Survival and buoyancy of Hygrophila costata stem fragments in salt, brackish and fresh water.

Abstract

Hygrophila costata is a perennial semi-aquatic plant listed as Category 3 Restricted Matter under the Biosecurity Act 2014, Queensland. It has been found growing in many creeks and rivers in Far North Queensland, where it is displacing native vegetation and colonizing/stabilizing otherwise transient sand banks. The impacts of H. costata are not yet fully understood but effects on hydrology and species composition are believed to be detrimental.
This paper summarizes two experiments investigating components of the weed’s dispersal ecology. In the first experiment, H. costata stem fragments were placed in salt, brackish, and fresh water for varying time periods over 21 days, then removed and planted in a suitable medium to observe survival and establishment. The fragments in the fresh and brackish water remained viable for 21 days and established readily when planted. The fragments in saltwater were viable for 14 days but dead by day 21.
A second study examining buoyancy of stem fragments with and without leaves concluded that fragments in freshwater remained floating for 21 days in either case. In brackish water, they floated for 21 days with leaves and 12 without. In saltwater, fragments floated for seven days with leaves and three without. Within the experimental period, free-floating fragments in fresh and brackish water produced new leaves and roots from the nodes.
These findings have implications for the spread and establishment of H. costata in instances where vegetative fragments become waterborne for extended time periods (e.g. flooding or storm events). The information may be useful in approximating dispersal patterns and distribution of the weed, especially when combined with existing modelling studies, and may influence choice or timing of control techniques.