Azolla

Scientific name

Azolla Lam.

Common names

pond fern, mosquito fern, water velvet

Family

Azollaceae

Could be confused with

not likely to be confused with any other plant genus

Native distribution

pan-tropical

Species commonly cultivated

Azolla caroliniana Willd. (America)
A. filiculoides Lam. (America, Africa, Asia to New Caledonia)
A. mexicana Schlecht. & Cham. ex K. Presl. (America)
A. nilotica Mett. (Africa)
A. pinnata R. Br. (Asia to New Caledonia)

U.S. Federal Noxious Weed: Azolla pinnata

Identification: Diagnostic characters: vegetative: triangular shape of plants, feathery appearance of roots; sexual: megasporangium has 9 floats; microsporangial massulae have no glochidia, but have vacuolated trichomes.

See Azolla pinnata disseminule fact sheet.

Adventive distribution

Some species are nearly cosmopolitan.

Weed status

Azolla pinnata is considered one of the world’s most troublesome weeds and an aquatic weed on the U.S. federal noxious weed list.

Habit

small, free-floating, with tiny, crowded, bilobed leaves

Brief description

Annual or perennial water fern. Stem floating, more or less pinnately branched. Roots simple, with root hairs present or absent at nodes. Leaves in two alternate rows, overlapping and covering stem, divided into two lobes; upper lobe aerial, thickened and photosynthetic (red or green colored), lower lobe submersed and non pigmented. A thin-walled pouch forms on the upper lobe, enclosing cavities containing nitrogen fixing cyanobacteria (Anabaena azollae Strasb.). Sporocarps borne in pairs in leaf axils of older branches; megasporocarps small, containing a single megaspore; microsporocarps with many microsporangia. Dispersal by spores or stem fragments.

Natural habitat

still waters of all types of water bodies

Additional comments

The genus Azolla comprises six species of small-leaved floating ferns. Azolla species form a symbiotic relationship with the cyanobacteria Anabaena azollae, which fixes nitrogen and grows in a cavity in the dorsal lobe of the fern's leaves.