Greenhouse Lighting: A Growing Concern

Image credit: Eddy Van Leuven
1167 words / 5-minute read

The news reports are at first ominous. Purple glows appear over towns. Some headlines are even comical: "Tomato factory lights mistaken for 'lovely aurora'". But they share a common origin. To extend growing seasons and further the agriculture industry, more food crops are grown in greenhouses. Artificial lighting used at night aims to increase yields. And that light is having an unwanted effect on neighbors and the night sky. Yet some are betting the farm that new technologies and better regulations can solve this problem. 

A bright idea

Greenhouse farming is on the rise around the world. In the U.S. alone, its economic output was almost $6 billion in 2024. Rising at an annual rate of almost 9% per year, analysts expect it to top $11 billion by 2030. Industry experts make a case for investing further into this sector. Last year Agritechture wrote: "The agricultural landscape in the United States is undergoing a transformative shift, driven by supply chain weaknesses identified during the pandemic, the changes in our climate making it harder for field farmers to grow consistently, and the increasing demand from consumers for more sustainable and locally sourced produce. Greenhouses offer a viable solution to meet this shift in consciousness by providing a controlled environment for year-round cultivation."

With year-round cultivation comes an effort to grow plants faster by lengthening the growing day. Illuminating plants during overnight hours can make them bigger and more productive. "Horticultural lighting" can also reduce the time from seed to crop by accelerating growth. Rapid improvements in lighting technology better support year-round growing seasons.

In particular, the arrival of LED lighting was as much of a game-changer for agriculture as in other sectors. Earlier lamps consumed a lot of electricity and radiated much of their power in the form of heat. Besides being much more energy-efficient, LED has remarkable color characteristics. Its properties are selectable to give plants only the light they need for photosynthesis, explaining the pink or purple colors reported in news stories.

Given the challenge of providing adequate food for the global population, no one doubts the value of greenhouse farming. Food insecurity is a serious threat to economic development. According to the World Bank, "food security continues to be at alarming levels in most low-income countries." That is particularly true in Africa, where famines and rising food prices put millions at risk.
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At the same time — and ironically in part due to light pollution — populations of pollinating insects are in decline. Thus far, machines cannot replace the "ecosystem services" pollinators provide for free. These factors combine to create distinct threats to humans through potential disruptions in the global food supply. 

Skyglow from greenhouse lighting reflecting from low-altitude clouds. Image credit: JW van Wessel / CC BY-NC 2.0

Making hay after the sun shines

Their clear glass walls and roofs that make greenhouse ideal during the day is the source of a problem at night if owners use light to extend the growing day. Even with good lighting design and best-in-class lighting products, greenhouses are the source of "obtrusive" light. Light scatters and reflects from glass, interior surfaces, and plants themselves. Some of that light emerges from the greenhouses sideways and can cause light trespass. Light leaving from transparent roof panels travels unimpeded into the night sky. This causes the strange glows reported in news stories. And it can have ecological consequences of its own. For example, one recent study found that greenhouse lighting is harmful to songbirds.

What can be done about this? The fundamental solution involves keeping light contained within the building at night. That's an obvious challenge for structures whose very nature is to let outside (sun)light in. To address that particular aspect, some have experimented with so-called 'smart glass'. This method uses exotic materials that are alternately transparent or opaque to visible light. The state of transparency changes when, for example, an electric current is applied to a glass panel. But these materials are rather expensive, and their opacity is usually not enough to keep all interior light contained.

​A lower-tech solution involves a much lower-tech approach: close the blinds. Roof-mounted machinery deploys physical window coverings at dusk and retracts them at dawn. As shown in the example below, these can be an effective mitigation for much of the light that would otherwise escape the greenhouse. Yet even this trick isn't inexpensive, and it involves moving parts subject to wear and tear.

Overhead views before (left) and after (right) shutters lining the inside of a small rooftop greenhouse are closed at night, showing a substantial reduction in light emitted into both the night sky and the surroundings. Images courtesy of Guillaume Poulin / Mont-Mégantic International Dark Sky Reserve / meganticdarksky.org

These mitigations can be combined with local regulations to enforce changes. The simplest laws impose a lighting "curfew" time, after which greenhouse owners must switch lights off each night. That may be most fair to people who live near greenhouses to reduce the impact of lighting. Of course, growers may take exception to such policies as unfairly limiting their operations. Zoning restrictions can help put some distance between greenhouses and their neighbors. And some jurisdictions may choose to prohibit greenhouses entirely within their territories.

​But regulation is challenging to put in place correctly and consistently. It often fails to completely address the problem. Like other examples of conflicting land uses, satisfying all those involved may be impossible. The benefits to society that greenhouse farming represents must be weighed against its social costs. In the ideal case, workable solutions respect the rights and wishes of all stakeholders.

A future clear as glass?

Modern greenhouse lighting is here to stay, and by all accounts will only be more important to the global economy in the future. Yet there are clear challenges to enabling its future development while reducing its effect on the nighttime environment.

The controllability of LED could be the key to solving this problem, alongside other tech. But good old-fashioned "shutting the blinds" is the best approach. It presents an added expense to operators, which could as easily be counted alongside other cost of doing business. In that sense it's not unlike complying with building codes or workplace safety laws. And it's up to each jurisdiction to decide how much (and how best) to regulate.

Recent experience suggests that jurisdictions should get out ahead of this issue before it lands on their doorstep. Often the first sign of a new greenhouse locals notice is the nighttime glow. Reactionary efforts at regulation have a habit of proceeding rather poorly. Proactive attention to the problem can reduce that tendency.

There may be also some lessons learned here about things like land-use zoning. They may point the way to how to deal with other outdoor lighting challenges from 'speciality' lighting applications. Because of its associated effects, greenhouse farming might be subject to geographic restrictions. Regulators should, however, be mindful of the fact that light pollution can drift far from its sources. Other interventions may still be necessary to avoid conflicts.

​Can the future of greenhouse farming remain bright without compromising the dark? Any serious discussion of that question must consider both lighting design and regulation. It must further weigh its economic benefits against potential social and environmental harms. As in many such instances, the ideal solutions find the right balance between the needs of both people and planet.