Life is a balancing act for plants

As a botanist and extension worker, I think a lot about how plants grow.

Understanding the method to their madness allows us to explain and diagnose the various concerns of citizens who come to us at the extension office. One of the many fascinating concepts of plant growth is the balance between root and shoot growth, referred to in some scientific publications as the root-to-shoot ratio, which scientists have found to be fairly constant for each particular species.

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As humans, we are primarily concerned with the sprout part of this equation, because that is the part of the plant that we can see. However, as has been mentioned in numerous articles by many volunteer extension workers and master gardeners, the health of a plant’s roots is just as much, if not more, of a concern to the overall appearance and production potential of the plant as a whole.

Vascular system

Plants are sophisticated organisms. Their ability to harness light energy from the sun makes most other forms of life on the planet possible.

However, only half of a plant is in sunlight, so the plants have struck a nice cooperative deal between roots and shoots – the roots will absorb water and nutrients in exchange for carbohydrates and other products of photosynthesis transported in the shoots. They share these products through the internal plumbing, or vascular system, found in the stems.

Since an increase in the number of shoots (leaves and stems) leads to a greater demand for water and nutrients, there must be an associated increase in root density. To increase the number of roots, the plant will have to supply more energy from the shoots. Therefore, a delicate balance must be struck for optimal plant health.

Roots/shoots ratio

Plant scientists have found that plants try to maintain a fairly constant root to shoot ratio, about 1:6 for trees in average conditions (five times more shoots than roots – by dry weight, so woody trunks add to the weight shoots) and react accordingly when either is significantly altered.

They also found that changes in the environment, such as heavy irrigation and fertilization, can reduce the root to shoot ratio (fewer roots) because the plant does not need to invest as much in roots to maintain shoot productivity. Although much of this science has been done to better understand food crops, it has practical applications to how we maintain and install landscape plants.

Excessive pruning of shrubs, especially hedges, leads to weakened root systems and, in some cases, reduced flowering.

Here are some examples:

Shaft size

When planting new trees in a landscape, we often want to buy the biggest we can find to make an immediate impact. Research has long shown that smaller plantings can actually catch up and reach full size before larger, more expensive selections.

The larger tree has many more shoots that need root support and so after planting the tree pauses on shoot growth to rebuild a root system to meet the demand. The smaller tree, having not been in a black plastic pot for years or having been dug out in the field, has a better balance between roots and shoots, which ultimately allows it to outgrow the most large specimen.

bonsai balance

A fun example of the root to shoot ratio is bonsai, the ancient art of growing miniature trees. Bonsai artists/producers know that to help keep these large, sometimes century-old trees alive and healthy in a small container, the roots need to be pruned regularly. This restricts and slows growth, creating frayed branches that make a knee-high tree look like an ancient giant.

Water and fertilizer

Since we know that a plant will reduce the root to shoot ratio (fewer roots) when fed abundant water and nutrients, this tells us that to make our plants more resilient to not-so-perfect conditions, we need to reduce the amount we water them and fertilize them. This encourages them to invest more in a strong root system that can access more water and nutrients stored in the soil.

Cutting the lawn too short puts excessive pressure on the roots, often leading to fungal and pest problems.

Pruning and mowing

The root to stem ratio also applies to our pruning and/or mowing practices. If we regularly prune the shoots – branches of a hedge or blades of lawn grass – there will be a reduction and stunting of root growth, stressing the plant and preparing it for the onset of disease.

The last two examples apply particularly to the management of lawns. By doing less work, we encourage the grass to invest in the roots and access the water and nutrients stored in the soil. Mowing at the right height avoids cutting the lawn by pushing back the roots.

Considering the root to shoot ratio reminds us of a whole other part of the plant that we rarely see – the roots. If we want to see healthy, productive shoots, we need to have healthy roots. Hopefully this encourages us to take care of the soil around these roots.

Marc Tancig

Mark Tancig is the Commercial/Residential Horticulture Officer at UF/IFAS Extension Leon County, an Equal Opportunity Institution. For gardening questions, email the extension office at [email protected]

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