Do Solar Panels Affect Crop Growth? Agrivoltaic Evidence UK

Farmers weighing solar on productive land want to know one thing first: what happens to the crop underneath. The honest answer is that panels take light, light drives yield, and the result depends on the crop, the design and the weather in a given year. This guide sets out what the research actually measured — and, as importantly, what it did not.

Light is the main thing panels take

Solar panels and crops want the same thing. Every unit of sunlight a panel converts is a unit that does not reach the leaves beneath it, so the starting point for any crop under panels is how its yield responds to light.

The clearest measurement comes from greenhouse horticulture. A 2006 review led by Leo Marcelis (Acta Horticulturae 711) tested the growers’ rule of thumb that 1% more light gives 1% more yield. It is close: each 1% of light added 0.7–1% to the yield of fruit vegetables such as tomato, cucumber and sweet pepper, and 0.8–1% to lettuce and radish. The review also found the relative effect of light is greater when light is scarce — the normal condition in a UK spring and autumn. Even a leafy crop, then, gives up yield when it is shaded; the old claim that salads simply do better under panels is not what the measurements show.

Under glass the effect is stark. In an Italian trial (Cossu and colleagues, Applied Energy, 2014), panels on half of a greenhouse roof cut the light reaching the crop by 64% over a year. Our guide to solar panels on greenhouses covers why growers put the array on packhouse roofs instead.

The best-known field trial: Heggelbach, Germany

The most widely cited field evidence comes from Fraunhofer ISE’s agrophotovoltaic project at Heggelbach, near Lake Constance. On one-third of a hectare of arable land at a Demeter farm community, 194 kW of panels were mounted on a structure five metres high, tall enough to farm underneath. The farm grew winter wheat, potatoes, clover and celery under the array and compared the yields with a reference field without panels.

The results changed from year to year:

• 2017. Fraunhofer reported a land-use efficiency of 160% for the combined system: the crops and electricity from one hectare would have needed 1.6 hectares if grown separately.
• 2018. In that year’s hot, dry summer, three of the four crops under the panels beat the reference field: celery by 12% and winter wheat by 3%, with potatoes also ahead, while clover fell by 8%. Land-use efficiency, based on the potato yield, rose to 186%. Fraunhofer’s explanation was that the shade under the semi-transparent modules helped the plants endure the heat and drought, while the sunny summer raised solar output by 8.4% (press release, 12 April 2019).

Two cautions follow. This is one site, with one design and two seasons reported in that release. And the 2018 gain came from drought: in a cool or dull season, the light the panels take matters more, which is what the Marcelis figures predict.

What UK research shows

UK evidence so far comes mainly from conventional solar parks on grassland, not from raised systems over crops. A study by Alona Armstrong, Nicholas Ostle and Jeanette Whitaker (Lancaster University and the Centre for Ecology & Hydrology; Environmental Research Letters, 2016) monitored a solar park near Swindon for a year. Under the panels in summer the researchers observed cooling of up to 5.2°C and drying; in winter, the gaps between rows were up to 1.7°C cooler than under the panels. Above-ground plant biomass and species diversity were both lower under the arrays, and so were photosynthesis and net ecosystem exchange in spring and winter.

In plain terms: grass under a conventional array grows less than grass in the open, because it gets less light and less rain. That is consistent with everything above, and it is why the realistic farming use under a low, conventional array is grazing.

What this means for different enterprises

Grazing. Conventional ground-mounted arrays sit low, so the farming use that can continue underneath is grazing — sheep keep the grass down without machinery. Expect less grass growth under the panels than in an open field, as the Swindon study found. Grazing does not change how the Rural Payments Agency classifies the land, though: land under a solar array has its own land-use code (UT01), which the RPA lists as a non-agricultural use.

Arable and field vegetables. Cropping under panels needs a raised structure like Heggelbach’s, tall and widely spaced enough for machinery — a much bigger structure than a conventional array. The yield effect then depends on the season: in a normal UK summer, expect the shaded crop to give up yield roughly in line with the light it loses; in a hot, dry summer, shade can turn that into a gain.

Salads and soft fruit. Shade can reduce heat stress, but the Marcelis figures show lettuce still loses yield as light falls. Any claim of higher yields under panels should come with the trial it is based on.

Greenhouses. Panels on the glass take light the crop would have used, in the darkest months most of all. Put them on solid roofs or on the ground nearby.

Design choices that change the result

• Height. Raised structures let machinery through and spread light more evenly; low arrays limit the land to grazing.
• Row spacing and panel density. Fewer panels per hectare means more light for the crop and less electricity; the right balance depends on what the crop is worth.
• Orientation. Vertical, east–west-facing bifacial panels leave wide strips of open ground between rows and generate more in the morning and evening than a south-facing array, at the cost of total output.
• Semi-transparent modules. Heggelbach used them. They pass more light to the crop and convert less of it.

The bottom line

Solar panels do affect crop growth, mostly by taking light. The best evidence says that in a normal season the crop gives up yield roughly in proportion to the light it loses, that shade can help in a drought year, and that combined land use can still be more productive than either use alone — Heggelbach’s 160% and 186%. For most UK farms the low-risk options remain roofs, which take no land at all, and grazing under conventional ground arrays. For the land-take side of the decision, see solar farm profit per acre.

Sources

• Marcelis, Broekhuijsen, Meinen, Nijs and Raaphorst (2006), “Quantification of the growth response to light quantity of greenhouse grown crops”, Acta Horticulturae 711, 97–104.
• Cossu and colleagues (2014), Applied Energy 133, 89–100.
• Fraunhofer ISE, “Agrophotovoltaics: High Harvesting Yield in Hot Summer of 2018”, press release, 12 April 2019.
• Armstrong, Ostle and Whitaker (2016), “Solar park microclimate and vegetation management effects on grassland carbon cycling”, Environmental Research Letters 11, 074016; Lancaster University news, 12 July 2016.
• Rural Payments Agency, land-use codes (UT01).

Related reading

• Agrivoltaics UK Guide — Combining solar generation with crop production on UK farms.
• Ground-Mounted Solar for Farms — Ground-mounted systems that can be combined with agricultural use.
• Solar for Arable Farms — How solar integrates with cereal and crop production.
• Planning Permission Guide — What you need to know about agricultural solar planning rules.
• Solar Panels for Agricultural Buildings — Roof-mounted alternatives that preserve all cropland.

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