Phenology of the environmental weed Achillea millefolium (Asteraceae) along altitudinal and disturbance gradients in the Snowy Mountains, Australia

This study examined the phenology of the weed Achillea millefolium over a growing season in the Snowy Mountains. Vegetative and reproductive characteristics of plants in 1 m² quadrats were compared among sites at four different altitudes (medium and high montane, low and high subalpine) and three types of infrastructure (primary road, secondary road and building, total 12 sites). Altitude, infrastructure and time of year did not affect percentage cover of vegetation. Flowering started earlier and lasted longer in the low montane sites compared to high subalpine sites. The type of infrastructure only affected the number of reproductive structures at the peak of flowering, with A. millefolium growing next to buildings having two to three times more inflorescences per m² than along primary and secondary road verges. At the peak for each reproductive stage, there was an average of 1.47 developing inflorescences, 21 inflorescences in bud, 24 inflorescences in flower, 4 inflorescences setting seed, and 3 releasing seed. Based on the maximum number of inflorescences present at any time at each site, there was an average of 36 inflorescences, giving an estimate potential seed production of 51 400 seed per one m² for A. millefolium in the Snowy Mountains. If the climate changes in the Snowy Mountains as predicted, then it is likely that yarrow will produce more inflorescences and seed in the higher altitude sites.