Sunflower straw as peat substitute is emerging as a promising solution in the search for more sustainable growing media. Within the PEATLESS project, several plant-based materials have been identified as potential alternatives to peat, and sunflower straw has been selected for further evaluation due to its availability and structural properties.
This article, prepared by ZHAW (Zurich University of Applied Sciences), explores how sunflower straw is being tested as a substrate component for vegetable propagation.
From Agricultural Residue to Growing Medium
In Switzerland, sunflowers are cultivated on more than 7,000 hectares. Since only the seeds are harvested, a significant amount of residual biomass remains in the field. This leftover material represents an underutilised resource with potential for sustainable substrate development.
Sunflower straw has a naturally heterogeneous structure, combining:
- Outer fibres: lignin-rich and structurally stable
- Inner pith: lightweight with high water-holding capacity
This combination makes sunflower straw particularly interesting as a substrate component, as it may provide both structure and moisture retention.
Processing and Initial Testing
To assess its suitability, sunflower straw was harvested in October 2026 from both organic and conventional systems. As standard harvesting machinery is not equipped to collect this material, it had to be manually gathered from the field.
The biomass was then processed through several steps:
- Shredding using a garden chipper
- Mechanical size reduction to a maximum particle length of 4 cm
- Sieving to improve uniformity
Cress test with referenced growing media (control) and two mixtures with sunflower straw, mixing ratio 50 % volume-to-volume (v/v; meaning 50% of the volume is sunflower straw) (Image: ZHAW)
A test substrate was prepared using a 50% sunflower straw and 50% fertilised reference growing media mixture. This was evaluated through a cress germination test. The results showed slightly reduced growth compared to a peat-based control substrate, indicating that while sunflower straw is functional, further optimisation is needed.
Chemical Characteristics and Key Considerations
Chemical analysis revealed several important factors influencing the performance of sunflower straw as a peat substitute:
- High pH (9.8)
- High electrical conductivity (65.4 mS)
- Acceptable C:N ratio (30)
The elevated pH and salt content may limit plant growth if not properly managed. However, the C:N ratio indicates that the material is suitable for blending with other substrate components.
Further Trials and Promising Results
In the next phase of the PEATLESS project, sunflower straw was incorporated into new substrate formulations. Two mixtures — containing 10% and 25% sunflower straw — were tested in a plant growth trial using Chinese cabbage.
Both mixtures showed promising results compared to the standard substrate, demonstrating that sunflower straw can be successfully used when applied in appropriate proportions.
Growth test on Chinese cabbage using a standard substrate and two mixtures containing sunflower straw (Image: ZHAW)
Improving Material Performance
To further enhance its properties, sunflower straw underwent mechanical fibre processing. The material was dry-defibrated at temperatures below 100°C, with the aim of:
- Improving fibre structure
- Increasing homogeneity
- Enhancing water retention behaviour
This processing step could play a key role in improving the performance of sunflower straw as a peat substitute in future applications.
Sustainability Considerations
While sunflower straw as peat substitute offers clear potential, its use must be evaluated within a broader sustainability context.
Removing crop residues from the field reduces the return of organic carbon to the soil, which may affect long-term soil fertility and humus formation. This creates an important trade-off between resource use and soil health.
Within the PEATLESS project, this balance is being carefully assessed to ensure that alternative materials support both horticultural performance and long-term sustainability.
