Azolla pinnata

Ferny azolla

Also known as:

Water velvet

Family: Azollaceae

Origin: Africa, Asia, Australia

A submerged plot of land covered in ferny azolla.
Forms a dense layer crowding out other plants.
Photo credit: Jack Craw

Regional Pest Management Plan (RPMP) status

  • Not a legally declared pest
  • Hauraki Gulf Controlled Area Notice pest

View more about the RPMP statuses

General description

Small, free-floating, mat-forming, aquatic fern < 3 cm long. The plant is triangular, with a straight midsection and pinnately arranged side branches. Roots are feather-like with fine rootlets. Leaves are minute, alternate, overlapping and green/red. Sporocarps are round and formed on the undersides of leaves.

What you need to know

Although ferny azolla is not a legally declared pest plant, it may still be invasive in some situations. Consider lower-risk alternatives for your garden, such as native plants.

Habitats

Lakes, ponds, wetlands, ditches, dams, slow-moving waterways.

Dispersal

Spores produced. Vegetative spread from fragmentation. Spores and fragments dispersed by water, waterfowl and wind. Human-mediated dispersal through the movement of contaminated boats, equipment and machinery.

Impact on environment

Forms dense mats, displacing native aquatic plants and reducing oxygen levels.

Control

Site Management

Increasing shading over the waterway can reduce infestation. Spraying aquatic weeds result in plants rotting in the water. This decreases dissolved oxygen in the water and in large amounts can kill aquatic life. Consider raking and only spraying regrowth in spring.

Recommended approaches

Physical control

Method: Rake onto dry land.

Plant parts requiring disposal: All parts.

Disposal options: Can be used as a fertiliser or mulch in the garden as it is high in nitrogen or can be fed to chickens.

Biocontrol

Biocontrol is currently not available for this species.

Community agrichemical control recommendations

Certified Handler/Experienced agrichemical user: Foliar spray emergent foliage with 100ml glyphosate green per 10L of water.

Caution: When using any herbicide or pesticide please read the label thoroughly to ensure that all instructions and safety requirements are followed.

Ferny azolla in a hand.
Each frond is made up of many rounded overlapping leaves each 1 or 2 millimeters long.
Close up of clusters of pinkish flowers of the ferny azolla.
The roots are hairlike and extend out into the water.
Photo credit: C. Howell