Beware: Alligator weed can invade our water resources

l by Dr. Lalith Gunasekera

(November 08, Melbourne, Sri Lanka Guardian) Alligator weed is a native plant of of Brazil and Argentina (from Amazon River basin) and originated in aquatic environment in the great Amazon river basin. It was first recorded in Australia in New Castle in 1946 where it was found growing on ballast dumped by wartime shipping.

Alligator weed is declared a noxious weed in all Australian States and territories. Alligator weed is highly suited to Australian climates and has become naturalised where it grows in Australia. 

I have been working on alligator weed in Australia since 1995 and first time located this plant growing in people’s gardens in all Australian states and territories. Alligator weed is amphibious and can grow in a variety of habitats ranging from damp soil along shorelines of rivers, canals, wetlands, ponds, lakes, streams and ditches but usually in fresh to slightly brackish water but can withstand some salinity. This amazing plant can be grown in wet soil or entirely terrestrial in dry lands without any water for several months.

This weed is now found in New Zealand, India, Indonesia, Malaysia, Thailand, China, USA, Sri Lanka. It has been found in home gardens growing as a vegetable by the Sri Lankan community in all Australian states and territories by mistake since 1960’s till I discovered the problem in 1995 (16 years back).

Alligator weed in a backyard in Australia

The health risk of this plant has not properly studied so far. Main problem with the plant is it’s ability of invading water resources like salvinia and water hyacinth. We do not want another invasive plant chocking in our water resources.

Alligator weed invade water resources

Alligator weed may be came to Sri Lanka from Australia via humans and planted as a vegetable by mistake.

• Alligator weed can exist in two forms (a) emergent aquatic perennial (b) terrestrial rhizomatous

• In aquatic situation, stems are creeping or floating and rooting at the nodes. Stems are hollow, branched, light green in colour with faint darker green parallel lines, extending from one node to the base of the next.

• When stressed, they may become pink to reddish.

• Terrestrial stems are solid rather than distinctly hollow. They produce two types of rhizomes. (a) a purplish horizontal rhizome that resembles stems with very short internodes. (b) fleshy white root like rhizome that looks similar to the taproot which may extend to a depth of 1 -2 m in soil.

• Aquatic plants only develop roots from nodes on the stem. Each node has two shoot buds at the base of the leaves. It has shorter finer roots than terrestrial plants.

• Leaves are simple, spear shaped dark green and have a distinct midrib. They arranged in opposite pairs along the stem. Leaves have short leaf stalk (1 cm), slightly hairy near the base, usually thick and fleshy. 

• Leaf blades mostly 4-11 cm long, 1-3 cm wide.

• Alligator weed has small clover like white flowers are produced on short stalks (4-9 cm long) attached on the leaf axils near the end of the stems. They are mostly 1-2 cm in diameter.

Alligator weed flower

Mukunuwenna plant

• Fruits are flattened and do not open to release the single seed. Seeds are not viable in Australia or Sri Lanka.

• Reproduction is entirely vegetative through fragmentation of rhizomes. New shoots and roots can be produced within 3 days from a rhizome fragment with only one node.

• Thick mat of alligator weed infestations can block waterways, causing flooding and impede flow in irrigation canals. It can be a weed in wet soil of agricultural land, pastures and in rice fields. Create a favourable habitat for mosquitoes.

Identification of alligator weed is easy. 

The flower of alligator weed is larger (1-3 cm wide) and carried at the end of stalks (4-9 cm long). But mukunuwenna flowers are small (less than 5 mm) and carried in clusters at leaf joints. But you do not see flowers during vegetative growth stages where people harvest these plants for sale. The stem is the better way to identify these two species. Alligator weed has a hollow (hole inside the stem) and soft stem specially growing in aquatic conditions. Mukunuwenna has a woody stem (no hole inside the stem). Better identification pictures would be a useful tool to recognise the two species. My recent visit to Sri Lanka found that alligator weed has been invading water resources in Nuwara Elia and Colombo districts.

Don’t stop eating mukunuwenna that is very healthy vegetable. Best thing is to get identified the “Alligator weed”.