Post-fire observations of Siam weed seed viability

Abstract

Siam weed (Chromolaena odorata) has been found in the wet and dry tropics in Queensland since 1994 and the Northern Territory since 2019. Many dispersal mechanisms allow this highly invasive large shrub to build up large infestations in remote areas, where broadscale management options include fire. Observations were collected on the viability of seed (achenes) after controlled burns (in June, August and October) and small research plot fires (June, July and August) of different years at several locations. Chromolaena odorata was scorched but not consumed by a fire in early October (2008) on the Pinnacles near Townsville, and the same percentage of viable seeds were present on scorched and unscorched plants after the fire. Highly viable seed was collected from a plant in the wet tropics near Mossman, which was scorched by a burn in mid-August (2013). No developed seeds were collected from scorched stems from a fire on Mount Stuart near Townsville in early June (2022). Small plots of six to eight C. odorata plants were raised at Charters Towers and burnt in 2022 and 2023. No viable seed was recovered from plants scorched by a fire in late June. Viable seeds were recovered from less than half of the plants burnt in plots in mid-August (2022). Another plot was burnt in mid-July (2023), and viable seeds were present before and after the fire on three of the six plants. The observations were used to build a pattern of seed viability after fires between June and early October. Two fires in June prevented the development of viable seed. Early ‘cool season’ fires prevent viable C. odorata seed forming. However, viable seed can be present on burnt and unburnt plants after mid-July. Viable seed may be present even if plants are scorched by fires from August onwards.