If you do a little research, you will usually find that Thuja Green Giant Arborvitae is listed as an evergreen that needs full sun. Yet look around, and you will see good specimens and hedges of this plant growing pretty well in shade – so what’s going on?
To understand this a little better, we need to look more closely at the different types of shade in gardens. Understanding this will help you grow not only Thuja Green Giant, but all your other plants too. Basically, there are three or even four distinct types of shade found in gardens, and each of them has a different effect on plants growing in them. Once you understand the differences you will see why it is that you see the same plant doing well in shade in one place, and badly in another, including plants of Thuja Green Giant.
Types of Shade – One: Open Shade
The first type of shade we find is what is often called open shade. What is ‘open’ about it? Simple – just look up. Can you see the sky, free of tree branches or any other obstructions? If you can, then you are looking at open shade. Even though the direct rays of the sun don’t come through, the atmosphere scatters and spread the light, and all the wavelengths of sunlight are present, in a very similar balance to direct light. This is important, and it explains why many plants will grow well in these locations. We find this kind of shade on the north side of buildings (or south side, if you are reading this in Australia, New Zealand, South Africa or most of South America) and in the shadow zone cast be trees, most noticeably between fall and spring.
Sunlight contains all the wavelengths and colors of visible light – all the colors of the rainbow. Plants use mostly red, and some blue light, but not green light. This is why they look green – the green part of the light is reflected back to our eyes, and the other colors are absorbed for photosynthesis and growth. Open shade gives plants all the necessary colors, just less of them, so growth is possible, even if it is reduced. Indeed, many plants grow very well in open shade – hydrangeas for example. Especially in hot states, and in the south, some plants prefer to be in open shade, since the very strong sunlight can burn the leaves and inhibit growth.
So if you want to grow Thuja Green Giant in a place in open shade, it may grow well. Not as fast as in full sun, and it may not be quite as dense, but this reliable plant will survive and grow, especially if it gets a few hours of direct sunlight, as we will discuss a bit further down.
Types of Shade – Two: Overhead Shade, Deciduous Trees
If you look up in this kind of shade, you will see that it comes from tree branches overhead. These may be solid and dense, or they may be more open and showing some blue sky. They may be close overhead, or high up above you. The denser and closer they are, the less light there will be. This kind of shade has another disadvantage. Most of the light that reaches the ground has passed through the leaves, and the valuable red and blue colors have been extracted by the chlorophyll. This creates that lovely cool look in shady spots – at least to us – but for plants growing in these areas the light has less value, because a lot of the ‘photon goodness’ has been taken out before it reaches them. This is why plants that will grow happily in open shade will grow less well in overhead shade.
Now, the shade from deciduous trees has some advantages, because in spring most trees are slow to leaf out, allowing much more direct sunlight through. In fall too, after the leaves have gone, light comes through, and in warmer, southern areas this is especially helpful in winter, where temperatures may be warm enough for plants to still be growing slowly. So a Thuja Green Giant, planted in the overhead shade from deciduous trees, especially in a warmer zone, may still do OK. It will certainly be a bit thin and more open, but with some trimming it should be possible to keep a reasonable look to it.
Types of Shade Three – Overhead Shade, Evergreen Trees
Now we come to the really difficult shade, the sort of thing you find underneath a big old spruce or fir tree, or a laurel bush. Not only are many evergreens very dense, allowing very little light through, but the shade is unrelenting, just as dense in fall, winter and spring as it is in summer. Gardeners know from experience just how few plants will grow in these conditions, especially if the branches are low and close to the ground. Here, Thuja Green Giant is simply not going to make it, so use something more shade tolerant, like Yew or Plum-yew (Cephalotaxus).
Types of Shade Four – Seasonal Shade
Most of us know that the sun is in the sky for longer in summer than in winter – pretty basic stuff. As well, the sun is higher in the sky in summer, especially between the two equinoxes, March 21 and September 21. Each day up until June 21 the sun is a little higher, and then it goes lower again, until everything turns around on December 21 and starts again. So if you look at your garden in winter you will see a lot of shade, from buildings and the long shadows of trees, especially evergreen ones. But as the shadows shorten, areas that were in shade are now in full sun, and this coincides with the growing season too. If you plant Thuja Green Giant in a spot that is only shady in winter, it will grow almost as well as in a spot that is sunny all year, because most of the growth happens between spring and fall, even though the plant is evergreen. If you want to know if you can make a hedge with this great plant, or plant a specimen or two, it is best to look at the available light during the summer period, as any shade in winter has very little effect.
Ah, I See. . .
It’s obvious that shade is a complex subject, and simply rules like ‘grow in sun’ have to be thought through in each garden, and the areas more closely identified. Then you will be able to plant more effectively and get ‘the right plant in the right place’. Hopefully that will mean that you can grow Thuja Green Giant in more places than you thought possible.