Hybrid Vigor and Thuja Green Giant

Anyone who has planted Thuja Green Giant, or seen its rapid growth, even in difficult conditions, will have realized what a remarkable plant this is. Many will have asked themselves why this would be – what makes this plant stand out among so many others? The reason lies in the origins, and genetic nature of this plant, so to understand it more, and see why it is that such a plant can exist, let’s look at the whole concept of hybrids and why so many of them have such special properties.

What is a Hybrid Plant?

A common mistake is to call these plants ‘High-bred’, thinking that the word means they have been bred in special ways to a certain high point – rather like the way we breed racehorses or prize-winning dogs. Although this certainly gets at the value of these plants, it does not accurately reflect how they have been created.

There is a big difference between plants and animals, and it is this. Because animals can move around and interact with other animals, there are barriers against one species breeding with another. Although this does happen, as with mules (horses bred with donkeys) or ligers (lions bred with tigers), it is very rare. Plants cannot move around, however, so these barriers are not common among them, and it is often easy for one species of plant to cross with another. In nature different plant species grow many miles apart, so crossing can never happen, and there is no reason for mechanisms to develop that would prevent it. Once plants from different places are brought together in a garden, there is often very little to prevent closely-related plants crossing. Our garden are full of hybrid plants, and many of them came about because plants from different places were suddenly being grown together

A hybrid plant is one with parents that are two different species. They usually share the same ‘first’, or genus name, but the ‘second’, or species name is different.

The Parents of Thuja Green Giant

For Thuja Green Giant, one parent is Western Redcedar, called by botanists Thuja plicata. The second parent is Japanese Arborvitae, called Thuja standishii. Notice that the first name is the same for both of these plants, but the second, species name, is different. This is common for all hybrid plants – crosses between a plant from one genus with a plant from another are very rare indeed.

Western Redcedar is a close relative of the White Cedar (Thuja occidentalis), which is also known as Eastern Arborvitae. Together these plants grow from one side of North America to the other, fromhe Pacific to the Atlantic. While White Cedar grows no more than 50 feet tall, and usually a lot less, Western Redcedar can grow to 200 feet, and it easily forms a tall tree. It is a relatively fast-growing tree, adding as much as two feet to its height each year when young.

Japanese Thuja, also called Japanese Arborvitae, grows high in the mountains of Japan, but it is only rarely grown in gardens. It grows between 60 and 100 feet tall, and looks a lot like Western Redcedar.

Notice that these two plants grow thousands of miles apart – 4,700 miles to be precise. There is absolutely no chance that they could ever naturally meet, and breed. So when they were grown near each other, as happened in a nursery in Denmark back in 1937, there was nothing to stop them crossing, and a seedling growing from that event. The resulting plant, which was spotted by the nursery owners and put aside as something interesting, took another 60 years to be seriously noticed, and analyzed. Using modern DNA analysis, scientists at the National Arboretum were able to establish that this interesting Thuja really was a true hybrid between those two parent plants, coming from opposite sides of the Pacific Ocean.

Hybrid Vigor Develops

What is remarkable about Thuja Green Giant is that it is tougher and faster growing that either parent. This is possible because as most people know, genes come in pairs (well, when did you see a pair of Levis with just one leg?). Over time, some bad genes accumulate in plants, slowing down their growth, and making them more susceptible to pests and diseases. If these bad genes are just one of the pair, then that is usually OK. It is when they become both parts of the pair that problems develop. When individual plants continue to cross within one species for thousands of years, it is inevitable that some bad genes will become ‘normal’ among some of the thousands of gene pairs.

However, when hybrids happen, a good gene from one plant replaces one of the bad genes, and each pair gets a new good gene – hiding the effect of the bad ones. So the hybrid plant has lots and lots of great genes, and the bad ones don’t show – the plant can really give its full potential. Since each plant will have its own, unique, bad genes, they will usually be different from the bad ones in the other plant. So each parent gives strong good genes to the hybrid plant, and they ‘mask’ the bad ones of the other parent.

Plant breeders have known about this for a very long time, because they have seen it happen with corn, food plants, and many other plants too. So while we may be surprised at the vigor and growth-rate of Thuja Green Giant, experienced plant breeders aren’t. They know the virtues of hybrid vigor when they see it.

The Vigor of Thuja Green Giant

We only have to notice the growth rate of this remarkable hybrid to see hybrid vigor at work. While the parent plants grow at the very most 2 feet a year, Thuja Green Giant easily manages 3 feet or more – 50% faster – with very little trouble. Even faster rates have been recorded in young plants, so we can safely say the hybrid vigor doubles the growth rate.

That is just a beginning. While both parent plants are a bit ‘fussy’ about where they grow, Thuja Green Giant grows well in all kinds of soils, be they sand or clay, wet or dry. So strong is the hybrid vigor effect, that we have here a plant the will outgrow any other conifer on the planet, and produce a long-lived, hardy tree that is pest and disease resistant too. Even deer stay away.


So when you are thinking about hedging plants, you hardly need to think further than Thuja Green Giant. Hybrid vigor really works, and this plant is living proof of that.