Germination and emergence of Ambrosia artemisiifolia L. under changing environmental conditions in China

Laboratory and greenhouse studies were conducted to determine the effects of key environmental factors on germination and seedling emergence of the invasive weed Ambrosia artemisiifolia L. (common ragweed) collected from Mudanjiang (temperate climate), Nanjing (temperate-subtropical) and Nanchang (subtropical) in China. Germination of seeds occurred at temperatures ranging from 5 to 40 degrees C, under both a 12-h photoperiod and continuous darkness. Germination success exceeded 48% in solutions with pH values between 4 and 12, with maximum rates occurring in distilled water at pH 5.57. Germination was greatly reduced in solutions with osmotic potentials below -0.8 MPa. Accordingly, the final germination ratio exceeded 69% at < 200 mmol/L NaCl, but only reached 8% at 400 mmol/L NaCl. Emergence was greater than 75% at burial seed depths of 1-4 cm; no seedlings emerged from a soil depth of 8 cm. Seeds collected from Mudanjiang, Nanjing and Nanchang had very similar germination traits, with the main differences occurring in relation to temperature. The great germination success of common ragweed over highly variable conditions throughout its Chinese distribution range explains its successful large-scale invasion.