The Botany of Thuja Green Giant – Part Two

Most gardening blogs are about how to grow plants, and here on Thuja Green Giant we certainly write lots of helpful and accurate information about that. But there is more to plants than growing them, and as we explained in the first part of this blog, garden plants are also a great door into the world of botanical science. In this and the previous blog we are looking at Thuja Green Giant with a botanist’s eye, rather than a gardener’s. In the first part we looked at the place of Thuja Green Giant is the bigger scheme of things – its position as a conifer, and what makes them special, and where in conifers it belongs.

In this part we will focus on the place of Thuja Green Giant in its genus, and on a botanical description of the plant itself, and its origins. Hopefully this will be of interest to horticulture students, professionals in landscaping and horticulture, and to interested gardeners who want to know more about the plants they grow.

A genus is the smallest grouping botanists use in their classification of plants. We can think of this as like a human family, with each member of the family being an individual species closely related to the other members. Don’t confuse this with the botanical concept of Family, which is a larger grouping of related genera – perhaps closer to our idea of nationality. Thuja is in the family Cupressaceae, which also contains cypress trees, juniper trees and redwood trees. In all there are close to 30 genera in that family, totally about 140 individual species.

Thuja itself is a small genus of currently five species, with two found in North America and three in Asia. These plants are commonly referred to as Thujas, Arborvitae, or Cedars. The last name is confusing, as there is a whole group of other conifers that are Cedars, in the genus Cedrus, and they are very different. Arborvitae is a much better common name, but its use is patchy. All the Arborvitae have stringy bark, which is reddish-brown in color, and grow into tall forest trees with a single trunk, although the Eastern Arborvitae (Thuja occidentalis) is often multi-stem, and it is also not especially tall. The branches end in sprays of small ‘branchlets’ which spread out in a flattened fan shape.

Thuja (and some other conifers) has two types of leaves. Young seedling plants have short, triangular, pointed leaves growing all along the stems. These juvenile leaves continue for at least a year, and we think they are like this to make the seedlings less attractive to grazing animals, so increasing their chances of survival during that difficult period.

As the plants grow taller, they develop adult leaves. These are decurrent – that is, their bases grow down along, and are attached to, the stems. The adult leaves are of two kinds. On larger branchlets they have a sharp tip 2mm (about one-eight of an inch) long, and that is turned up away from the stem. On the flattened branchlets the leaves the leaves are pressed tightly onto the younger stems, like scales, which is why it looks green and ‘leaf-like’. There are four vertical rows of leaves that cover the younger stems completely. So there are two rows of leaves facing outwards, called ‘facial’ leaves, and two rows facing sideways, called ‘lateral’ leaves. The facial leaves are flat, but the lateral ones have a ridge along them, like the keel of a boat.

Botanists use detailed differences between the relative lengths and forms of the leaves to distinguish one species of Thuja from another, while experienced gardeners can often tell by the overall ‘look’ of the tree which species it probably is. Only a botanist can give a definitive answer!

Because Thuja are conifers, they produce cones, although these are not much like the pinecones most of us know. There are separate male and female cones. The male cones are small, at the ends of branches, and consist of just a few thick scales, making a tiny ‘cone’. Pollen is produced in spring from between these scales, and they are carried by the wind to female cones on other trees. The female cones are lower down on the sides of the branchlets, and they are larger, 9 – 14 mm (about ½ an inch). The female cone has 8 to 12 scales, arranged in pairs rotating around a central axis. When young they are greenish and the scales are closed, making a pea-like object. As they mature – which they do in a single year – they become brown and the open, releasing one to three seeds from the inner surface of each scale. Remember? These Gymnosperms produce ‘naked’ seeds, right on the surface of the seed-producing organ, not inside an ovary, as flowering plants do. This means that we see the seeds right there, sitting on the scale. The seeds have a wing of thin, papery tissue on either side, to help them blown away in the wind and spread to new places.

There are two species of Thuja growing wild in North America. In the east we find Thuja occidentalis, a tree of wet areas, and very winter hardy. It grows up into Canada, as well as all through the eastern states, especially around the Great Lakes and in the Appalachians. Trees are typically about 50 feet tall when mature, but there are exceptional trees that reach 125 feet in height. It can be a long-lived tree, where deer and fire do not reach it, and specimens over 1,500 years old have been found growing on rocky cliffs in Southern Ontario. This makes them the oldest trees in eastern north America. This tree is called northern white cedar, or eastern arborvitae. The name ‘arborvitae’ means ‘tree of life’, because a tea made from the foliage is rich in Vitamin C, and Native Americans taught early European explorers and settlers to use it in winter. It prevents scurvy.

 In the west we find Thuja plicata, a larger forest tree, usually called western red cedar or western arborvitae. This tree is much taller than its eastern brother, and it often grows over 200 feet tall, in time, with a massive trunk that can be 10 to 13 feet in diameter just above the base. This tree also grows old, well over 1,000 years, with the oldest specimen known being 1,450 years old. This tree is one of the parents of Thuja Green Giant.

The second parent is one of the three Asian species, Thuja standishii. This tree is called Japanese Thuja, and it grows in southern Japan, on the islands of Honshū and Shikoku. It is a forest tree that can grow over 100 feet tall. We have described in other blogs the fascinating story of the origin of Thuja Green Giant as a hybrid between western arborvitae and the Japanese thuja. The other two Asian species are Thuja koraiensis, a species found in Korea and part of northeastern China. It is a small, shrubby tree growing no more than 30 feet tall. The final species in this genus is Thuja sutchuenensis, the Sichuan cedar, from western China. It is a small tree, that may reach 60 feet, although no trees alive today are that tall. This tree was thought to have become extinct, because it was harvested for its fragrant wood. Then a small group was discovered in 1999, and the area is now protected.

These are the trees of the genus Thuja, and we hope this helps you understand more that hedge or screen you have of Thuja Green Giant.