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Gardening column: How light intensity affects plant growth

Effects of light intensity on plants

In an experiment by the horticulture program at Trident Technical College, you can see the effects of different amounts of light on plants. Tony Bertauski/Provided

You may be bringing plants inside to avoid the winter, but the indoors can be a tougher setting for plants than you might think. It appears like the perfect environment with plenty of light. Or so it seems.

Plants are autotrophic, and leaves are their solar panels. They make their own food in the presence of light. When a photon strikes a leaf, it kicks loose electrons that power the production of glucose through photosynthesis. The intensity of the light plants require is just as important as the quality.  

When plants are exposed to low light, photosynthesis is reduced, and less glucose is produced. Growth slows down and foliage appears weak. We demonstrated the impact of light intensity in botany class at the horticulture program at Trident Technical College where sunflowers were seeded under grow lights.

In one treatment, the lights were kept 3 inches above the plants. In a second treatment, they were 12 inches above plants. After three weeks, sunflowers under 3-inch lighting had fully expanded leaves with short, sturdy stems. Sunflowers under 12-inch lighting were twice as tall and struggled to remain upright. Leggy, thin growth is a typical response to low light as plants stretch in search of more light. And because photosynthesis was lower at 12 inches, there were half as many leaves that were smaller and lighter green compared with the 3-inch treatment. Just raising the lights a mere 9 inches made significant difference.

A third treatment was in a well-lit office with fluorescent lighting, not much different than typical room lighting. The sunflowers were lying over the sides of the pots and failed to produce more than two tiny leaves. While the office appeared to have plenty of light, the intensity was, in fact, low. And unlike the grow lights, the light quality was poor. The plants in the office were nearly dead by the end of the experiment.

It’s difficult to measure light with the naked eye. However, a light meter could have predicted what was going to happen to our sunflowers before we planted them. A light meter is an inexpensive tool easily purchased online. You don’t have to buy a fancy one. You can even download a light meter app on your phone.

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Light meters measure the amount of light falling on a surface in units of lux or footcandles. But first, there are lumens. A lumen is a unit of visible light. Lux is the measure of lumens that fall over a square meter. Footcandles measure lumens over a square foot. Each lumen that falls on a leaf will increase photosynthesis.

Plants adapted to direct sunlight require about 11,000 lux or 1,100 footcandles. Low-light plants will tolerate with less than 2,500 lux or 250 footcandles. We used sunflowers, a sun-loving plant, in our experiment to exacerbate the results. With the lights 3 inches above plants, we were getting 5,000 lux. At 12 inches, it dropped to 1,000 lux. And the office lighting was less than 500 lux. The sunflowers didn’t stand a chance.

If you have poor lighting indoors or heavy shade outdoors, there’s nothing you can give your plants if they aren’t adapted to those conditions. Fertilizers won’t help. In fact, fertilizer can cause more harm because it forces plants to use what little amount of glucose they’re able to produce.

Aside from adding grow lights, the key to low light is proper plant selection. Shade-loving plants are adapted to poor light quality and low light intensity. A sun-loving plant, such as sunflower, will be sentenced to a long, slow death in the shade. If you’re decorating your office, use low-light loving plants such as pothos or snake plants. If you have deep shade outdoors, use things like cast iron, fatsia or aucuba.

If you’re bringing plants in for the winter, find the brightest spot in the house with a light meter. You likely won’t get much growth and probably even get some leaf drop, but it’s nothing a little sunshine won’t cure.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, e-mail him at tony.bertauski@tridenttech.edu.

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