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Black alder (Alnus glutinosa)

Black alder is introduced in Australia and can invade damp ecosystems, especially wetlands and riparian zones. Its fast growth, nitrogen-fixing root nodules and ability to form dense stands can let it alter invaded sites and displace other species.

Key facts

Type: Weed, tree.

Native to: Eurasia and northern Africa.

Established in: NSW, ACT, VIC, TAS.

Other names: European alder, Common alder, Sticky alder, Alder.

Best time to act: Look for flowers before leaf emergence in early spring and inspect for cone-like fruits that open gradually over winter.

Identification

  • A fast-growing tree that can reach about 50 feet high, with a single or multi-stemmed trunk.
  • Bark is initially smooth and greyish-green, becoming speckled greyish-brown with warty horizontal stripes and shallow fissures.
  • Leaves are dark green, leathery and oval to nearly round, with serrated edges and blunt tips; young leaves are gummy.
  • Reddish-brown male catkins appear before the leaves in early spring, alongside smaller, cone-like female catkins.
  • The woody cone-like fruits ripen from green to brown and contain many small, winged nutlets.

Where it grows

In Australia it can grow in damp habitats near water, including wetlands, riparian zones and the margins of ponds and lakes.

Why it matters

  • It can form dense, single-species stands in less than 10 years, excluding other species and shading native plants.
  • Its dense root network can increase sediment accumulation and alter water flow.
  • Nitrogen fixation, leaf fall and decomposition can increase soil nitrogen, while soil phosphorus availability can also increase.

How it spreads

  • It reproduces mainly by seed and can produce prolific seed crops every three or four years, averaging 240,000 seeds per tree.
  • Cut stumps can resprout, and fallen green branches can take root in soft, swampy wetland areas.
  • It can also reproduce vegetatively through tissue culture; root suckers are rare.

Control options

  • Remove young plants physically where practical, and fell established trees where this is an appropriate method.
  • Treat cut stumps with a suitable herbicide registered for the use and site, following the product label.
  • Glyphosate stump treatment and triclopyr triethylamine foliar application are reported control options; check current Australian registration and label directions before use.
  • Inspect treated sites for resprouting and follow up as needed; contact the relevant state or territory biosecurity agency for local reporting and control advice.

Hygiene and washdown

  • Clean boots, machinery and other equipment of soil and plant material after work, and avoid carrying cones or nutlets from infested sites to clean areas.
  • 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: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • 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: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • Australian Capital Territory: Declared pest, prohibited. It is a declared pest throughout the ACT. Dealing with it, such as selling, moving or keeping it, is prohibited.

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

Sources