A multi-year study has concluded that the cup plant, Silphium perfoliatum, is a more sustainable and ecologically beneficial alternative to silage maize for producing bioenergy. Research from the University of Bayreuth in Germany demonstrates that the cup plant not only produces more biomass than maize after its initial establishment period but also drastically reduces the leaching of harmful nitrates into groundwater, a significant environmental problem associated with intensive maize cultivation.
The findings, published in the journal GCB Bioenergy, come as European nations seek sustainable agricultural solutions to meet climate neutrality goals by 2050. In Germany, maize is the dominant crop for biogas production, but its cultivation leads to soil erosion, nutrient runoff, and significant nitrate contamination of water supplies. The cup plant, a long-lasting perennial native to North America, presents a viable solution that mitigates these ecological damages while ensuring a reliable energy source, proving resilient even under conditions of moderate drought that hamper maize production.
A Multi-Year Comparative Experiment
Researchers at the University of Bayreuth conducted a rigorous four-year comparative study to evaluate the cup plant’s potential. The experiment took place in a lysimeter facility, a specialized setup that allows scientists to grow plants under highly controlled conditions while precisely collecting and analyzing all water and substances that drain through the soil. This system enabled the team from the Agroecology research group and the Botanical Garden to monitor the growth and environmental impact of both cup plant and maize under two distinct scenarios: an adequate water supply and moderate drought stress, simulating the variable conditions imposed by climate change.
Superior Biomass and Nutrient Retention
Outpacing Maize in Growth
The study’s results showed a clear performance advantage for the cup plant after its first year of establishment. From the second year onward, it consistently produced a greater amount of biomass than silage maize. In one year of the study, for example, the mean shoot biomass of the cup plant was measured at 2,696 grams per square meter, vastly outperforming maize, which produced only 266 grams per square meter in the same period. This high yield is critical for biogas production, which relies on a large volume of plant matter for conversion into methane.
A Drastic Reduction in Water Contamination
The most significant ecological finding was the cup plant’s remarkable ability to protect groundwater from contamination. Its deep and extensive root system efficiently captures nitrogen from the soil, preventing it from leaching out as nitrates. In the third and fourth years of the experiment, nitrate leaching under the cup plant was reduced by as much as 99% compared to the levels recorded under maize cultivation. This demonstrates a profound potential to improve water quality in agricultural regions. Prof. Dr. Johanna Pausch, Professor of Agroecology at the University of Bayreuth, noted that the deep root system allows the plant to access water and nutrients in lower soil layers, contributing to its efficiency.
Resilience in a Changing Climate
The cup plant also proved to be a more reliable crop in the face of increasing climate variability. While maize production suffered under simulated moderate drought conditions, the cup plant’s biomass output remained comparatively stable, showing only minor losses. Its deep roots are again a key advantage, enabling it to draw water from soil depths that maize cannot reach. This drought resistance suggests the cup plant is a more dependable long-term energy crop as weather patterns become less predictable. The study’s lead author, Anna Hollweg, emphasized the plant’s ability to efficiently absorb nitrogen, which contributes to its robust performance.
Ecological and Agricultural Co-Benefits
Enhancing Soil Health
As a perennial crop that can remain productive for 15 years or more, the cup plant offers substantial benefits for soil health that annual crops like maize cannot. Because the soil is not tilled and reseeded every year, the risks of soil erosion and carbon loss are significantly reduced. Furthermore, the permanent root structure promotes the activity of soil microorganisms, which improves overall soil quality and structure over the long term. This stands in stark contrast to maize, which requires frequent cultivation that can degrade soil and results in emissions and higher costs.
Supporting Local Biodiversity
Beyond its role as an energy source, the cup plant provides distinct advantages for local ecosystems. As a member of the Asteraceae family, its daisy-like yellow flowers offer a valuable source of pollen and nectar for pollinators, including bees and butterflies. The unique structure of its leaves, which join at the stem to form a “cup” that holds water, also provides a resource for insects. This makes fields of cup plant far more valuable to wildlife than the monocultures typical of maize cultivation.
From Research Findings to Field Application
Practical Adoption in Biogas Production
The benefits identified in the laboratory are being put to the test on farms across Europe. In the United Kingdom, farmers have begun planting Silphium perfoliatum as a feedstock for anaerobic digestion (AD) plants. Its perennial nature is a major draw, as it eliminates the annual cost and effort of cultivation. Farmers can use the same harvesting equipment for both cup plant and maize, making the transition straightforward. To further ease the process, researchers from the Bayreuth study suggest a combination of maize and cup plant during the initial establishment phase to harness the benefits of both crops.
Evaluating Methane Yield and Viability
While the ecological and cultivation benefits are clear, the ultimate success of an energy crop depends on its energy output. Some field experience in Germany suggests the methane potential of cup plant silage may be slightly lower than that of maize, perhaps around 90% of the yield. However, this potential reduction in energy density is likely to be offset by the crop’s higher biomass yields, significantly lower input costs over its long lifespan, and powerful environmental advantages. A separate Life Cycle Assessment study confirms that cup plant represents a lower environmental load per unit of production and area, making it a highly recommendable option for sustainable farming.