Ferny Azolla
Water Velvet
Salviniaceae - Azolla pinnata

What does it look like?

Ferny azolla is a tiny free-floating aquatic fern.  It forms red coloured mats (green in shaded areas) on top of the water surface, with its roots hanging into the water.  Plants are 25 x 20mm and triangular or 5 angled in outline with regular branches.  The roots are up to 5cm long and have fine lateral rootlets.

Azolla is common on ponds, lakes, dams, drains and swampy areas and only thrives in areas protected from the wind.  Ferny azolla spreads from plant fragments and it can also reproduce by spores.  It spreads downstream via water currents and also on machinery, equipment and waterfowl.

Why is it a problem?

Azolla can form dense mats which can completely cover small sheltered areas of water.  It decreases surface water mixing, lowers oxygen levels, causes stagnation, can block water intakes, pumps and hinder stock access to drinking supplies.

Often the presence of Azolla is a symptom of a greater problem of high nutrient levels in still or slow flowing water.  Azolla absorbs nutirents from the water and grows rapidly in water with excessive nutrients.  Run off from orchards, gardens and animal grazing areas can fuel spectacular Azolla growth, especially in warm weather when water evaporation increases nutrient concentrations.  Stock access to dams and waterways can make the problem worse.

Control Methods

Ferny Azolla is very hard to eradicate as it re-establishes quickly and control is likely to need ongoing treatment.  Long term control of Azolla requires changing the conditions that favour it, particularly reducing water nutrient levels.  While the water conditions continue to favour Azolla growth it will almost certainly appear year after year.  It is only by changing these conditions and taking an intergrated approach to the management of this plant, that Azolla can be effectively managed.  This can be done by excluding livestock from dams, establishing deep rooted perennial vegetation on the up-slope from dams and reducing and diverting run off.

Control options include:

  • Increase shading on sides of narrow waterways
  • In dams and ponds, raising spring-summer water levels exposes the water surface to wind to help prevent it becoming established.
  • Rake or net weed onto dry land.  This is easier to carry out when windy to assist the drift of Azolla.  A floating boom will concentrate plants into one area.  Once removed from the water, Azolla makes excellent fertiliser or mulch for your garden as it is high in nitrogen.  It can also be fed to chickens.
  • Azolla can be sprayed in spring with glyphosate (use a glyphosate formulation approved for use over water).

The use of herbicides should be carefully considered as undesirable off-target damage and the consequences of a sudden accumulation of rotting plant material can be more detrimental than the living Azolla.  It may be more effective to rake or net out as much of the Azolla as possible, then spot spray small amounts of regrowth, rather than attempting to spray the whole pond.

 

CAUTION: When using any herbicide or pesticide, PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.

More Information

Spraying over water may require resource consent as many herbicides can have devastating effects on water-life including plants, fish and insects.