Roots and shoots: For plants, life is a balancing act from top to bottom | Mark Tancig
As a botanist and extension agent, I think quite a bit about how plants grow.
Understanding the method to their madness helps to explain and diagnose the various concerns of citizens that come our way at the extension office. One of the many fascinating concepts in plant growth is the balance between root and shoot growth, referred in some scientific literature as the root to shoot ratio, that scientists have found is pretty constant for each particular species.
As humans, we are mostly concerned with the shoot part of this equation, as that is the part of the plant we can see. However, as has been mentioned in numerous articles by many extension agents and Master Gardener Volunteers, the health of a plant’s roots are just as much, if not more, of a concern for the overall look and production potential of the plant as a whole.
Plants are sophisticated organisms. Their ability to harness light energy from the sun makes most other life on the planet possible.
However, only one half of a plant is in the light of the sun, so plants have come up with a nice cooperative agreement between the roots and shoots – the roots will absorb water and nutrients in exchange for the carbohydrates and other products of photosynthesis carried out in the shoots. They share these products through the internal plumbing, or vascular system, found within the stems.
Since an increase in the number of shoots (leaves and stems) leads to more demand for water and nutrients, there must be an associated increase in root density. To increase the number of roots, the plant will need to provide more energy from the shoots. Therefore, a delicate balance must be achieved for optimal plant health.
Ratio of roots to shoots
Plant scientists have found that plants try to maintain a pretty consistent root to shoot ratio, around 1:6 for trees in average conditions (five times as many shoots than roots – by dry weight, so woody trunks add to shoot weight) and respond accordingly when one or the other is changed significantly.
They have also found that changes in the environment, such as plentiful irrigation and fertilization, can decrease the root to shoot ratio (less roots) as the plant does not need to invest in roots as much to keep the shoots productive. While much of this science has been done to better understand food crops, it does have practical applications to how we maintain and install landscape plants.
Here are a few examples:
When planting new trees in a landscape, we often want to buy the biggest we can find to have an immediate impact. Research has long shown that smaller plantings can actually catch up and reach full size before the larger, more expensive, selections.
The larger tree has much more shoots that need root support and therefore, after planting, the tree takes a pause on shoot growth to reconstruct a root system to meet the demand. The smaller tree, having not been in a black, plastic pot for years or dug from the field, has a better balance of roots and shoots, leading to it eventually surpassing the larger specimen.
A fun example of the root to shoot ratio is bonsai, the ancient art of growing miniature trees. Bonsai artists/growers know that to help keep these large, sometimes century-old, trees alive and healthy in a small container, the roots must be regularly pruned. This limits and stunts the shoots, creating scraggly branches that make a knee-high tree seem like an ancient giant.
Water and fertilizer
Since we know that a plant will reduce the root to shoot ratio (less roots) when supplied with plenty of water and nutrients, this tells us that to make our plants more resilient to not-so-perfect conditions, we should reduce how much we water and fertilize them. This makes them invest more in a strong root system that can access more water and nutrients stored in the soil.
Pruning and mowing
The root to shoot ratio also applies to our pruning and/or mowing practices. If we regularly prune the shoots – branches from a hedge or blades of lawn grass – there is going to be a reduction and stunting in root growth, stressing the plant, and setting it up for disease to occur.
The last two examples especially apply to lawn management. Just by doing less work we are encouraging the turf to invest in roots and access stored water and nutrients in the soil. Mowing at the right height prevents buzz-cutting the lawn, setting back the roots.
Considering the root to shoot ratio reminds us of a whole other part of the plant we rarely see – the roots. If we want to see healthy, productive shoots, then we must have healthy roots. Hopefully, this encourages us to take care of the soil around those roots.
Mark Tancig is the Commercial/Residential Horticulture Agent at UF/IFAS Extension Leon County, an Equal Opportunity Institution. For gardening questions, email the extension office at email@example.com.
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