Magenta Solar Panels Kept Broccoli Yields—But Delayed Harvest by 25 Days

A Swedish field experiment has put a useful number on the central trade-off in agrivoltaics: broccoli grown beneath semi-transparent magenta solar panels reached a similar harvest weight to open-field plants, but it needed 25 extra days to get there. The result is more informative than a simple claim that crops and electricity can share land, because it shows both the productive potential and the operational cost of reduced light.

The peer-reviewed study, published October 2 in Cell Reports Physical Science, tested two 20-by-20-metre agrivoltaic systems at Kärrbo Prästgård farm near Västerås, Sweden, during the 2024 growing season. One canopy used panels with 50% transparency and the other 70%; a third plot grew broccoli in full sun. The team monitored temperature, humidity, soil moisture, photosynthesis, nutrient composition, crop yield and electricity production.

Yield held up, but time became the constraint

Open-field broccoli reached maturity 79 days after transplanting, while plants beneath both panel systems matured after 104 days. At harvest, average head weight was 187.6 grams in the open, 168.5 grams under the 70%-transparent panels and 173.7 grams under the 50%-transparent panels. The researchers reported that those yield differences were not statistically significant under the trial conditions.

That distinction matters. The study does not show that shade accelerated broccoli growth or increased harvested mass. It shows that the plants eventually compensated well enough to reach a comparable final size despite receiving less light. For growers, an extra 25 days can affect labour planning, pest exposure, seasonal rotations and the risk that a crop meets cold weather before harvest. A result can be biologically promising while still requiring a different commercial calendar.

The plants’ radiation-use efficiency—the biomass accumulated per unit of intercepted solar energy—was 4.5 times the open-field value beneath the 50%-transparent system and 2.8 times the open-field value beneath the 70%-transparent system. This does not mean the shaded broccoli produced four times as much food. It means the plants converted the smaller amount of available radiation into biomass more efficiently.

Why the panels are magenta

The experimental modules use cadmium-telluride thin-film photovoltaic strips embedded in semi-transparent glazing. A coloured interlayer changes the transmitted spectrum: it allows relatively more blue and red light, where chlorophyll absorption is strong, while reducing green light. The design is intended to divide the incoming spectrum between photosynthesis below and electricity generation above, rather than treating every wavelength as equally valuable to both uses.

The 50%-transparent array was estimated to produce about 447.6 kilowatt-hours during the cultivation period, with a photovoltaic conversion efficiency of 7.2%. Normalised to a hectare, the researchers reported roughly 224.7 megawatt-hours over the growing period. That is a modelled scale-up from the research installation, not output measured from a commercial one-hectare farm.

The team also calculated a land-equivalent ratio of 1.18 for a hectare-scale scenario that assumed 40% of the site could not be cropped because of structural and access requirements. In practical terms, the model suggests the combined crop-and-electricity system could deliver 18% more land productivity than producing the same outputs on separate areas. The ratio depends on those assumptions and should not be read as a guaranteed farm return.

What the experiment does—and does not—prove

The strongest finding is that two panel configurations with substantially different transparency produced broadly similar broccoli outcomes. That opens a design question: if crop performance can be maintained with the denser photovoltaic arrangement, a system may be able to allocate more of the site’s light to electricity without an obvious yield penalty. But the trial was not an economic study, and it did not establish the capital cost, maintenance burden, grid value or payback period of the prototype.

The evidence also comes from one crop, one location and one completed growing season in central Sweden. Broccoli is relatively suited to cool conditions; grains, fruiting vegetables and other crops can respond differently to shade, spectral changes and delayed development. The authors explicitly call for trials across additional seasons, crops, climates and configurations before large-scale deployment.

For now, the most realistic near-term applications may be smaller plots, community gardens or greenhouse roofs, where semi-transparent modules can serve as both enclosure and power generator. The broader lesson is not that magenta panels have solved the land competition between food and solar energy. It is that spectrum, panel density, crop choice and harvest timing can be engineered together—and that the right agrivoltaic design must be judged on the complete production system, not yield alone.