Drivers of invasion: The spread of pink-flowered Nymphaea rubra Roxb. ex Andrews in paddy ecosystems of Kuttanad, South India

Kuttanad, the rice bowl of Kerala, is a unique agro-ecosystem practising below-sea-level farming, declared as a globally important agricultural heritage system by the Food and Agriculture Organisation (FAO) in 2013. Its paddy landscapes and waterbodies face severe aquatic weed invasion, intensified by nutrient influx due to leaching from surrounding paddies and high inherent soil fertility. Since the 2018 flood, pink-flowered waterlilies (Nymphaea rubra Roxb. ex Andrews) have spread widely, producing substantial biomass-about 65300 kg ha-1 fresh weight. Infestation occurs during rice fallow when fields remain inundated, delaying land preparation for the next crop. At the same time, the bloom is promoted as a hub for waterlily tourism. A study in the Malarikkal region of Thiruvarp Panchayat, Kottayam, investigated the biology and growth dynamics of the invasive pink waterlily N. rubra and the native Nymphaea pubescens to identify drivers of proliferation. Experiments assessed flowering behaviour, biomass and the influence of rhizome size on plant morphology under controlled conditions. A field investigation from June to September 2024 documented seedling emergence, growth stages and morphological traits across varying water depths, revealing how flooding and depth gradients affect the propagation and blooming of the species. Nymphaea rubra is a night bloomer, opening at 6.00 pm and closing by 11.00 am, with flowers lasting 4–5 days before submerging. Fruits mature underwater, dispersing seeds that are water-mediated, while seedlings produce stolons that develop into rhizomes. Rhizomes segmented and grown under net house conditions showed that intermediate sizes enhanced growth efficiency, with the highest mean leaf number (18.95) and petiole length (24.14 cm) recorded for 0.035 g of rhizome slice. Fragments of rhizome as small as 0.005 g could regenerate fully. Farmers apply systemic herbicides thrice-post-harvest, pre-sowing and post-emergence-eradicating rhizomes, confirming seeds as the primary propagule. Seedling growth under varying water depths revealed that greater depth increased petiole length and leaf area but reduced leaf and flower numbers, with shallow waters producing more flowers. These findings highlight the growth dynamics of N. rubra in Kuttanad’s paddy ecosystems and the need for sustainable weed management strategies.