There is a very good reason that Thuja Green Giant is the top-selling evergreen for hedges and screens – it’s growth rate. Proven in trials to grow faster than any other evergreen, during its early years you can expect growth of 3 feet a year or more, so that solid, tall plants will be yours in just a few short years. But plant growth is complex, and when rapid growth is not seen gardeners are understandably frustrated and look for answers. Let’s examine in detail the different factors that control the growth of those Thuja Green Giants you just planted or are thinking of planting.
Some of the factors that control the growth of plants are a given of their location and can’t really be controlled by us. Let’s start with those, as they alone can often answer the question, “Why aren’t my plants growing faster?”
Although we rely mostly on the USDA hardiness zones to look at this, these only measure winter low temperatures, and don’t take into account other factors, such as rainfall. How do these things control the growth of your plants?
Winter minimums for your zone are averages, and in bad years the real numbers can be much lower. Severe cold snaps kill many plants considered hardy in a zone, while in other, milder years borderline plants thrive. Thuja Green Giant is hardy throughout zone 5, but if there is a severe period of lower temperatures, they can suffer. Foliage may burn, so that plants in spring are now smaller than they were the previous fall. With evergreens, the ability to take up water from the ground all winter long is essential to keeping the foliage green, and especially if plants are new, and a lack of snow cover during a cold spell allows the ground to freeze, then foliage burn can easily follow, even in ‘safe’ zones. If you see dead leaves and burned foliage in spring, then a hard winter is often the simplest explanation.
As well, the amount of sunlight and dry winds in winter are a big factor in foliage burn. Cold dry winds and bright sun will cause damage at temperatures that will be just fine in humid, cloudy conditions.
If you garden in a zone that typically has hot and dry summers, you will not see the same growth on your plants compared to places where they receive a regular water supply. Even watering cannot completely make up for those periods gardeners often call, ‘growing days’, when gentle rain encourages rapid growth. Only an Arizona-style full irrigation system can produce a lush oasis in hot, dry areas, and that is beyond the reach of most of us. So if growth happens mostly in spring and fall, with a long, dry, dormant summer period, you will not see as much growth on your Thuja Green Giant.
In exposed, windy places plants naturally respond by hunkering down, and creating broader forms, with shorter extension growth. Although Thuja Green Giant is often used for windbreaks – and it’s a good choice – the growth rate will be slower than in a sheltered area, which encourages the longest stem extension.
Every garden is different, so your Thuja Green Giant plants may not have ended up in the perfect spot for maximum growth. There are two main limitations that can only be fixed in theory, since were we plant is often determined by the layout of our gardens.
Thuja Green Giant grows best and fastest in full sun, yet in gardens many places, including where you want a hedge, may be shaded, at least partly, by trees or buildings. Not only will the growth be slower, it will inevitably also be more open, and creating a dense, solid hedge will be more difficult. Like wind exposure, you can rarely control this, since a hedge has to go where a hedge is needed, so if there are significant periods between early spring and late fall where your hedge is in shade, the growth rate will be reduced.
Competition from Other Plants
While shade from surrounding plants is obvious, their underground activity is not, and we underestimate how far the roots of trees spread. Those planting holes you created, filled with good rich earth and fertilizer, are magnets for surrounding mature plants, which will have their roots into them in a couple of years. Thuja Green Giant will not grow at its maximum rate when it is fighting with tree roots for water and nutrients.
How to Improve the Growth Rate of Your Plants
All the above elements are pretty much out of our control, but there is lots we can do, with a little work, to get the best from our Thuja Green Giant plants, even if there are intrinsic limitations.
This is number one. Digging deeply, adding organic material and basic nutrients, as well as started micro-organisms, all give your plants the best possible start. This will be reflected in the speed they grow. Some people are disappointed when they do all this and still see limited growth during the first year after planting. Remember that until they spread out into that inviting soil, your new plants are dependent on the root ball that came out of the pot, so slower growth in the first year is, unfortunately, something we can do little about. The maximum period of growth for newly-planted trees is usually from year 2 to year 5, and it is during those establishment years you will see the biggest changes.
How, and how often, you water depends on where in the establishment cycle your Thuja Green Giant plants are. During that first year there are two goals. The first, basic one is to keep your plants alive, and that means keeping that root ball moist. A common mistake is to thing that if the soil looks damp everything is good. In reality your trees are at first dependent on that limited volume of root ball, so keep it moist by watering close to the stem.
Equally, you want to encourage your trees to spread their roots outwards, and they will only do that if the surrounding soil is moist. See that as a separate job, and keep the nearby soil damp, especially deeper down. Since a lot of root growth happens in fall, early winter and early spring, watch out for drought at those times, not just during spring and summer.
After that establishment period watering should be deep, and spaced out, so that the ground dries between watering, especially in the top few inches. You want to encourage deep rooting, so moisture needs to be lower down in the ground.
For maximum growth, regular fertilizer application, either traditional or organic, is needed. This releases nutrients directly around the trees, so they have rapid access to them. In the first two or three years, liquid applications are best, as that really does get them down to the limited root area. After that, granular fertilizers work well, and they are a lot easier to use, especially if you opt for a slow-release type that only needs one application a year.