Differences in soil seed longevity for two subspecies of the invasive weed, Chrysanthemoides monilifera (Asteraceae)

The persistence of a soil seed bank influences weed management options. However, for many species, the degree of persistence of a soil seed bank is unknown. We assessed how long seeds remain in the soil for two invasive weeds, Chrysanthemoides monilifera subsp. monilifera (boneseed) and C. monilifera subsp. rotundata (bitou bush), as seeds of these species were predicted to have different longevity in the soil associated with differences in seed coat thickness. Seeds of both subspecies were buried at two depths at two sites in eastern Australia. Seeds were tested for germinability and viability regularly over an 8‐year period. Freshly collected seeds had quite low viability, and there was a rapid decline in viability in the first couple of years after burial, with C. m. rotundata declining faster than C. m. monilifera. Some seeds, however, remained viable in the soil at some sites for up to 8 years, with high site‐to‐site variability. The variability in viability from site to site may be due to the length of time soils remain moist: a combination of rainfall quantity and humidity levels. The low numbers of seeds remaining viable at 8 years indicate progressively lower probabilities of recruitment at sites following weed control but also suggest that monitoring for both subspecies needs to occur for a long period to minimize the risk of reinfestation from the seed bank.