Management of the invasive cord grass, Spartina alterniflora, in the Groot Brak Estuary: challenges in a complex estuarine system

Management of the invasive cord grass, Spartina alterniflora, in the Groot Brak Estuary: challenges in a complex estuarine system         
Ernita van Wyk1, Virgil Jacobs1, Janine Adams2 and Taryn Riddin2
1Invasive Species Programme, South African National Biodiversity Institute, Kirstenbosch Research Centre, Claremont 7735, South Africa
2Department of Botany, Nelson Mandela Metropolitan University, PO Box 7700, Port Elizabeth 6031, South Africa

Spartina alterniflora is an invasive cord grass native to the Atlantic and Gulf Coasts of North America. The species is adapted to living in salt marshes and estuaries and was found growing in the Groot Brak estuary between Mossel Bay and George in 2004. Research shows that, if left unmanaged, the population in the Groot Brak estuary can spread at a rate 0.162 ha per year and that it can cover 41% of the total vlei area. Currently, the plant occupies 0.9 ha of the intertidal marsh. Experiences from other management attempts, particularly from Washington State, suggest that small populations of this species can be eradicated. An additional advantage for management of the species in the Groot Brak and beyond is that this population does not produce viable seed. Despite these points favouring possible success, management of the species in a seasonally open-closed estuary is challenging. Mechanical removal encourages spread of the plant and therefore herbicides must be used. The use of a glyphosate-based herbicide (Kilomax) was approved in 2010, but the application of the herbicide has been challenged by various factors: financial constraints and the transition of operational responsibility from one agency to another, administrative delays in putting contractors in-field and a  relatively short treatment season influenced by  whether the  estuary mouth is open or closed, such that when the mouth is closed, the plants are submerged for many months and inaccessible for treatment. Treatment must also be confined to low tides, low wind speeds and dry conditions, i.e. no rain. Various treatments are possible and are being tested within the constraints imposed by the estuarine system. These management options and a system to monitor the response of the Spartina population to the treatments are described.