Introduction | Impacts | Classification | Control

Introduction

Olive Hymenachne is a semi-aquatic grass that threatens wetland biodiversity values across northern Australia. It was introduced from South America as fodder in ponded pastures and was planted and promoted in tropical wetlands of Queensland and the Northern Territory. Olive Hymenachne has since escaped from cultivation and is now a serious environmental and crop weed. Olive Hymenachne is a prolific seeder and can also regenerate from broken stems or roots. Seeds and broken plant parts are readily spread by water, livestock and birds, making it a highly invasive weed of wet or seasonally flooded areas.

There is a native Hymenachne species that occurs in the Northern Territory which is also an aquatic or semi-aquatic perennial grass with thick, trailing stems. Olive Hymenachne is very similar in appearance to native Hymenachne but is readily distinguished by its broader leaves and stem-clasping leaf-bases.

Impacts

Olive Hymenachne invades permanent water bodies and seasonally inundated wetlands where it has the potential to smother native vegetation by forming dense monocultures. Native wetland plants are out-competed and displaced and wildlife habitat is degraded as weed biomass creates a physical barrier for aquatic and semi- aquatic animals.

By blocking waterways, Olive Hymenachne potentially causes flooding and threatens drinking water supplies and access to bush tucker resources for Aboriginal people. As well as impeding drainage and agricultural irrigation channels, fish habitat and nursery areas are also at risk if choked by Olive Hymenachne.

Olive Hymenachne has the potential to impact significant areas of tropical northern Australia, including national parks, water reservoirs and agricultural areas such as sugar cane plantations.

Classification

Olive Hymenachne is listed as a Weed of National Significance and is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts. Olive Hymenachne is classed as a weed in the Northern Territory, Western Australia and Queensland. The Northern Territory Parks and Conservation Masterplan lists Olive Hymenachne as a significant environmental weed in nine bioregions in the Top End. It is listed as a high impact weed in the Field Guide to Assessing Australia’s Tropical Riparian Zones. Olive Hymenachne is recognised as a Key Threatening Process to biodiversity in northern Australia.

Although a declared weed, Olive Hymenachne can continue to be used for pastoralism, subject to its containment.

Control

It is important that Olive Hymenachne not be confused with Native Hymenachne to ensure that only the weed is being treated.

It is also very important that catchments that are presently free of Olive Hymenachne should be protected from infestation. This can be done by identifying and monitoring at-risk catchments, and eradicating small infestations as they occur. Although difficult to propagate, Native Hymenachne has value as a pasture species in such areas.

Physical removal of Olive Hymenachne is problematic due to the plants highly effective vegetative reproduction from small fragments. Heavy grazing in the dry season can reduce seed production. Aim to reduce plant bulk by burning or grazing prior to wet season flooding and then attempt to drown it.

Olive Hymenachne can be controlled by spraying with glyphosate. Applying a foliar spray on actively growing plants in April/May or November/December achieves the best results. It is best practice to avoid spraying plants growing in water.