Q&A: Making way for the sun

Sunlight is free, but it’s not always available in the amount greenhouses need it. To compensate, many growers install supplemental lights. Now, research shows that dimming lights based on sunlight levels can optimize crop growth and save growers money.

Marc van Iersel, professor of horticulture at the University of Georgia, is conducting research in this area. He also started his own lighting control company, Candidus, to help growers with dimming and other lighting needs — both with LED and HPS lights.

Greenhouse Management recently caught up with van Iersel to speak about light dimming best practices and the latest in lights.

Greenhouse Management: Broadly speaking, how can dimming lights help ornamental growers?

Dimming and taking advantage of the dimmability of LEDs can help in making sure that you provide light in the amount that the plants can efficiently use when they can best use it. So, when you’re growing in a greenhouse, you constantly have changing levels of sunlight and supplemental light is used much less efficiently when you have a lot of sunlight. By measuring how much sunlight you have and making the LEDs respond to that, you can make sure that you don’t provide light when the plants cannot use it efficiently.

GM: From a daily light integral (DLI) standpoint, do you have any general figures or ranges for when additional light is no longer beneficial to plants? I know it varies by species and variety.

MvI: That’s really hard to say. One thing that we have found — and this also ties in with dimming LEDs and controlling them — is that DLI by itself may not be all that good of a guideline. We are growing Rudbeckia seedlings in the greenhouse right now, and we’re doing that with different photoperiods. So they all get the same amount of light over the course of the day — they all get 12 mols — but it’s spread out over times, I think, ranging from … 13 to 22 hours. So if you give the plants the same amount of light but you spread it out over a longer period, then you get a lot more growth. That also has the advantage that from a practical perspective, you don’t have to install as many lights. So, it decreases the capital expense.

GM: How does dimming ensure that plants get the same amount of light every day? Does the amount by which lights need to be dimmed depend on how much sunlight there is?

MvI: The dimming that we do is always in response to sunlight, but we do it in such a way that by the end of the day, the crop receives ... a specific daily light integral. But through the algorithm that we use, we make sure that that light is provided preferentially when there’s little sunlight and the plants can use that light most efficiently. One really big advantage of this precise control of lighting is that it makes production more predictable. And since in our industry, almost everything is grown based on a contract with a specific shipping date, if you know that you have good control over the environmental conditions, it becomes much easier to make sure that your crop is ready when it needs to be ready.

GM: You started a new lighting company, Candidus, with Erico Mattos, a recent PhD graduate of UGA. What is Candidus and how does the technology work?

MvI: Candidus makes lighting control systems. We don’t make lights because there are plenty of other companies that do that much better than we ever could. So, what we do is we interface with existing lighting systems. We have a really simple interface for growers where they can set the photoperiods [like], when do the lights come on, when do they go off, what daily light integral do they want? Then, our control system takes over from there to make sure that those goals are achieved. So, our initial setup was designed for dimmable LED lighting systems, but right now we’re also working on systems that can do the same with high-pressure sodium lights or non-dimmable LEDs.

GM: With Candidus, can different sections of a greenhouse be dimmed at the same time?

Yes, you can have different lighting zones.

GM: Did you say you’re looking at dimming high-pressure sodium lights as well?

MvI: No, we wouldn’t dim them, but we would turn them on and off. Some high-pressure sodium lights have some dimming capability, but it’s not like with LEDs where you can dim very precisely and immediately.

GM: In addition to the research that’s available now, what more do you think needs to be done to ensure that growers provide their plants with the right amount of light and the right types of light?

MvI: What I would really like to see — and we’re working on — in addition to having a lighting system is to have some kind of imaging system that can actually monitor how fast a crop is growing. So then you can use benchmarking techniques where you know that after 25 days, your crop should have reached a certain stage. And, if for some reason, your crop hasn’t, then you can adjust the lighting [and] environmental conditions to help that crop catch up.

GM: Is there anything else you would like growers to know about light dimming and how it can help them in their production?

MvI: I think the big advantage of dimming [lighting] systems is the predictability [they provide] and providing light when plants can best use that light. So there are energy savings associated with it as well as the big benefit of making your production more predictable.