The Importance of Monitoring Natural UV Light Levels in Your Greenhouse

Authors: Dr. Fadi Al-Daoud¹, Sharon Kitur¹, and Dr. Caroline Strang²

¹Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA), Harrow, Ontario

² Department of Biology, Western University, London, Ontario

Summary

Most Ontario greenhouse producers monitor light levels outside and inside their greenhouses to optimize their growing conditions for maximum yield. This article discusses how ultraviolet (UV) light levels inside greenhouses are affected by greenhouse coverings, and how this may impact bumblebees used for pollination of many greenhouse fruit and vegetable crops. It demonstrates how monitoring natural UV light levels inside greenhouses can benefit producers and allow them to make more informed decisions about their production practices.

Introduction

Commercial insect pollinators, like bumblebees (Bombus impatiens), are used to improve pollination and increase the yield of many greenhouse fruits and vegetables, including tomatoes, peppers, eggplants, and strawberries (Figure 1). Like other insects, these pollinators use ultraviolet (UV), blue and green light to navigate the complex greenhouse environment. Unlike human eyes that see visible light mostly in the photosynthetically active radiation (PAR) range (400-700 nm), bumblebees’ light receptors detect light in three main peaks: 347 nm (UVA), 424 nm (blue) and 539 nm (green). Insufficient light levels in these three colours may negatively affect the bees’ ability to forage, resulting in less flower pollination.

Figure 1: A) Image of a typical bumblebee colony in a commercial greenhouse. B) Video recording clip of an active bumblebee hive in a demonstration box (video provided by Cara McCreary, OMAFA).

The natural light environment in a greenhouse is mainly determined by its covering, coating, and curtains. Glass has higher light transmission than polyethylene (poly), and double poly has higher light transmission than triple poly. Ontario greenhouses also apply coating, a.k.a. whitewash, to their glass to exclude infrared radiation and diffuse sunlight. This reduces heat buildup and light intensity inside greenhouses during late spring, summer and early autumn when there’s an excess of sunlight in southwestern Ontario. Diffused light also scatters around the greenhouse, which reduces shading and increases light levels within the canopy. Newer diffuse glass technology reduces light intensity and scatters light more than traditional clear glass, but it also requires whitewash to reduce IR transmission and heat buildup in the summertime. Whitewash is not typically applied to plastic covering because greenhouse poly includes additives that allow it to diffuse light and reduce sunlight transmission.

Manufacturers of greenhouse poly coverings include UV-blocking agents in their formulas to increase their longevity by reducing UV transmission. Exposure to environmental and weather conditions, like UV from sunlight, precipitation, and temperature fluctuations, takes a toll on poly greenhouse coverings and reduces their integrity over time. As poly ages, it allows more light inside greenhouses, and its ability to retain heat in the greenhouse in cold weather deteriorates. That is why it is recommended to change greenhouse poly coverings every four or five years, or as recommended by the manufacturer.

UV-blocking poly reduces UV levels in greenhouses year-round. This can affect plant physiology, plant health, bumblebee pollinators, pathogens, and beneficial and pest insects. Overall, UV-blocking greenhouse coverings improve plant health by increasing photosynthesis and transpiration, which leads to bigger plants and greater yields. However, they also negatively affect fruit quality by reducing secondary plant compounds such as phenolic compounds, flavonoids and carotenoids. The effect of these materials on pests and pathogens is mostly inhibitory. They suppress the activity of insect pests and the ability of some fungi to sporulate. However, they also negatively affect pollinating and beneficial insects in the same way.

A greenhouse tomato producer in southwestern Ontario observed that his bumblebees were not pollinating as well in a range that had newer triple poly covering compared to ranges with older double and triple poly. We measured light levels in the ranges to determine if any differences in UV levels may be affecting his pollinators. A spectrometer (340-1010 nm) was used to measure light between two middle rows at different heights in the canopy (top, middle, and bottom) in each range at midday on several days in the spring and summer of 2025. This provided data for light irradiance (W/m²), photosynthetic photon flux density (PPFD, µmol/m²/s), partial UVA range (340-400 nm), PAR range (400-700 nm), far red (700-800) and partial infrared range (800-1010 nm). The ratio of the partial UVA to PAR levels, shown here, was calculated to account for differences in time, crop height in the different ranges, and changes in weather conditions as we moved from one range to the next. Measurements were also taken outside the greenhouse to compare outdoor sunlight levels. The data shown here is from one sunny day, but these trends were also observed on multiple sunny and cloudy days.

Results

As expected, the highest levels of irradiance (Figure 2), PPFD (Figure 3), and UVA/PAR ratio (Figure 4) were observed outdoors. The overall light levels were lower in the older triple poly range than in the other ranges, most likely because it was measured first on this day when sunlight was not as strong as it was later in the day. As expected, all three ranges showed a similar pattern of less irradiance (Figure 2) and PPFD (Figure 3) in the middle and at the bottom of the canopy than at the top of the canopy. The UVA/PAR ratio, however, did not change much at different heights in the canopy in each range (Figure 4).

When the UVA/PAR ratios of the different ranges were compared, some differences were observed. Of all the greenhouse ranges, the older double poly range had the highest UVA/PAR ratios at all heights in the canopy. The older triple poly range had lower UVA/PAR ratios than the older double poly range. The lowest UVA/PAR ratios at all heights in the canopy were observed in the newer triple poly range, although it had similar or higher levels of total light (irradiance and PPFD) than the other ranges. In fact, the newer triple poly range had approximately half the UVA/PAR ratio of the older triple poly range and a third of the ratio of the older double poly range (Figure 4). This trend was observed consistently on sunny and cloudy days. Therefore, there was an association between lower UV levels in the newer triple poly range and lower pollination of the crop.

Figure 2: Irradiance levels (W/m²) outside and inside greenhouse ranges with older double poly, older triple poly and newer triple poly on a sunny day. Each bar represents the average of three technical replicates.

Figure 3: PPFD levels (µmol/m²/s) outside and inside greenhouse ranges with older double poly, older triple poly and newer triple poly on a sunny day. Each bar represents the average of three technical replicates.

Figure 4: The ratio of partial UVA (340-395 nm) to PAR (400-700 nm) (UVA/PAR) outside and inside greenhouse ranges with older double poly, older triple poly and newer triple poly on a sunny day. Each bar represents the average of three technical replicates.

Conclusions and Recommendations

Studies have shown that lower UV levels could negatively affect crop pollination by bumblebees in a number of ways:

Bumblebees locate and identify flowers using UV, blue, and green light. Low UV conditions can disrupt flower identification and reduce the efficiency of flower visitation by bees, though it has been shown that bumblebees can adapt to low UV conditions over time.
Bumblebees are less active in low UV environments. The number of foraging trips that bees take in a day is lower in greenhouses with less UV than in those with higher UV levels. The activity of other insects, including pests and beneficials also decreases in low UV greenhouses.
Bumblebees are attracted to UV light, so they will move from lower UV to higher UV environments. This is known as positive phototaxic behaviour. The greenhouse where we conducted our study had gutter vents with no insect netting. When the vents were open, the higher UV levels outside of the greenhouse may have been visible to bumblebees foraging inside. Bees may have been attracted to the higher UV levels outside the greenhouse, or the higher UV levels in the other ranges of the greenhouse, and escaped out of the newer triple poly range to the higher UV environments. Bumblebee escapes can reduce colony size and crop pollination in the range with lower UV levels.

Many greenhouse producers monitor the daily light integral (DLI) and PAR levels outside and inside their greenhouses, but they don’t necessarily monitor natural UV levels. This study shows the importance of monitoring the amount of natural UV light inside greenhouses, especially in poly greenhouses. Doing so would allow producers to assess the integrity of the poly covering over time. Because poly allows more UV transmission as it ages, producers can use increasing UV light levels as an indicator of when poly coverings should be replaced. When replacing greenhouse poly, producers can use UV level information to assess if/how the new poly covering changes the UV levels inside their greenhouses. If the new poly significantly reduces UV levels, as it did in this case study, it might signal to the producer the need to increase their pollinator hive density to compensate for the reduced ability of the bees to pollinate in the lower UV environment, and to compensate for some of the bees escaping to higher UV environments. As the poly ages, UV levels will increase inside the greenhouse, allowing the producer to reduce the density of the hives.

As always, it’s important to think of any changes you make to your greenhouse with a systems approach by asking yourself how this change will affect the greenhouse environment as a whole. For example, reducing natural UV levels in your greenhouse may be good for suppressing some pests and pathogens, but it may also negatively affect beneficials and pollinators. The ideal level of UV required inside greenhouses to keep insect pollinators happy and reduce pests and pathogens at the same time is currently not known. Future research will hopefully shed some light on this topic.

Acknowledgements

We would like to thank the tomato greenhouse producer who collaborated on this study, without whom this would not have been possible. We are grateful to Cara McCreary, OMAFA, for providing the photo and video of a demonstration bumblebee hive, and to researchers at Agriculture and Agri-Food Canada’s Harrow Research and Development Centre for their guidance and feedback. Funding was provided by OMAFA, including the Summer Experience Opportunity (SEO) program, which was used to hire the co-author Sharon Kitur as a summer student.