Spring and early summer are peak times for plants to grow. Some plants only grow at that time and spend the rest of their time maturing buds for the following year. Others grow continuously, but more slowly and less vigorously, outside of that early peak period. When we grow plants in our gardens we often want to have maximum growth, so our screens and hedges mature rapidly, and so our gardens look full and lush. This means that even if we take good care of our soil, so that most of our plants need nothing more, in a few parts of the garden we want to boost growth with something extra.

Supplementary fertilizer is especially useful with plants where we take something away from the plant regularly. This could be fruit or flowers, but it also applies to lawns and hedges. Whenever we clip plants they need extra food to make up for that, and to replace it with new growth. This is where fertilizers come it – to give that extra fillip to those plants we stress by harvesting – even if that ‘harvest’ is just a bag of hedge clippings.

Evergreen bushes in particular – both broad-leaf and conifers – need a particular balance of fertilizer elements, so when you go to the store for hedge food, you see that the makers have created something the call ‘evergreen food’, or something similar. So what is special about the needs of evergreens, and what is in that bag? Why should you buy this one, and not something else? These are all good questions, and ones that lots of gardeners ask, so let’s try and answer them.

Young growth needs Nitrogen

By law, fertilizer bags are labelled with the amount of the three essential plant nutrients in them. This ‘fertilizer ratio’ is the three numbers you will find somewhere on the labelling, that looks like ’18-5-11’, or some other set of three numbers. First of all, you will see that it doesn’t add up to 100%. That doesn’t mean the rest of the bag is ‘filler’, as some people believe. Those numbers are the pure elements, and the rest of the bag is the other elements that are bound up with them to create the specific compounds used. Take a look at the ratio on a bag of evergreen fertilizer, and you will always see that the first number is the biggest. This is nitrogen, the element that plants need to make proteins and DNA, and especially the pigment called chlorophyll, which is the magic chemical plants use to trap energy from sunlight, and then turn it into sugars for growth. Right at the heart of every chlorophyll molecule are nitrogen atoms, and since green plants have lots of chlorophyll, they need lots of nitrogen to make it all.

All those clippings from your evergreens that you rake up or blow under the hedge are green, so you are taking chlorophyll away from the plant, and with it the nitrogen needed to make more. It’s that simple. Of course the plant also needs nitrogen for the structural proteins in its growth too, but chlorophyll is the big user, and so plants lacking in nitrogen will have pale green or yellowish leaves, instead of the rich, deep-green foliage we admire – and want.

In artificial fertilizers that first number is often big – between 7 and 20, depending on the formulation. If you look at organic fertilizers, it is rarely more than 4. That doesn’t mean these fertilizers are weaker, it is instead a result of the measuring system. That number is the amount of nitrogen in a form that the plant can use immediately. Much of the nitrogen in organic fertilizers is locked up in the material, and it takes the activity of bacteria and soil organisms to release it. This is why the top manufacturers of organic fertilizers are adding those special organisms to their fertilizer, to guarantee that all the goodness will be released. It’s a useful extra, especially in poor soils.

Potash is good too. . .

The third number in the formula is potash, and especially if you live in an area with colder winters, higher levels of that nutrient are very useful, as they make your evergreens more resistant to low temperatures. This is because potash (or potassium as it should be called) raises the pressure of the sap inside the cells, making the walls grow thicker and tougher, and so better able to resist low those icy days. It also makes them more resistant to pests, so its always good to see a higher level. If you are trying to choose one food over another, it makes sense to go for higher potash, if everything else is similar.

What else to look for?

You might wonder why some bags of fertilizer are more expensive than others. There are a couple of possible answers – for example it could be that one is slow-release, saving you going out and applying it several times – but another common reason is that some contain micro-nutrients as well as the ‘big three’. These are especially important to get top rates of growth, especially in evergreens that you are clipping regularly.

The most important ones to look for are iron and magnesium. These will be listed lower down and they must be shown on the bag if they are there. Both of them are involved in that chlorophyll molecule again – so vital for good growth. Iron is part of the enzyme system that makes chlorophyll – so low levels mean the factory slows down, and production falls below the necessary levels for top growth. Magnesium is right inside the chlorophyll molecule itself – surrounded by that nitrogen already mentioned – so it’s a raw material, and without enough, again, production will have to fall if the supply chain is disrupted.

Finally. . .

That’s it really. When checking the bag, look for a high first number – nitrogen – or if the material is organic, look for microbes to help release it. Also check if it contains iron and magnesium, and a higher potash number (the third one) helps too. All the rest is window dressing, but now you can make the best choices, and give your evergreens the best possible care.