When is ‘food’ not a food?
We regularly talk about ‘feeding’ our plants, but what we’re actually doing is giving them nutrients. These supplements can to a degree help encourage growth, which is why we perceive that ‘feeding’ them helps and why the British manufacturers label these nutrients as ‘food’. However, this isn’t what is actually happening and in the long term these nutrients could cause the plant to decline from exhaustion.
Plant nutrients are a bit like the vitamin pills we take. They supplement essential elements that might be lacking in a normal diet. However, vitamin pills don’t keep us alive on their own and this is exactly the same for nutrients in plants.
Both we and plants need a supply of energy that comes in a large part from carbohydrates in the form of glucose which in humans mainly comes from fats and proteins as we have a higher need to store energy for later use. Normally plants take most of the nutrients via their roots and then distribute them through their stems and branches.
Their roots aren’t generally extensive enough to extract all the moisture and nutrients they need, so they have a symbiotic relationship with mycorrhizal fungi that significantly enlarges
the area of absorption. This arrangement between plant and fungi is beneficial to both with the plant giving glucose in exchange for moisture and nutrients from the mycorrhiza, (see figure 1).
So, when we say we are going to ‘feed our plants’ we should really say we are going to give them some nutrients. It would really help educate us all if labelling of most ‘plant feeds’ in English (though not usually in other languages) was changed to ‘plant nutrients’.
How do plants produce energy if they don’t eat it like we do?
They make it themselves by converting six carbon dioxide molecules (6CO2) + six water molecules (6H2O) into glucose and oxygen (C6H12O6 + 6O2) using energy from the sun via photosynthesis. The carbon dioxide is taken in through leaf pores with water absorbed through the roots and mycorrhizal fungi attached to them. The glucose that is produced is then used by the plants in a process called cellular respiration to release the energy for the plants’ cells to use as they multiply, causing the plants to grow (see figure 1).
Why is this important for boxwood?
Every time we clip or shear our plants, we remove leaves and branches that are made up of the cells necessary for life-giving photosynthesis and respiration. These leaves were created by the plants to produce more energy so they can maintain themselves and continue to grow. Removing them, therefore, has a negative impact on the plants’ growth.
Growing each boxwood leaf takes roughly the amount of energy that a leaf creates during its growth over two years. Therefore, if we clip off the new leaves repeatedly, and there aren’t enough other leaves on the plant, then slowly the energy levels available for growth will reduce and, in the end, run out.
Imagine a fuel tank with enough fuel in it to grow one leaf (see figure 2). As the leaf grows the fuel tank empties until there is no fuel left when the leaf has grown to full size.
To fill up the fuel tank again, the leaf needs to produce fuel which it does from the sun over a two-year period (plus some whilst the leaf is in its growth stage). See figure 3.
This partly explains why the same variety of plant can have differing sized leaves depending on location, environment and how it has been treated. If the plant is unable to produce enough energy due to lack of sun or constant clipping, it will grow smaller leaves because the energy will run out before they reach full size. These underdeveloped leaves then generate less energy, so the plant is only able to grow even smaller leaves in a self-defeating attempt to stay alive. In the end it won’t have enough energy left and will expire.
What can we do to increase energy production?
The potential energy your boxwood can produce is directly related to the number of leaves available to photosynthesis and its root health.
One way of increasing the number of leaves is to leave the new growth unclipped. However, from a garden-design point of view this doesn’t work for long as the plant would lose its shape and become too large. As a degree of clipping/shearing is therefore necessary, the best option is to make sure there is as much internal leaf growth as possible. This can be achieved through careful pruning and thinning as the plant grows allowing increased light and air into the centre of the plant to encourage internal leaf growth. As these internal leaves won’t get clipped off when the plant is shaped, they will provide an ongoing source of energy production during their life span of around three years.
Make sure the pH of the soil is between 6.8 to 7.5, which is the ideal range that will allow absorption of the nutrients in the soil. To test your soil, either purchase a test kit from a garden supplier that will give you a rough indication or send off soil samples for professional accurate testing. Depending on the results, the soil can then be amended to bring it to the correct pH level by using sulphur as an acidifier to lower the pH, or a form of lime to increase the pH. This process generally takes a couple of years if your soil is significantly outside the ideal range.
Encouraging good root health is also very important as it will allow the plant to take up the moisture it needs. This can be done by adding decomposed organic matter (compost) to improve soil structure and oxygen and moisture levels and encourage earthworms and other microbial activity. Mulching will also help retain moisture, keep the soil cooler in our warming climate, and prevent weeds that would otherwise use up valuable nutrients. It will also reduce potential box blight spores splashing back up into the plants.
This article was written by Chris Poole with graphics by Abigail Dodd and was first published in Topiarius 2022 Vol.26