by Andrew McAllister

Plants are terrible listeners. I’ve told the plants on my windowsill, “Grow taller, grow faster! Make more delicious leaves for me to put in my food!”, but they just grow in the same slow way they’ve been doing since I bought them.

Farmers and florists share my pain, but on a larger scale. Sometimes, no matter what you do the squash isn’t big enough to be a prize winner and the roses flower a little too early or too late for Valentine’s Day. If only plants could hear our requests. Alas, while the old rumor about talking to your plants to help them grow might be a little bit true, using sound waves isn’t specific or effective enough to change how we grow plants on a large scale. To do that we’re turning to a different kind of wave to get through to plants — light.

Different “Kinds” of Light

Plants use light to do many things. The first that probably comes to mind is making food, or energy. Plants convert light into energy through photosynthesis. But they also use light to decide which direction to grow, how long to make their stem, how large to make their leaves, when to flower and more.

To tell plants to grow in a particular way, farmers need to know two things. First, what kinds of light lead to which response. Next, how do we give our indoor plants just those kinds of light.

These “kinds” of light I’m referring to are the different wavelengths of light (Figure 1). When we’re talking about light we can see, we’re talking about different colors of light. Longer wavelengths appear to our eyes as red while shorter wavelengths appear to our eyes as violet. But there is also light that our eyes can’t see, but that our bodies can feel. Wavelengths even longer than red light, or infrared light, makes us feel warm. While wavelengths smaller than violet are called ultraviolet (or UV) which causes sunburn.

Finding the Light Plants Need

All of these wavelengths of light come from the sun during the day, but plants are picky and don’t need or want all of these different kinds of light. The trick for scientists and farmers is to understand what kinds of light plants transform into food, what kinds lead to more fruit or flowers, and what kinds get ignored.

That understanding comes from examining the chemicals that plants use to interact with light. The most important one is a pigment called chlorophyll, which is used in photosynthesis to make energy for the plant. Chlorophyll is what gives plants their healthy green color, and this gives us a big clue as to the kinds of light that plants will like. Our eyes see green because that’s the light the plant doesn’t want — it gets reflected away. Instead chlorophyll prefers red and blue light.

But while chlorophyll makes energy for a plant to grow, it doesn’t tell the plant how to grow. That job falls on chemicals called phytochromes and cryptochromes. Phytochromes are sensitive only to red light and tell plants how many leaves to make and how big to make them. On the other hand, cryptochromes are only sensitive to blue light and control how plants respond to a daily cycle of light. Cryptochromes help tell plants when to flower.

Using Light to Make Better Plants

Farmers and researchers took this information about different chemicals in plants and did an experiment to test how light affects plant growth. They tried to grow strawberry plants totally indoors so they could better control the light they were giving their plants. While strawberries could grow under just red light, they were bigger when blue and red light were both included. But that wasn’t all they found. The strawberries also had different nutrients, sugars, and “taste chemicals” based on the proportions of red to blue light that were used. They also ripened faster or slower depending on that ratio of red to blue light. So not only could researchers grow strawberries indoors using a few wavelengths of light, but they could control how healthy and how tasty the strawberries were and grow them faster or slower.

Another group of researchers were interested in flowering plants. They found that the tussock bellflower will flower later when exposed to certain kinds of red light, but pansies flowering time won’t change at all under the same red light. Instead, pansies grow longer stems but fewer overall flowers. This understanding could help florists get plants to flower at just the right time for their customers.

The difference between the bellflower and the pansies illustrates a big challenge for researchers — each kind of plant will have preferences for different wavelengths. But there is another problem here — once we know a specific wavelength of light will tell plants what to do, how do we shine just that kind of light on them? This is where LEDs come in.

LEDs: Good for More than Just Lighting your Living Room

For most of history we haven’t been able to easily create light of an exact wavelength. Light sources like flames, incandescent light bulbs, or fluorescent light bulbs are difficult to finely tune to emit exact wavelengths of light. In a lab, difficult or complicated can be okay, but trying to run an entire indoor farm requires simpler technology. Luckily, LEDs can be tuned to emit precise wavelengths of light, allowing us to easily change what light we’re shining on our plants.

Moving farming indoors might seem silly with all that free sunlight, but there might be good reason. Indoor farms could be closer to urban areas, reducing both the monetary and carbon cost of shipping large quantities of food from the countryside to cities. The climate can also be unpredictable, and controlled indoor farms could be vital if the weather becomes too variable to easily grow the food we need. Plus there is the benefit of a high degree of control in how our food tastes, how nutritious it is and how fast it grows. Rather than leaving it up to chance with giving plants the same light they get outside, we can create nutrient dense and delicious foods.

Lucky for indoor farmers, LEDs are good for more than just communicating with our plants. LED bulbs are much more efficient than other types of light bulbs. If you have to run the lights for 12 hours a day, you’ll start to rack up quite the electric bill using older light bulbs. Plus, you’ll have to change those bulbs very frequently. With LEDs, you save on long term electric bills and from having to change the bulbs as frequently (with a bit more upfront cost). LEDs also don’t generate much heat, so they can be put very close to the plants without making them too hot. Then there is less wasted light illuminating walls or other areas that aren’t the plants leaves.

Using LEDs in combination with what we’ve learned about how plants use light is a wonderful merging of the disparate fields of engineering and biology. As we learn more about the biology of how plants function, we can become better at telling plants what we want from them — grow taller, grow faster, be more nutritious and tasty (like the strawberries).

Right behind these biological developments are developments in how we can actually implement these findings in settings that are important to us: indoor farming to help with food production in smaller and diverse spaces is one possibility. Another is to grow plants on long space trips. While LEDs are an exciting technology for the daily, taken-for-granted task of lighting things up in our homes and workplaces, they also will change the way we live in other ways. We’ll be able to be healthier, with plenty of nutritious food and (maybe eventually) less frustration in our home gardens. Because instead of begging our plants to grow, we’ll be able to tell them exactly what we’d like from them.

Andrew McAllister is a PhD student in the Applied Physics program at the University of Michigan. He can be found on his personal webpage or on Twitter.

This piece was written with guidance from Friends of Joe’s Big Idea.

For more information:

Get more in depth about the benefits of LEDs for indoor farming and the environment

Learn how Phillips, a lighting company, is experimenting with plants and LEDs