east Indian hygrophila

Hygrophila polysperma
terrestrial habit (Photo: Sheldon Navie)
infestation in a waterway (Photo: Sheldon Navie)
infestation on muddy soil (Photo: Sheldon Navie)
submerged habit (Photo: Sheldon Navie)
stems with roots at their joints (Photo: Sheldon Navie)
emergent leaves (Photo: Sheldon Navie)
close-up of younger emergent leaves (Photo: Sheldon Navie)
Aquatic
Opposite
Simple
White
Purple
Blue
Green

A submerged or emergent herbaceous aquatic plant. Its paired leaves are pale green or reddish in colour when submerged and bright green on emergent stems. Its stalkless tubular flowers borne in the leaf forks on the emergent stems. These tiny white, bluish or purplish flowers (5-9 mm long) are surrounded by two small hairy bracts. Its small narrow capsules (6-9 mm long) split lengthwise to release their minute seeds.

Common names 
Also known as: east Indian hygrophila, dwarf hygrophila, east Indian swampweed, green hygro, hygro, Indian swamp weed, Indian water star, Miramar weed,
Family 
Acanthaceae
Deciduous 
No
Flowering time 
Autmn - Winter
Native/Exotic 
Exotic
Origin 
Native to the Indian Sub-continent and south-eastern Asia.
Notifiable 
No
Council declaration 
SIL – Special Investigation List
Known distribution 

Currently only naturalised at a few locations in south-eastern Queensland and north-eastern New South Wales. Also naturalised overseas in eastern USA (i.e. Florida, Texas and Virginia).

Habitat 

A potential weed of slow-moving waterways and the margins of other waterbodies (i.e. dams, lakes and ponds).

Habit 

A small, much branched, herbaceous plant with creeping (i.e. procumbent) to upright (i.e. erect). It grows underwater (i.e. submerged aquatic) or with stems emerging above the water surface (i.e. emergent). Underwater stems may grow up to 2 m long, while emergent stems are usually less than 30 cm tall.

Impact and control methods 

"East Indian hygrophila (Hygrophila polysperma) is regarded as an emerging environmental weed in Queensland and New South Wales. This species was first collected in Australia in August 2005 in the Caboolture River north of Brisbane. It was growing along the riverbank and in the water, both as a submerged aquatic and terrestrial plant. East Indian hygrophila (Hygrophila polysperma) is thought to have escaped cultivation as an aquarium plant and has since been found along the Tweed River and Clarence River in coastal northern New South Wales and along Enoggera Creek in suburban Brisbane.

This species has the potential to become a serious weed of freshwater lakes, ponds and dams in eastern Australia. It prefers flowing water in warmer environments, but may also be found in slow moving waters and lakes. East Indian hygrophila (Hygrophila polysperma) is fast growing invasive plant that out-competes native aquatic plants. It can occupy the entire water column and also creates problems as an emergent plant along the margins of waterbodies."

Stem and leaves 

"The stems regularly produce roots at the joints (i.e. nodes) where they come into contact with the soil underwater. The younger emergent stems are square in cross-section (ie.. quadrangular) and finely hairy (i.e. pubescent) near their tips.

The paired leaves are almost stalkless (i.e. sub-sessile) and either hairless (i.e. glabrous) or sparsely hairy (i.e. puberulous). The underwater leaves are narrowly oval (i.e. elliptic) to oblong in shape and usually 4-8 cm long. These leaves are usually pale green, yellowish or reddish in colour. However, the leaves on emergent stems are smaller (1-3.5 cm long and 4-10 mm wide) and darker green in colour. All leaves have entire margins and pointed or somewhat rounded tips (i.e. acute or sub-obtuse apices)."

Flowers and fruits 

"The white, pale bluish or purple flowers are arranged in the forks (i.e. axils) of the emergent stems. These small tubular flowers (5-9 mm long) are stalkless (i.e. sessile) and subtended by two hairy bracts (5-15mm long). Each flower has five small narrow green sepals (about 4 mm long) that are fused together at the base into a tube (i.e. calyx tube). Their five petals are also fused together into a tube (i.e. corolla tube) about 3 mm long and separate into two lobes at the tip (i.e. they are two-lipped or bilabiate). The upper lobe has two smaller lobes at the tip, while the lower lobe has three smaller lobes. Each flower has two fully-formed stamens and two partially-formed stamens (i.e. staminodes), and an ovary topped with a slender (i.e. filiform) style about 5 mm long. Flowering occurs only during autumn and early winter.

The fruit is a narrowly oblong capsule (6-9 mm long) that is hairy towards the tip. These capsules split lengthwise when mature to release about 20-30 seeds. The minute seeds are flattened and rounded in shape (i.e. orbicular)."

Reproduction and dispersal 

"Its main method of reproduction is by stem fragments, which can easily and develop into new plants via the production of roots at their joints, but it also produces seed.

Most spread of the seeds and stem fragments is by water movement or with the assistance of watercraft. The sticky seeds may also be spread by birds and other animals or by vehicles. Stem fragments have also be dispersed to new areas in dumped aquarium waste."

Similar species 

East Indian hygrophila (Hygrophila polysperma) may be confused with hygrophila (Hygrophila costata ) and a similar native species (Hygrophila angustifolia). These species can be distinguished by the following differences:

East Indian hygrophila (Hygrophila polysperma) is a relatively small plant (usually growing less than 30 cm tall) and has stems that are not hollow. Its leaves relatively small (1.5-8 cm long) and have entire margins. Its small flowers are tubular in shape (5-9 mm long), stalkless (i.e. sessile), and borne in small clusters in the leaf forks (i.e. axils). These flowers are either white, bluish or purple in colour and the mature fruiting capsules are relatively small (6-9 mm long).

Hygrophila (Hygrophila costata ) is a relatively large plant (sometimes growing 1-2 m tall) and has stems that are not hollow. Its leaves are relatively large (up to 18 cm long and 5 cm wide) and have entire margins. Its flowers are tubular in shape (5.5-10 mm long), stalkless (i.e. sessile), and borne in small clusters in the leaf forks (i.e. axils). These flowers are usually whitish in colour and the mature fruiting capsules are relatively small (7-13 mm long).

Hygrophila angustifolia is a relatively small plant (usually growing 15-45 cm tall) and has stems that are not hollow. Its leaves are very narrow (usually 2.5-16 cm long and only 2-8 mm wide) and have entire margins. Its flowers are tubular in shape (9-20 mm long), stalkless (i.e. sessile), and borne in small clusters in the leaf forks (i.e. axils). These flowers are either white with darker markings, violet, blue or mauve in colour and the mature fruiting capsules are relatively large (10-18 mm long).
It may also be confused with Australian water horehound (Lycopus australis). However, this native species has leaves with coarsely toothed (i.e. serrate) margins."