The Trouble with Growing Tomatoes Indoors Using LEDs

“Gardening is cheaper than therapy and you get tomatoes.” — Anonymous

Anyone who ever found themselves while losing themselves in a tomato garden knows truer words were never spoken. Unfortunately, growing tomatoes indoors with LED lights isn’t therapeutic at all.

Up until the late 1800s, and for years after, the tomato was considered poisonous by the majority of people across the world. In Europe, for example, the upper crust of society ate from plates containing lead. The highly acidic juice from the tomato would leach lead from the plate and, as a result, a high number of people died from lead poisoning. Naturally, they blamed the tomato.

Being botanically classified in the deadly Nightshade family didn’t do much to instill consumer confidence, either. Once confirmed they were not poisonous but delicious and so culinary friendly, the love affair with the tomato began in earnest. Tomatoes have become one of the most popular vegetables (technically a fruit) in the world and grown in some 85 per cent of outdoor gardens.

Growing tomatoes indoors, however, is another matter.

Tomato Plants and LED Grow Lights

To better understand the relationship between growing tomatoes indoors and light emitting diode (LED) lights, I sought the expertise and wisdom of lighting maven and researcher Carey Mitchell, professor of horticulture at Purdue University in West Lafayette, IN. The good professor was kind enough to cram me into his hectic schedule between his teleconference with NASA and grading finals.

Professor Mitchell has spent years experimenting with LED lights and their viability as an alternative light source to high intensity discharge (HID) lights. These include metal halide (MH) and the high-pressure sodium (HPS) lamps used in commercial indoor growing.

I explained to the professor the article I was writing involved growing tomatoes indoors for the home gardener using LEDs. His first words were: “Well, they’re going to have a hard time doing that. Tomatoes are a high-light species and you’d have to put all the LED lights in the world to those. That’s why the commercial hydroponic tomato production industry is greenhouse-based. They merely use electric lighting to supplement sunlight, especially in times of the year when the photoperiod is very short or it’s very cloudy or it’s snowing. Additionally, 98-99 per cent of that market is high pressure sodium lighting because it costs so much less to get into that initially.”

LED lights are becoming more efficient as their ability to economically convert electricity into photons increases. While currently somewhat low, they are getting better at it. According to Mitchell there are advantages that LEDs hold over other light sources. He said the secret is they emit practically no heat and can thereby be placed much closer to the plants without scorching them, which results in reduced electrical costs.

Mitchell went on to say LEDs are so expensive to begin with that it is still cheaper to use HPS lighting, despite the fact they cost 75 per cent more to operate. He also noted HPS lights help heat the greenhouse. I chimed in that seems like an expensive way to heat a greenhouse. The professor agreed, but also noted heat was one of the perks of using HPS lights. He stated their initial fixture costs are so much less than LEDs and that they get “good stimulation of yield,” so it is still more economically feasible. Moreover, the residual radiant heat is used to offset the enormous cost of heating a greenhouse with natural gas or additional electricity when the photo period is very short or during cold or cloudy periods.

Another drawback to using LEDs in a greenhouse is they can block out the natural sunlight from the glass panels, so it becomes a trade-off as to when the benefit of their light is overshadowed by the sunlight they block out.

“Completely indoor, like vertical farms, are almost all LEDs...or at least they are going to be,” Mitchell says. “This is because with LEDs you don't have to worry about the fixtures blocking solar. The problem with LEDs right now is the fixture costs. I think they are three to five times more expensive per fixture than HID lights like MH and HPS, but they are getting cheaper.”

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Leafy Greens Shine Under LED Light

Mitchell stressed these aren't fruiting vegetables being grown commercially with LEDs, these are quick-turning, leafy green crops like lettuce, kale, and arugula. Quicker yet are baby greens and the even-faster-maturing microgreens, as they require practically no light.

“For the most part, tomato, cucumber, and eggplant are high-wire plants and tend to require a lot of light,” Mitchell added.

When I asked what mistakes indoor growers seem to make most often, Mitchell was hesitant to think of them as mistakes, but more a lack of knowledge. Not having the right light prescription for your plants combined with high humidity can create some leaf burn and other issues. This is bad news for the commercial grower because this renders the greens unmarketable.

“You can get lighting prescriptions from the different companies, but you know, it takes more than light to grow plants. It takes carbon dioxide. It takes nutrients, water; it takes the correct temperature; it takes humidity and so all those things factor together,” says Mitchell. “It’s still a fairly young industry so there are risks involved in any new crop you might try to grow without having the experience. So, a lot of these companies are doing what are called light recipes, but I’d say it’s really growth recipes you need to work out that include that litany of environmental factors that interact with light.”

Mitchell stated that because the photoperiod of tomatoes is so long, they are one plant that doesn't do well indoors. For the home grower, if you wanted to grow some grape or cherry tomatoes or a few dwarf species with LEDs you may get some tomatoes. There are light banks available (even LEDs) that may work on those types but not so much the standard tomatoes we see in the garden.

They just don't do well under solely artificial light conditions.

“So right now, the cards are not there for growing indoor tomatoes productively and certainly not commercially. For the home grower, as I said, you could probably grow some cherry tomatoes on a very small scale in your basement, that might work,” said Mitchell. “If you ask someone else, say someone at an Extension office, they may see growing tomatoes indoors differently.” As a researcher, Mitchell says his views are purely objective. “I just know the state-of-the-art for LEDs right now and it's just not there for growing tomatoes. Sole-source lighting works well for greens and that’s why they grow them. For the homeowner, if you have a greenhouse and supplement the natural light with LEDs you can get a decent yield.”