Thuja Green Giant is an evergreen conifer, widely planted for hedges and screens. Its vigor and reliability come from its hybrid parentage, marrying the best features of each parent, and masking their limitations.

Let’s look at those parent plants and see what features they have that produced this remarkable plant that has been planted in millions over the last few decades.

The Basic Story

Thuja Green Giant began its life in Denmark, at the Poulsen nursery in Denmark. This family nursery dated back to 1878, and it has once specialized in roses. In 1937 an unusual Thuja plant was found there, but because of the Second World War very little attention was paid to it. In 1967 the National Arboretum in Washington, D.C. bought a shipment of plants from Poulsen, to look for possibly valuable introductions for American gardens. Among that shipment were several Thuja plants, which were put out in a nursery to grow. By the 1990s, one plant was catching the attention of visiting nurserymen who were amazed to see it had grown 30 feet tall in just 25 years. They were given pieces to grow and test, and a nurseryman from Tennessee called Don Shadow suggested the name ‘Green Giant’ for the plant. A major grower – Wayland Gardens – produced many plants, and promoted it as a replacement for older, diseased hedges, and the rest, as they say, is history. But how this plant developed was at first a mystery.

DNA analysis was a new thing in the 1970s, but some scientists from the National Arboretum, The New York Botanical Gardens and the Holden Arboretum took on the challenge, and they found this plant was something that had never happened before – a hybrid between two species of Thuja, one from Japan and one from America. We don’t know how it came about – probably a chance cross-pollination – but the result was a demonstration of something well-known to plant breeders. They call it ‘hybrid vigor’, and it is seen in many of our food crops for example. When two closely-related plants are crossed together the strongest genes dominate, and most of the weaker ones are hidden. As a result, the offspring are far more reliable and vigorous than either parent. But our interest here is in Mom and Dad – the two parent species. Let’s look at them in more detail.

Japanese Arborvitae – Thuja standishii

Known as nezuko (クロベ ) in Japanese, this tree is an important timber tree in Japan. Light and soft, and therefore easy to work by hand, but durable, waterproof and pleasantly scented, it is used for sake kegs, tubs, and other bent-wood items. It is one of the Five Sacred Trees of Kiso, along with Sawara cypress, Hinoki cypress, umbrella pine and Hiba arborvitae. These were, and still are, the trees used to build Shinto shrines, temples and palaces, so for common people cutting one down was punishable by death.

Japanese Arborvitae grows 65 to 100 feet tall in the forests of Japan, with broad trunks up to 18 feet in diameter 4 feet above the ground. The foliage has a pleasant lemony smell, and in appearance it is typical of arborvitae. Only an expert can quickly tell one species of arborvitae from another.

This tree is found in southern Japan, mostly on the islands of Honshu and Shikoku. We might expect it to enjoy hot weather, but in fact it grows only at high altitudes, in mountainous areas up to 8,000 feet high. This means it grows well only in cooler, moist climates, an in North America it grows best in the northwest and northeast. It will not grow in dry areas, and in fact if you wanted to grow one it would be hard to find a plant except in a specialized nursery, or if you grew it from seed. If you live in a part of the country where it will grow, you might be able to find one at a local botanical garden or horticulture school in a college or university.

Western Redcedar – Thuja plicata

Since we don’t know how Thuja Green Giant came about, we don’t know for sure which plant Mom was – the seed parent, and which was Dad – the pollen parent. But it is nice to think that it might have been Western Redcedar, the rugged American meets the Geisha Girl. Sexual stereotypes aside, the American side of this meeting is not such a different plant, although much more widely known to the average American.

Even non-gardeners know the lumber called ‘red cedar’. Waterproof, and so very popular for outdoor construction, it has a well-deserved reputation as the lumber of choice for gardens. It needs no preservatives, and ages gracefully to a soft gray color, while weather brings out the grain in the surface, creating over the years that ‘lived in’ feel to the garden. Don’t confuse it with cheaper white cedar, from eastern arborvitae, Thuja occidentalis. That wood lacks much smell, and it is generally inferior. Red cedar has a very characteristic smell, of spicy decongestant chest rub, from the camphor in it, which fades over time.

Red cedar lumber is cut from the Western Redcedar tree, the other parent of Thuja Green Giant. This is an even taller tree than its Japanese cousin, and old trees approach 200 feet in height, and they can be 1,000 years old. The foliage when crushed smells like pineapple, and it has the same stringy reddish bark as the Japanese Arborvitae. It grows on both sides of the border between Canada and the USA in the northwest, and on Vancouver Island. In the US it grows through Oregon and Washington State. It also grows as a forest tree around the world, from Australia to Hawaii, and in Great Britain it has become a naturalized tree, spreading by itself. It grows along rivers and in the lush forests of those areas, kept constantly moist by rainfall in all seasons.

Like Japanese Arborvitae, you would have to search for a tree to plant, certainly outside of its native states. In Europe and Great Britain it is widely grown, especially for hedges, as the mild, damp climate suits it well.

Thuja Green Giant

So given the parents, both of which thrive only in damp, cool places, it is remarkable what a tough child they had. From zone 5 to zone 9, in a wide range of growing conditions, including drier zones, this reliable plant certainly shows us what hybrid vigor looks like in trees. For consistent rapid growth and reliability, it is still the number one choice almost everywhere.