In almost every garden situation, evergreens benefit from a fertilizer program. This is especially true when you are developing a screen or hedge – there you want maximum growth in height, but density too, not just tall, skinny shoots. It is also true for specimen plants, which we want to always look their best, with rich green foliage and healthy growth. Even mature plantings benefit from fertilizer programs, which keep them healthy, and encourage rejuvenation, especially following trimming. Plants that are regularly trimmed must constantly replace the growth removed, so their nutritional needs are much greater than in plants that are just left to grow. This explains why feeding the lawn is the most common fertilizer activity among gardeners – all those clippings must be replaced.

Fertilizer doesn’t replace caring for your soil

Always look at your fertilizer use as a supplement to your overall soil care, not a replacement for it. Good soil preparation before planting, and maintenance of soil fertility afterwards, are the key to good gardening. It is perfectly true that such management is all you really need – Mother Nature has been gardening that way for millions of years. They key to good soil preparation and maintenance is organic material – rotted plants, and the waste from animals. Garden compost, farm and stable manure, the by-products of seed oil production (corn, soya, etc.) – all these are good sources for garden use. Materials like peat moss, for all their availability, are low on the list of suitable materials, but if that is all you can find, it is better than nothing.

Not only should you add plenty of organic material when preparing the area for evergreen planting, but established plantings should be mulched with these kinds of materials regularly. How regularly depends on your soil. Sandy soils need it every year or two, loam soils every 3 or4 years, and heavier soils perhaps every 5 years. Heavy clay does benefit from plenty of organic material, because it improved drainage, but it is not truly needed very much as fertilizer in those very rich soils.

What’s so special about fall?

But we digress. . . so to return to our subject, what is special about fall when it comes to fertilizing evergreens like Thuja Green Giant, Arborvitae, Cypress, or other evergreens? While deciduous plants solve many of the problems of winter by just shedding their leaves and going to sleep, evergreens keep their foliage, so they can face more problems as a result. Evergreen foliage has evolved to protect itself from severe cold, but in garden situations the very act of gardening changes the plants – particularly when we accelerate growth. The high levels of nitrogen applied in spring and summer create larger cells in the foliage, with thinner walls. It doesn’t make much difference if you use chemical or organic sources of nitrogen either. Anything that accelerates growth will do this.

Those thinner walls are easier for insect pests to feed through, and for diseases to penetrate, and even more importantly in fall, those thinner walls are more easily damaged by cold. The sap inside the leaf is thinner too, so it freezes to make large ice crystals, which in turn rupture the cell walls, killing that part of the plant.

This means that in fall we want to reduce the amount of nitrogen in the fertilizer for our hedges and evergreens. This is the first number in the fertilizer formula, and while in an evergreen fertilizer for spring and summer you will see a high number, in fall mixes it should be smaller.

Special fall mixes that prepare for spring

Now there is a complexity that steps in at this point. Some more sophisticated formulations of chemical fertilizers use special sources of nitrogen that are only released when the soil temperatures are above about 45 degrees. These are sometimes used in fall fertilizers, so if you look at high-end products promoted for fall use, that first number can be surprisingly high. Don’t be concerned. The purpose in using these materials is to get an early start in spring – as soon as the soil warms that nitrogen will be released, and you don’t need to make an early fertilizer application. So you are saved a job at a busy time of year – a great idea, yes?

Make those cell wall tough and hardy

But usually you will see a low nitrogen number in fall fertilizers. The next number to look at is the last one – the element potassium, also called potash. For a fall fertilizer for your evergreens, like Thuja Green Giant, look for a big number here – 10 or more. Potassium is a slightly strange fertilizer element. It doesn’t take part in building proteins, as nitrogen does, or in making DNA or fats, as phosphates do. In fact, it doesn’t appear in any structural parts of plants (or humans for that matter). It is found, however, in the sap of plant cells, where it plays a vital role. It causes the cells to absorb water – and the nutrients that water contains. It is essential for the plant roots to take up plant foods, and in the cells of the leaves it keeps them full-up with water. This way the leaves stay rigid, stand up and catch the sunlight. Plants low in potassium will often look like they are wilting, even when they have enough water available at the roots.

For plants strength and resistance, potassium is important too. By ‘pumping up’ the cells, pressure is applied to the cell walls. Remember we said they are softer than normal in plants that are fertilized for quick growth? Well potassium in fall reverses that. The pressure of the cell contents on the wall makes it grow thicker (rather like resistance training for people), and restores its protective functions, giving better resistance to cold, wind and diseases. That pressure also sucks in more minerals and sugars, lowering the freezing point of the cell contents, and creating small, mushy ice crystals that won’t break the cell walls. So the cells don’t die.

Take care of the roots too

Finally, all that rapid upper growth can make the top parts of the plant outgrow the roots. Fall is when plants renew their root systems, and roots have a high need for that middle number in fertilizers, phosphate. So look for a reasonable number here – over 10 – so that you feed the roots as well, strengthening and extending them, so that come spring they can deliver everything the top-growth needs.

Fall fertilizer is often overlooked, but as you can see it should be a vital part of your annual program, if you want the very best Thuja Green Giant and other evergreens can give you.