Olive hymenachne

Olive hymenachne is a declared Class B and Class C weed and a Weed of National Significance.

Go to the Weeds of National Significance website for more information.

Another name for this plant is Hymenachne amplexicaulis.

Olive hymenachne - infestation

Identification

You should use this as a guide. There may be other plants or weeds that look similar.

If you are unsure, contact the Weed Management Branch.

Habit

These features describe the habit of this plant:

  • perennial, robust grass
  • up to 2.5m tall
  • can grow above or below water with its roots in the ground.

Olive hymenachne - habit

Stems and branches

These features can identify the stems and branches:

  • stems float but are not hollow
  • contain white pith
  • can produce runners along the ground that root at the nodes.

Olive hymenachne - stems and branches

Leaves

These features can identify the leaves:

  • 10 to 45cm long
  • leaf base may be up to 3cm wide
  • leaf base may be covered with long hairs
  • upper part of the leaf is narrower and without hairs
  • leaf blade is heart-shaped at its base where it clasps around the stem - this is a key characteristic of this species.

Olive hymenachne - leaves

Flowers

These features can identify the flowers:

  • occur as a cylindrical cluster at the end of a spike
  • occasionally branched
  • 20 to 40cm long
  • made up of numerous spikelets that are short stalked
  • 0.3 to 0.5cm long and broadest below the middle (lance-shaped).

Olive hymenachne - flower

Fruit and seeds

These features can identify the fruit and seeds:

  • produces large numbers of viable seeds
  • one study reports 98% viability of seed
  • seeds are very small and difficult to distinguish individually.

Olive hymenachne - fruit and seeds

Impact

Olive hymenachne is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts. It invades permanent water bodies and seasonally inundated wetlands.

Olive hymenachne can have all of the following impacts:

  • blocks waterways, potentially causing flooding and threatening drinking water
  • threatens fish habitat and nursery areas
  • forms dense stands that reduce plant diversity
  • reduces available habitat for native animals
  • can affect water quality
  • has the potential to severely detract from the high conservation and tourism value of natural wetland systems (eg Kakadu National Park)
  • reduces open water for water foul
  • rapidly replaces wild rice and other native species used by magpie geese.

Habitat and distribution

Olive hymenachne was originally introduced from South America to provide pasture in ponds for cattle in central Queensland. It has since demonstrated invasive characteristics, by forming dense monocultures in natural wetlands and on agricultural (predominantly sugar cane) land.

It was planted in the tropical wetlands of northern Queensland and the Northern Territory, and is now found mainly in low-lying areas along the edges of permanent water. It can withstand prolonged flooding (40 weeks) by growing above floodwaters.

It is currently well established across areas of pastoral land located on the floodplains of the Top End, some of which were deliberate planting where it has been used in mimosa control programs as a competitive cover crop.

Spread prevention

Olive hymenachne is a prolific seeder. Seeds are spread by water, including flooding events, and when transported in mud by animals and birds. It can also spread vegetatively from small plant parts.

You can prevent the spread of olive hymenachne by doing all of the following:

  • contact the Weed Management Branch for advice on controlling large infestations
  • map infestations to help develop a management plan
  • control minor infestations, isolated outbreaks or seedlings first
  • designate wash down areas and actively work to prevent contamination of clean areas
  • monitor areas that you have treated and watch for re-infestations.

Olive hymenachne - spread

Control

Heavy grazing in the dry season can decrease seed production. Mechanical or physical removal is ineffective due to highly effective vegetative reproduction from small fragments. 

The use of heavy earth moving machinery to remove hymenachne from drains has met with some success in north Queensland. Aim to reduce plant bulk prior to wet season flooding and drown it. For large infestations contact the Weed Management Branch.

Chemical control

The best time to treat olive hymenachne is from April to May, and then from November to December. Below is a list of treatment methods that can be used.

Chemical and concentration Rate Situation, method and notes
Glyphosate 360 g/L
Various trade names and formulations
10 mL / 1 L Seedling or adult (individuals or infestation):
Foliar spray - apply when actively growing

Last updated: 27 June 2017