Seed Germination Ecology of Burning Nettle ( Urtica Urens ) and Common Groundsel ( Senecio Vulgaris ) Populations from Leafy Salad Production Systems in Australia

Abstract Burning nettle ( Urtica urens L.) and common groundsel ( Senecio vulgaris L.) are widespread, economically important weeds in horticultural systems. This laboratory and glasshouse study evaluated the effect of temperature, photoperiod, salinity, osmotic stress, and pH on seed germination, and the effect of burial depth on seedling emergence of these species. A significant interaction between temperature and photoperiod showed that germination depended on combined thermal and light cues rather than either factor alone. Urtica urens germinated higher in complete darkness (53%) than in light/dark (3%) across three temperature regimes studied. Germination peaked at 90% at 25/15C in complete darkness, while no germination occurred for this species at 15/5C and 35/25C day/night temperatures. In contrast, S. vulgaris preferred light/dark conditions (52% germination vs 29% in darkness), reaching 96% at 25/15C under day/night conditions. Salinity and osmotic stress inhibited germination of both species in a non-linear, sigmoidal pattern. Urtica urens germination was completely inhibited at ≥200 mM of sodium chloride (NaCl) and at ≤−0.6 MPa osmotic potential. Senecio vulgaris showed greater tolerance, with complete inhibition at ≥250 mM NaCl concentration and at ≤−1.0 MPa osmotic stress. Urtica urens germination peaked at pH 6 (92%) and declined under more acidic and alkaline conditions, while S. vulgaris germinated >97% across all pH levels (4 – 10). Urtica urens showed overall low emergence with 11% from soil surface and <1% when buried at different depths. In contrast, S. vulgaris showed up to 90% and 38% emergence from surface in sand and soil, respectively, but its seeds could not emerge beyond 0.5 cm burial depth. These findings can help refine integrated weed management in baby leaf crops by identifying periods when emergence is most likely, supporting timely stale seedbed or pre-emergence interventions, avoiding unnecessary soil disturbance when conditions favor germination, and prioritizing surface-focused control tactics for S. vulgaris .