To adapt to climate change and water scarcity during dry, hot summers, more sustainable, or even deficit, irrigation is required in the ornamental sector, as it uses large amounts of water to sustain high-value crop production. Biostimulants, especially seaweed extracts, could offer a sustainable solution against drought stress as they are known to increase plant tolerance to abiotic stress. The effect of four seaweed extracts based on Ascophyllum nodosum, Soliera chordalis, Ecklonia maxima, and Saccharina latissima and one microbial biostimulant were tested on container-grown Hydrangea paniculata under drought stress conditions for two years. During the first trial year, in 2019, overall irrigation was reduced by 20%. In 2021, plants were subjected to repeated drying and wetting cycles. In general, less irrigation, and thus a lower substrate moisture content, reduced stomatal conductance, biomass production, and root development, but increased plant compactness. The biostimulants showed minor effects, but these were not observed in both experiments. Treatment with the A. nodosum extract resulted in longer branches and more biomass under deficit irrigation but tended to accelerate flowering when repeated drying and wetting cycles were applied. The E. maxima extract negatively affected the branching of Hydrangea under repeated drying and wetting cycles.

Over the past years, we experienced more extreme weather conditions during the growing season, April till October, with prolonged droughts. Rain-fed production of high-quality forest trees was possible, but recent droughts proved to have an economic impact on the plant quality. Therefore, the hardy nursery sector demands irrigation thresholds and suitable tools including soil and plant sensors to schedule irrigation based on crop water demand. Two trials were conducted with Acer pseudoplatanus liners (1 + 0) grown in a sandy soil in 2022 and 2023 at Viaverda (Destelbergen, Belgium). A rain-fed treatment was compared with a sprinkler irrigation treatment in both trials. Irrigation doses were evaluated with a soil water balance model, which is based on reference crop evapotranspiration (ET₀), rainfall, and soil hydraulic properties. The soil water balance model was calibrated based on the measurements of soil sensors and soil samples. Simultaneously, stem water potential at solar noon, tree length, and growth were measured. The irrigation treatment had a positive effect on the stem water potential of Acer in both trials with a less negative value, ±0.7 MPa, compared to the rain-fed treatment. Irrigation increased growth with 28.4% in 2022 and 5.8% in 2023 compared to the rain-fed treatment, resulting in trees of higher commercial quality that could even be classified into a superior grading range in 2022.