The Botany of Thuja Green Giant – Part One

One of the great things about gardening is its natural interface with the science of botany. There, in your garden, is a whole display of the breadth of the Plant Kingdom. (Or at least the more advanced parts of it, since algae don’t figure much in most gardens.) Mostly on this blog we think and write about the practical side of gardening, the “How Too” stuff, but looking more closely at your plants is an easy way to open the door into the science of nature and understand more about the plants on which we depend. Since Thuja Green Giant is found in many, many gardens, why not start with it? After all, the entrance to science has many doors. With that in mind, for gardeners who want to know more, and for students of horticulture and botany, young and old, here is a botanist’s look at this popular evergreen.

Evergreen or Conifer?

Let’s start right there. In the garden we usually refer to all plants that stay green all year as ‘evergreens. But it doesn’t take much observation to notice that really there are two distinct types of plants in this group. First there are the plants that have similar leaves to deciduous trees and bushes – flat, green leaves that are broad and thin. These are often called ‘broadleaf evergreens’ to distinguish them from the other kind, those with needles, like pine trees, or with green stems that don’t really have what look like leaves at all, but that are green. The first group are just plants that live in warmer areas, and they see no particular need to drop their leaves for winter, so they don’t. Their leaves may live two or three years, and often drop in spring, when new growth comes, rather than in fall. Often they are closely related to plants that do lose their leaves for the winter.

Thuja Green Giant belongs in that other group, which is more correctly called conifers. These plants are very different. To begin with, they don’t have flowers, which botanists define as a structure with enclosed seeds. Instead, these trees are Gymnosperms, and they have their seeds sitting open on a leaf like structure. It is this fundamental difference that put them into their own group.

The History of Conifers

The first Gymnosperms appeared around 350 million years ago, during the Carboniferous age, when the great coal and oil deposits we use today for energy were being formed. Their exact origin is not clear, but they may be evolved from earlier plants that were fern-like, but that produced seeds on their leaves. If you have seen Cycads, those strange plants that look half-fern and half palm tree, you have seen the earliest forms of these plants. Despite their appearance, these ancient plants have large cones that look very like those on pine trees!

Shortly afterwards two other groups formed. Of one, all we have left is the beautiful maidenhair tree, Gingko biloba. Although we might think this was a flowering tree, the structure of the wood and of the ‘fruit’ shows it to be part of the Gymnosperms.

The Different Conifer Groups

The other group formed by that division is today the most widespread and common – the conifers. There are about 1,000 species still with us, and many more extinct ones, since the rise of flowering trees pushed these plants to areas like the far north, where they could compete successfully. Within the Conifers (called Pinidae), are three large groups, called ‘orders’. These are, firstly, the pines and their relatives, like firs, spruce and true cedars – all tree with needle-like leaves. Then the South American tree called Monkey Puzzle, Araucaria, and the Asian Buddhist Pine, Podocarpus, are grouped together with several other conifers not normally seen in gardens. Third we come to the group containing many of our garden conifers, including redwood, bald cypress, hinoki and sawara cypress, and arborvitae. Yew is also included in this last group, which is called the Cupressales. Within it are three ‘families’, and it is in the one called Cupressaceae that we find Thuja Green Giant.

The ability to test the DNA of plants has led to many changes in our picture of their relationships. It is usually not the DNA of the plant itself that is studied, but the DNA of the chloroplasts – those green parts in the cells that carry out photosynthesis. These structures have their own DNA, and since they reproduce inside the cell by simple division, their DNA changes much more slowly than that of the plants they live in. So they are the perfect guide to long-term relationships between different plant groups. Carefully analysis has allowed us to create clusters of plant groups, and to see exactly how they are related. The resulting changes have bothered many gardeners and nursery growers, but they give us a much better picture of the plants around us.

The Genus Thuja

As a result of all this work and analysis, we now know that Thuja, the ‘genus’ of Thuja Green Giant, is most closely related to a whole cluster of genera of popular garden conifers. In that cluster are Chamaecyparis, which has the important and very varied Japanese cypress trees. As well, in that same group, we find Cupressus, the true cypress trees, from both Europe and North America, as well as the Junipers (Juniperus), which are found all across the Northern half of the world. This family is called Cupressaceae, the cypress family.

We can think of a genus as being like a human family which contains, instead of individuals, groups of individuals very similar to each other, called species. Some genera (the plural of genus) are large, while others are small, sometimes containing just one species. The closest relative of all to Thuja is a single Japanese tree called Thujopsis dolobrata, the asunaro tree, which is very similar in appearance to the Hinoki Cypress. Remember that these divisions are in the end arbitrary, and it depends on how big a difference botanists think is necessary to create a new genus, especially when it contains just one species, as Thujopsis does. For the moment at least botanists are keeping it separate, but that could change in the future. The organization of plants into groups is a human activity, which plants are ignorant of. As gardeners we are best to remember that, so that we see the regular changes that occur as man-made, while nature itself remains just what it is.

In the next part of this blog we will look more closely at Thuja and explore its structure and relationships. Who would have thought that Thuja Green Giant could open so many doors into the science of botany!