No fertilization or reduced irrigation of hydrangea in autumn: an eco-friendly tool to favour leaf senescence without consequences on flowering quality

With global climatic warming, autumns in Anjou (49, France) are hotter, so container-grown hydrangea fertigation needs to be extended later. Even if growers use a lower nitrogen formulation for autumn fertigation, plants receive sufficient nitrogen to promote green growth, this causes a delay in nutrient remobilization which leads to leaf senescence and fall. Furthermore, in case of rainy periods in autumn, these nitrogen supplies increase nitrogen leaching risks. As at the end of October, companies store all hydrangea production in fridges (around 2°C for at least 2 months to allow vernalization and to avoid frost damage to flower buds), the leaves, still green and turgid, fall in fridge, needing costly manual evacuation by blowing. One solution to solve these problems could be to promote leaf senescence by reducing irrigation and/or by stopping fertilization supply earlier in autumn. During two successive years, in September and October, we applied different fertigation protocols (halving of water supplies, suppression of fertilizers). Experiments were carried out on three vigorous hydrangeas cultivars representing a panel in terms of Botrytis cinerea sensibility. We measured consequences on plant morphological and physiological parameters during autumn and after fridge storage, as well as on plant quality at blooming. Stopping autumn fertilization tended to accelerate the senescence onset. Halving irrigation also tended to advance senescence but was unsuitable for the most sensible cultivar to botrytis, due to necrosis of young leaves on hot and dry days, increasing fungi infestation risks. Neither halving autumn irrigation nor stopping autumn fertilization affected qualities of hydrangea growth and flowering during next spring. While, yet insufficient to allow full leaf fall before fridge storage, stopping earlier in autumn hydrangea fertilization and reducing irrigation seem to be promising approaches to increase the sustainability of hydrangea crops by reducing both the synthetic fertilizer uses and the nutrient leaching risks.