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Myrtle-leaf milkwort (Polygala myrtifolia)

Myrtle-leaf Milkwort is an invasive evergreen shrub that can form dense stands and dominate the shrub layer. It reduces native plant diversity and needs follow-up control because seeds can germinate over several years.

Key facts

Type: Weed, shrub.

Native to: South Africa, from the Western Cape to KwaZulu-Natal.

Established in: NSW, ACT, VIC, TAS, SA, WA.

Other names: Butterfly Bush, Parrot Bush, Bellarine Pea, Sweet-pea Shrub.

Best time to act: Look for flowers mainly from August to December and seedlings germinating in May to June; fruits mostly ripen from November to March, and follow-up control is needed over several years.

Identification

  • An erect or spreading evergreen shrub that can grow to about 3 metres tall.
  • Its hairless leaves are oval to broadly elliptic or obovate, with smooth edges, and are crowded alternately along the stems.
  • Clusters of white, pink or mauve pea-like flowers grow at the tips of younger stems, mainly from August to December.
  • The flattened, notched capsules turn brown and straw-coloured when ripe and contain two hairy black seeds.
  • Look-alike: Native peas such as Templetonia, Podolobium, Pultenaea and Mirbelia can look similar when not in flower.
  • Look-alike: Austral Indigo (Indigofera australis) has similar-coloured, similarly shaped flowers, but its leaves have several leaflets attached to a central stalk.

Where it grows

In Australia it occurs mainly in coastal and near-coastal areas, especially in sandy soils and coastal shrublands, across NSW, SA, TAS, VIC and WA.

Why it matters

  • Dense stands shade other plants, reduce botanical diversity and can become monocultures that exclude other desirable plants.
  • It can invade intact vegetation and eventually dominate the shrub layer.
  • Seedlings can establish in disturbed, exposed sites and in low light beneath established shrublands; mass germination can follow fire or mechanical vegetation removal.
  • It is not known as a weed of agriculture or other primary production.

How it spreads

  • It spreads by seed, and seeds can remain viable in the soil for two to three years; reports indicate viability may last 10 years or more.
  • Seeds are not suited to long-distance dispersal by wind or water, but may travel short distances by wind or animals and potentially longer distances by birds.
  • Seeds can also be moved in contaminated soil and dumped garden waste; ants or other invertebrates may move them short distances.

Control options

  • Young plants can be hand-pulled or shallow-dug, particularly in sandy soils; this is unlikely to be practical in heavy infestations.
  • Use registered herbicides or other approved methods; foliar spraying is reported as effective on seedlings and mature plants, and cut-and-swab treatment is effective but labour-intensive.
  • If using fire, kill some or most plants before burning, as green plants do not burn well; no biological control agent is available in Australia.

Hygiene and washdown

  • Avoid moving soil or garden refuse that may contain seeds, as both can spread Myrtle-leaf Milkwort.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources