called branchlets, and these grow as flattened, fan-like sprays. Branchlets live for several years, before turning brown and falling off in a piece from the branch, which will by then have new, green branchlets further along it.
Emerald Green Arborvitae is a conifer, so it does not produce flowers, but it may produce cones, although these do not very much resemble more well-known cones such as those of pine trees. Cone production is relatively rare, and they are usually only produced on older trees, and usually not on trimmed plants, but this can occasionally happen. Young cones are of two kinds. Male cones produce pollen, and these are very small, reddish colored, and carried on the tips of the shoots of branchlets in spring. Female cones are larger, beginning in spring yellowish-green, and maturing over summer to be ½ inch long, rounded, and with small spikes on them. When they mature, they turn brown, and open to release the seeds, leaving the cone looking like a miniature rose flower. Seeds may be grown, but they will not have the characteristics of this particular variety, and they will more likely resemble the wild parent tree.
Uses in the Garden
Emerald Green Arborvitae is often used as a green vertical accent specimen, placed on a lawn, in a planting hole in a paved area, or in beds. It may be planted as a single specimen, or in groups. Aesthetically, groups arranged in odd numbers (3, 5, 7, etc.) are more attractive. The spacing between plants in groups can be varied – close spacing will develop into a single clump, with multiple growing crowns, while wider spacing will retain the individual character of each plant. Specimens can also be grown in large planter boxes and pots for several years, for screening or decoration on a terrace, patio or balcony. Specimen plants have a naturally-dense habit, or they can also be trimmed for a more formal appearance.
This tree is also widely used to create hedges, spacing plants approximately 2 feet apart for a solid, flat-surfaced hedge, or 3 feet apart for a more informal hedge. It can also be planted in a row at wider spacing to give a screening effect without creating a solid wall. Hedges and screens may be trimmed up to three times a year, for a very tight, formal appearance, or less often – annually or bi-annually – for less formal effects. Because of its compact form, good screening can be created without any trimming at all.
Emerald Green Arborvitae contains Thujone, an essential oil, which is also found in other arborvitae, some junipers, oregano, sage, and wormwood (Artemisia). The oil extracted from Thuja or these other plants is sometimes used in herbal medicine, although higher doses can cause convulsions, and its presence in food and alcoholic drinks is strictly limited (European Union, Canada) or banned entirely (America).
A tea made from the foliage of American Arborvitae was used by native Americans to prevent scurvy, and this was taught to early French explorers, preventing many deaths and allowing early colonies to survive the harsh winters. This led to the tree being called ‘arborvitae’, which is Latin for ‘tree of life’.
The Emerald Green Arborvitae is an easy plant to grow in a wide range of garden conditions and climate zones. It is particularly useful in colder zones, and in moist to wet soils.
This plant grows best, with the densest foliage and fastest growth, in full sun, or in light partial shade. A minimum of 4 hours per day of direct sunlight during the growing season is needed – levels below that will produce poor, straggly and open growth. Plants placed in very low light will deteriorate over time, losing most of their foliage and growing poorly. The best growth is seen in plants receiving 6 hours or more of direct sunlight each day.
Emerald Green Arborvitae grows well in a wide range of soils, with a preference for damper soil. It will grow in soils from acidic to alkaline, and from sandy loam to clay. The best plants are grown on moist, well-drained soil, but wet soils are also tolerated well. Only dry, very sandy soil is less suitable, as regular dryness will limit growth and cause yellowing of the foliage
Newly planted trees should be watered once or twice a week for the first few months, until the root system has become established. This tree benefits from regular watering throughout its life, but once well-established it will tolerate periods of normal summer drought.
This plant is very winter hardy, tolerating minus 50o Fahrenheit, making it hardy to zone 2 in the USDA plant hardiness rating system. The foliage of the parent plant turns brown at very low temperatures, but the Emerald Green Arborvitae remains green, which is a major factor in its popularity among gardeners.
The American Horticultural Society has recently introduced a Heat Resistance rating, based on the average number of days per year above 86o Fahrenheit. In this rating system, the Emerald Green Arborvitae has a rating of 7, which corresponds roughly to a line running from North Carolina to Utah and Northern Nevada. Alternatively, it is rated to zone 7 or 8 in the more-familiar winter hardiness rating, meaning it begins to grow less well when winter lows never fall below 0oF.
Pest and Disease
Emerald Green Arborvitae does not normally suffer from any serious pests or diseases. When plants become infected or diseased it is usually a sign of poor care, particularly lack of water and fertilizer.
Occasional pests and diseases can include:
Cedar Leaf Miner – caterpillars of this small moth may cause the death of branchlet tips by burrowing inside the young stem. Trimming in early spring and encouraging strong growth will usually prevent it becoming a significant problem.
Bagworms – this moth caterpillar makes a protective case out of plant debris and hangs it from a branch. They are easily controlled by hand-picking.
Scale & Mealy-bugs – these small, sap-sucking insects accumulate on stems and foliage. They look like brown or white bumps, as white ‘fluff’, or a sticky, roughness on stems. Although relatively common, they are only rarely significant, and well-grown plants usually don’t suffer any significant damage from them.
Spider Mites – these microscopic insects create tiny holes in the leaves. Heavy infestations can make the foliage look bronzy or yellowish. They are most common during hot, dry weather. Watering the roots and spraying the foliage with water will usually prevent significant problems.
Tip blight – this fungal disease can cause branchlets to brown and die. It is usually seen in heat-stressed plants, and it is unsightly but rarely a serious problem.
Deer – this tree is unfortunately often selected by deer for winter grazing.
Young plants benefit from fertilizing in spring and summer. Mulches of organic material will provide significant levels of nutrients, and this is usually enough for untrimmed trees, unless they are growing in poor, sandy soil. Chemical or organic sources of nitrogen will accelerate growth, and they are often needed for trimmed trees and hedges, since trimming removes nutrients which need to be replaced.
A wide range of blended fertilizers, made from either natural or synthetic materials, are available, normally labelled as ‘hedge food’, ‘evergreen tree and shrub food’, or similar designations. All are equally effective, although those that include micro-nutrients may give better results, especially on poor soil. Slow-release formulations reduce the time spend applying fertilizer in a season. Avoid feeding after early fall, particularly in colder zones, as this may encourage new growth that can brown in winter.
Trimming and pruning
Emerald Green Arborvitae can be trimmed between early spring and early fall, as needed. Avoid trimming too early or too late in colder areas, to avoid frost damage to new growth. Do not trim during hot, dry weather, particularly if the soil is dry. Use sharp tools, either hand-shears for single plants, or power hedge trimmers for hedges or multiple specimens. Untrimmed plants remain neat and compact, but any loose branches can be cut back with hand pruners (secateurs).
Like other arborvitae, this plant does not have the ability to produce new growth from old, leafless branches, so it should not be cut back below the level of the foliage. Bare limbs will never re-sprout. For this reason, begin trimming hedges when they are still young, to develop a dense structure, and trim regularly, as large amounts of excess growth cannot be safely removed.
Emerald Green Arborvitae originated as a selected seedling of the American arborvitae, also known as eastern white-cedar, eastern arborvitae, northern white-cedar, or Thuja occidentalis.
Natural distribution and ecology
Thuja occidentalis is one of two species of Thuja growing in North America. This is a small genus, containing just five species in all, and the differences between them are minor, and not easily distinguished. American arborvitae grows in eastern Canada, from Ontario to New Brunswick, and in the north-eastern states, from Minnesota to Main, and as far south in mountainous regions as Tennessee. It grows generally on rocky, alkaline soils, around lakes and along streams, on cliffs, mountains, and in wetlands. Trees are typically around 60 feet tall, but they may occasionally be taller. This species is very long-lived, and some specimens are among the oldest trees in North America – over 1,000 years old.
History of breeding
American arborvitae has the distinction of being the first North American plant to be introduced into Europe, in 1540, by the French explorer Jacques Cartier. It is widely grown in colder parts of Europe as an ornamental tree. In 1950, at a Danish nursery called D. T. Poulsen, an unusual seedling was noticed, with a more compact habit of growth, and with rich green foliage all winter. It was named ‘Smaragd’, and it became widely popular. When it was introduced into nurseries in America it was given the informal name of Emerald Green, since ‘smaragd’ is Danish for ‘emerald’. That is the name it is usually known by today.