Hydraulic Behavior and Plant Performance of Redwood Bark–Coir and Palm Fiber–Peat Substrates under Deficit Drip Irrigation

Reducing reliance on peat-based substrates is a growing priority in containerized horticulture, but viable alternatives must sustain water, nutrient, and plant performance under commercial irrigation. We evaluated three substrates: a conventional peat–perlite mix, a peat-free redwood bark–coco coir blend, and a palm fiber–peat blend using recycled palm biomass. Greenhouse experiments with Impatiens walleriana and Pentas lanceolata used a 3 × 3 factorial design (three substrates × three irrigation regimes) over two 8-week production cycles. Substrate hydraulic properties, moisture dynamics, plant growth, salability, water use, and nitrogen dynamics were assessed. Substrates differed substantially in hydraulic and chemical behavior. Redwood bark–coir exhibited greater air-filled porosity and a steeper decline in water content at low matric suctions, whereas palm fiber–peat showed intermediate container capacity but elevated electrical conductivity and evidence of nitrogen immobilization. Peat–perlite produced the greatest plant biomass and salability, particularly for the water-sensitive Impatiens . Both alternative substrates supported commercially viable Pentas across irrigation treatments, indicating potential suitability for more drought-tolerant crops. Nitrogen dynamics diverged among substrates. Unaccounted nitrogen was greatest in peat–perlite, whereas palm fiber–peat retained nitrogen, suggesting immobilization. Redwood bark–coir showed limited lateral water redistribution using single-emitter drip irrigation, indicating that this irrigation method may be particularly important for substrates with low unsaturated hydraulic conductivity. Overall, partial peat substitution with palm fiber produced viable plants at a lower cost, but substrate-specific irrigation and fertility management are required to account for contrasting hydraulic and nutrient dynamics. Graphical Abstract