Exploring Coffee Silverskin as a Sustainable Peat Additive in the Plant Nursery Production

Sustainable alternatives to peat in horticultural substrates are increasingly sought. This study assessed the use of coffee silverskin (CS), a byproduct of coffee roasting, as a substrate component for rooting and growing ornamental plants—Buddleja, Lythrum, and Veronica. Plants were cultivated in peat-based substrates with 0%, 25%, 50%, and 75% CS addition. In order to determine the effect of substrate modification with CS, the following parameters were analyzed: rooting efficiency, plant growth, pigment content, physiological indices (SPAD, Fv/Fm, NFI), and substrate properties. A 25% CS addition supported high rooting success (94.4% on average) without negatively affecting root or shoot traits, and even improved flowering earliness. At 50% CS, Buddleja showed moderate tolerance, while Lythrum and Veronica performed poorly. The substrate with 75% CS addition significantly reduced rooting and growth across all species. Elevated pH and electrical conductivity in high-CS substrates likely contributed to plant stress. Physiological indicators confirmed minimal stress at 25% CS, but increased stress response at 75%. Overall, CS can replace up to 25% of peat in substrates without compromising plant performance, offering a sustainable alternative in nursery production. However, higher CS levels require structural or chemical adjustments to reduce compaction and stress effects. Further research is needed to improve CS-based substrate formulations for broader horticultural use.