Linking nutrient dynamics and phenology in Lupinus polyphyllus to identify the right timing for population control

• Biomass and nutrients of Lupinus polyphyllus roots are lowest during flowering. • Shoot:root ratio peaks during flowering and the seed ripening phase. • Mowing during flowering and before seed dispersal will weaken the plants the most. • Planning of control by mowing should be based on the species’ phenology. In light of the decline of semi-natural grasslands, road verges have become important refuges for grassland plants, benefiting from the management regimes that resemble those of traditional grasslands. However, the value of road verges for grassland plants is threatened by the presence of invasive alien plant species such as Lupinus polyphyllus . Mowing is most effective for controlling invasive plants when the shoot:root ratio is high or when below-ground reserves are exhausted but seeds have not yet matured. The aim of this study was to identify the optimal time for mowing of L. polyphyllus based on the species nutrient dynamics and phenology. We photographed and collected whole lupine plants throughout one growing season, divided them into roots, leaves, and stem + inflorescence, and analyzed each fraction for three growth-limiting, phloem-mobile nutrients (N, P and K). The nutrient contents in roots were lowest during the flowering phase, while the nutrient contents in stems + inflorescences were lowest during the early flowering phase and increased until the seed ripening phase, before declining again in the seed dispersal phase. Leaf nutrient contents showed a general decrease throughout the season. Shoot:root ratios of nutrient contents were also maximized during flowering. Our conclusion is that mowing will weaken L. polyphyllus most when carried out between phenological phases 2 (less than 50% of flowers open) and 4 (more than 50% of flowers withering), with the most optimal timing being during phase 3, i.e., when more than 50% of flowers are open. Concerning the heterogeneity within L. polyphyllus populations, the phenological assessment before management should prioritize individuals that have progressed furthest in their development.