When we plant a Thuja Green Giant hedge or screen, we want fast, strong growth. That is why we chose such a fast-growing tree. Usually hedges are put in for privacy, and it is hard to really feel at home in the garden until that lush green wall goes up, keeping the outside world truly outside, and screening us from unsightly views, traffic noise, wind, drifting snow, and even the entry of unwanted animals into our gardens. So it makes sense, right from the start, to have a well-planned fertilizer program in place, so that the growth of our new hedge is fast, lush, healthy and durable. There are many products on the market, all claiming to be ‘the best’ fertilizer available. It is a great help to understand the basics, so that the label can give us some real information, and we can read past the advertising and get exactly what we need. Let’s begin with some basics:

The Big Three Plant Nutrients

Plants are very different from animals, and the first thing to realize is that the food groups and vitamins we need have no relevance at all to plant nutrition. Plant live on minerals from rock, dissolved in water, making everything else they need from them, from carbon dioxide in the air, and from the energy of the sun, unlocked by the process of photosynthesis. The only part of that we have any control over is those minerals, and plants use just three in significant quantities. These are the represented by the three numbers seen in the fine-print on fertilizer packaging, called the Fertilizer Ratio, and looking like ’20-20-20’ or ’12-5-15’.


If we want an analogy with our own diet, nitrogen is the protein of the plant diet. It is the nutrient used the most, and indeed, it does go into all the proteins needed by plants, which are not as numerous as they are for us. The main plant proteins are all enzymes for growth. As well, nitrogen makes DNA, and the pigment chlorophyll for photosynthesis. From a practical viewpoint, we can see immediately that for plants to grow, cells must divide, and each dividing cell needs DNA. As well, without chlorophyll, no growth can take place. So nitrogen is the growing nutrient, encouraging new shoots, green leaves, and in our Thuja Green Giant, nice long stems, of a rich green color, and quick recovery after trimming.

High levels of nitrogen are found in all general-purpose hedge fertilizers, and you should look for a big first number in the Fertilizer Ratio.’20’ or ‘30’ are numbers that indicate a fertilizer bursting with nitrogen. When you use a high-nitrogen fertilizer, it is easy to overfeed, and encourage soft growth, easily damaged by cold or insects. In extreme cases it is even possible to kill plants completely, as is seen when you spill some lawn-food on the grass, leaving a dead patch. So always follow the directions for dilution and frequency of application. In this case a ‘little extra for good luck’ can have exactly the opposite effect.


This mineral is the second most important one for your hedge, because it develops strong root-systems, and balances the tendency of nitrogen to stimulate soft growth that is sensitive to cold, and more easily attacked by pests. Phosphorus is used by your plants as another essential component of DNA, so it is found in all the growing tips of both roots and shoots. Since a plant has a lot more roots than shoots, it needs a lot more phosphorus for the root-system than it does for the top growth. Phosphorus is the second number in that Fertilizer Ratio, and you will find big numbers, sometimes as high as 52.

Your soil conditions are just as important as how much phosphorus there is in the box, because soil pH – its acid/alkaline balance – plays a big part in making soil phosphorus soluble, and so able to be taken up by plants. Slightly acid soils, with a pH of 6.5, have the best uptake, and if you have very alkaline or very acidic soil, you may not get the full benefit of the phosphorus fertilizers you use.

Thuja Green Giant uses phosphorus to make strong roots, establishing itself well when newly planted, and sending roots deep into the ground to give drought resistance and the ability to absorb all the nutrients needed for optimal growth. For this reason, extra phosphorus, in the form of superphosphate, is recommended when preparing your planting areas. Work all phosphorus fertilizers well into the soil. they dissolve slowly, and only move a few inches a year through the soil. So scattered on top as an after-thought, once planting is over, really is a complete waste of time.

Fertilizers sold as ‘starter’ or ‘planting’ aids usually have lots of phosphorus, and they are ideal for feeding freshly-planted trees. Use them during the vital first few months of growth. They really get your plants off to a flying start, and any extra remains in the soil for decades, so it is almost impossible to over-use phosphorus fertilizers.


The last number in that Fertilizer Ratio stands for Potassium. This mineral is not used by the plant to make any structural components of its cells, but it is used inside the cell to keep everything rigid and strong, and its presence stimulates plant cells to build big, sturdy cell walls. This protects them from insects and cold. So you will see large numbers for potassium in ‘fall fertilizers’, designed to bring the growth of your hedge to a conclusion for the season, strengthening and thickening the stems, and increasing cold-hardiness and resistance to being pushed over by snow and ice. Plenty of potassium is especially important if you grow your Thuja Green Giant in colder zones, because cold-hardiness is an important aspect of good health for your hedge.

Enough for now. . .

That’s a lot to absorb, if  you will excuse the pun, so in the second part of this blog series we will take a look at the minor nutrients, which play an important part in the color and vigor of your hedge, even though they are not shown among the ‘big three’ nutrients in your fertilizer.