Greenhouse Gas Emissions of Organic Strawberries


The demand for organically grown food has increased over the past few decades, with environmentally savvy consumers concerned about how the food they eat affects both their health and the planet. However, organic farming  usually produces a lower yield when compared to ‘conventional’ methods, and rely on the same CO2 emitting machinery.


Professor Sean Clark of Berea College, Kentucky examined the carbon footprint of organic strawberries grown in high tunnels, otherwise known as polytunnels, and how they compare to traditional farming methods or to controlled ‘vertical’ farms.


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Image credit: Jax10289/





Hello and welcome to Research Pod.  Thank you for listening and joining us today. In this podcast we are exploring the research of Professor Sean Clark of Berea College, Kentucky.  Professor Clark examined the carbon footprint of organic strawberries grown in high tunnels, otherwise known as polytunnels.  At Berea College, Clark teaches a diverse range of courses in agriculture, horticulture, and food production, and conducts research on the college’s teaching farm.


The demand for organically grown food has increased over the past few decades. Consumers are more environmentally savvy and concerned about how the food they eat affects both their health and the planet. It is generally assumed that organically farmed produce would always be the better option, after all, the whole point of organic produce is to maintain the soil quality and biodiversity of farmland. Since organic farms avoid the use of synthetic pesticides and fertilisers, one would assume they are always the greener option , but are they? The impact of organic farming on greenhouse gas emissions and climate change is a little more complicated than you might think.


One drawback of organic farming is that it usually produces a lower yield when compared to ‘conventional’ methods.  Manufactured fertilisers and pesticides, while energy-intensive to produce, can boost crop yields by supplying essential nutrients as well as suppressing weeds, insect pests, and pathogens. But using these fertilisers comes at a cost to the environment and human health. The technology to produce consistent high yields organically simply doesn’t exist yet.  Organic farming still results in soil disturbance and depends upon the same CO2 emitting machinery to raise the plants which, when compared to non-organic crops, produce less fruit, grain, or vegetable per hectare of land.  This means that each unit of product from an organic farm may have a higher individual carbon footprint than the equivalent grown conventionally.  If organic crop yields are lower than their non-organic counterparts, organic farms must increase their land use to compensate, generating more greenhouse gases.


One of the proposed solutions to this issue is the use of controlled-environment agriculture or C.E.A. Also known as plant factories with artificial lighting or vertical farming, C.E.A. facilities control all aspects of a crop’s growing conditions.  The humidity, lighting, and temperature can all be precisely manipulated within a space that is densely packed with crops.  Although they are also energy and material resource intensive, C.E.A. systems boast high yields per hectare, which in theory may result in a lower carbon footprint per unit of product.  However, recent research has shown C.E.A. facilities consume vast quantities of resources, and produce a large carbon footprint, due to the materials used in their construction, and the energy required in maintenance. Proponents of this strategy argue that vertical agriculture reduces our agricultural land use while protecting crops from weather and pests, but if the system produces a carbon footprint larger than that of organic agriculture, it may not be the greener alternative, even despite higher yields.



Another option for organic farming is the use of unheated polytunnels, or high tunnels.  It is a simple technology with a much lower carbon footprint than both conventional farming or plant factories.  Professor Clark and his team studied the carbon footprint of organic strawberry production using this unheated polytunnel system to document greenhouse gas emissions and assess the global warming potential of such a setup.  Strawberries were grown on raised beds protected from weeds with woven plastic fabric. The polytunnel system was much cheaper to implement, while still employing all the benefits of organic agriculture and a semi-protected environment.  The aim of the project was to determine whether a polytunnel method for organic strawberries could produce a yield high enough to justify investment of inputs.


To accurately assess the carbon footprint or other environmental impacts of an agricultural product, all aspects of production need to be quantified.  The per-unit impact on global warming can be measured with a life cycle assessment. This type of evaluation is a ‘cradle to grave’ calculation of all the resources employed in a product’s creation, use, and disposal. It  should account for all environmental impacts of a resource, or in this case, crop.


Life cycle assessments have already been applied in many industries, and have become more popular for agricultural assessments in recent years. Results are not always completely conclusive, but they can still provide useful insights.   A common pattern revealed by these life cycle assessments is that organic fields typically generate fewer carbon dioxide, nitrous oxide, and methane emissions per hectare, but they struggle to compete with conventional farms per unit of product due to their lower yields.


It was important for Professor Clark’s team to be able to produce a sufficiently high yield of strawberries under certified organic conditions, as this would be a powerful testament to the sustainability of organic farming. By using polytunnels, the organic farm can achieve a yield as high as the conventional farm, but without the massive infrastructure and energy input of a vertical factory setup. Previous life cycle assessments have shown that strawberries grown in open field or polytunnel systems in a Mediterranean region, such as Spain, Italy, and Turkey had the lowest greenhouse gas emissions, while open-field and greenhouse production systems in the USA, Germany, United Kingdom, and Japan had the highest emissions. Open field production was generally greener in warmer, semi-arid climates and more detrimental to the environment in cooler climates, regardless of strategy.


By applying life cycle assessment to the research site in Kentucky, Clark’s group wanted to establish an accurate picture of the greenhouse gas emissions of small-scale organic strawberry production. To calculate this, the team measured the total energy input, all materials consumed, equipment used, as well as the direct emissions associated with the harvest and post-harvest operations, and compared the total to the strawberry yield. One kg of strawberry fruit was defined as a unit and the amount of carbon sequestered by the soil was also factored into the calculations. Overall, the strawberry production of this study contributed fewer greenhouse gas emissions than most others reported for conventional production around the world.   And although the average yield over two years was generally lower than others reported for commercial production areas, they were comparable to those achieved in conventional outdoor production for the region. The results suggested that unheated polytunnels could successfully increase the yields of organic production to match those of conventional open-field production. The team also noted that the construction of the polytunnel setup, which used aluminium and plastic, contributed the most to the overall carbon footprint.  Perhaps the polytunnel system could become even greener in the future if more efficient building materials are developed.


This strategy for growing organic strawberries could be competitive with conventionally grown berries in Kentucky and the surrounding region of the US, using the simple technology of unheated high tunnels. Many companies are still motivated to build energy-intensive vertical-farming plant factories with artificial lighting to protect their crops and increase yields in a world with an increasingly unpredictable climate. More studies like this one, which assess the environmental costs and benefits of each system, will help identify the best approach.


Growers are always keen to optimise their methods to achieve higher yields, preserve the environment, and lower the costs of production.  Organic farming practices have demonstrated success in preserving soil health and using energy more efficiently, but challenges still remain. Agricultural science can help improve organic farming techniques aimed at enhancing soil fertility, protecting biodiversity, conserving natural resources, and addressing climate change.  In-depth analysis, such as the life cycle assessment, can provide answers and identify key areas for improvements.  By collaborating with researchers, organic farmers can continue to develop more sustainable food production systems for their consumers and the environment.


As Professor Clark sees it: ‘Agriculture is major contributor to climate change, so finding appropriate methods of producing food while reducing emissions is urgently needed. That requires unconventional thinking and collaboration.’


That’s all for now. Thanks for listening and stay subscribed to ResearchPod for more of the latest science. See you again soon.

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