Seed germination of cultivated, naturalised and floristically traded Cortaderia species in South Africa

• Cultivated and naturalised Cortaderia populations have higher seed viability and germination success than traded Cortaderia populations. • Some inflorescences sold as 'sterile' had significant numbers of viable seeds and should be viewed as posing a risk. • The trade in inflorescences is potentially an important pathway for the spread of Cortaderia species in South Africa. • Despite the legislation being updated, the trade of Cortaderia inflorescences is done illegally and legislation should be enforced. Cortaderia species (commonly known as pampas grass), C. jubata and C. selloana, have become invasive in South Africa. The National Environmental Management: Biodiversity Act (NEM:BA) (Act 10 of 2004, updated 2020): Alien and Invasive Species regulations (A&IS regs) forbids the trade of pampas grass. The growing popularity of pampas grass inflorescences for home décor and events is a concern. This study aimed to test the seed viability of cultivated, naturalised, and traded inflorescences of Cortaderia in South Africa to determine whether the trade in inforescences might lead to viable seeds being dispersed and thereby increasing opportunities for these species to invade. Seeds were collected from inflorescences of cultivated and naturalised populations from across nine provinces, and traded inflorescences were bought from retail and informal traders. Seed viability and germination of seeds of both species, cultivated, naturalised, formal and informally traded inflorescences were assessed using: (1) triphenyl tetrazolium stain, (2) germination trials in a growth chamber (triplicate repeats, randomised placement), and (3) germination trials in potting soil in a greenhouse (randomised complete plot design). There was no significant difference between C. selloana (71.67 %, 68.89 %, 79.15 %) and C. jubata ’s (57.00 %, 52.62 %, 62.54 %) ( P = 0.277, P = 1.00, P = 0.362) seed viability and germination across triphenyl tetrazolium stain, growth chamber and greenhouse trials, respectively. Seed viability of cultivated and naturalised Cortaderia inflorescences differed significantly to formally traded inflorescences ( P = 0.022 and P = 0.018, respectively). Naturalised populations of both Cortaderia species had a significantly higher germination (65.69 %, 74.35 %) than the formally (25.00 %, 5.33 %) and informally traded populations (28.75 %, 35.56 %) under growth chamber and greenhouse experiments, respectively. These results suggest that traded inflorescences of Cortaderia might often not be sterile, as evident from the observed seed viability and germination success of seeds from traded inflorescences and thus promote invasion and further spread of Cortaderia species. Effective management of Cortaderia species must focus on enforcing legislation to stop this pathway of spread, and reduce new seed recruitment, particularly during their short flowering season, and also explore more options to effectively treat inflorescences.