The challenges of controlled environment hydroponic farming: a life cycle assessment of lettuce

Purpose: Lettuce and other vegetables are predominately produced with open-field methods in agriculturally optimal locations and transported long distances to markets; alternative methods can produce vegetables in the regions where they will be consumed. This study aims to comprehensively assess the environmental impacts of lettuce cultivation methods: ranging from uncontrolled open-field farming to lightly controlled high-hoop greenhouse cultivation, to fully controlled hydroponics with artificial lighting and climate control systems. The overarching purpose is to illuminate the impact of cultivation methods on vegetable production, including the effects of transport, and to support efforts to reduce impacts across production methods. Methods: We constructed a life cycle inventory for cultivating lettuce, including energy, water, nutrients, pesticides, carbon dioxide, nitrous oxide, nitrogen oxides, ammonia, and particulates. The system boundary is cradle-to-gate, including the production and utilization of electricity, fertilizers, irrigation water, and the utilization of pesticides. We included the energy use of production and post-harvest transportation at a continental and regional scale to market. Excluded from the system boundary are the retail sale of the lettuce, residential or commercial preservation, preparation, consumption, and end-of-life. We assessed the impacts using ReCiPe 2016 impact assessment methods (hierarchist perspective (H)) which includes both midpoint and endpoint evaluation. We investigated regional and long-distance lettuce production with the US state of Georgia representing the regional state and California and Arizona serving as examples of long-distance states for lettuce cultivation to the US East Coast (e.g., Georgia) region. Results and discussion: Lettuce produced through controlled environment hydroponic farming has relatively high energy consumption, primarily due to the use of artificial lighting. In contrast, open-field farming and low-energy greenhouse farming have higher impacts across other environmental categories, due to higher use of fertilizer, pesticides, land, and irrigation water. Using detailed data on fertilizer, pesticide, and water consumption, we show that open-field production has higher impacts in every midpoint category other than climate impact. Endpoint analysis suggests that the climate impacts of controlled environment hydroponic agriculture have greater impacts on human health and ecosystems than all of the other impacts of both open-field and greenhouse production. Conclusion: To match conventional agriculture, energy consumption and greenhouse gas emissions of controlled environment hydroponic agriculture must be substantially reduced. This could be achieved with use of low carbon electricity combined with use of natural light.