Weed of the Month July 2023: Water lettuce

Water lettuce - Pistia stratiotes is a floating aquatic plant native to tropical regions, which grows and spreads incredibly quickly when introduced to dams, wetlands and rivers in our region.

Like its weedy friends Water hyacinth, Salvinia and Frogbit, Water lettuce is capable of completely smothering farm dams within a few weeks during warm weather. This dense growth leads to a reduction in water quality and dissolved oxygen, which degrades habitat for other life in the dam and encourages mosquitos. The weight and density of these infestations also blocks irrigation pipes, damages infrastructure and blocks access for people and livestock.

Long feathery roots

Water lettuce plants are a rosette of wedge-shaped pale green leaves looking like an open head of lettuce floating on the water. One plant can be up to 30cm across. The leaves are spongy with long ribs and a velvety surface. The feathery roots float in the water below the plant up to 60cm. Flowers, fruit and seeds are inconspicuous and hidden within the centre of the plant.

Spreads in floods

Water lettuce may be present in ornamental ponds or large aquariums. It may be spread deliberately or via floods to new waterways and dams where it can rapidly overtake. It produces daughter plants continuously, and can also reproduce from seed.  The plants start flowering and seeding after they have four to five leaves.

Water lettuce on pond in Sancrox Road
Eradication required

Landowners must eradicate Water lettuce if it is present. Early intervention to physically remove plants from the water is most effective, and herbicides can also be applied to help control infestations. The use of an aquatic weed harvesting machine can be highly effective on large infestations, whilst a water lettuce weevil is also available for long term biological control. Water lettuce must not be sold or traded in this region. If you think you have Water lettuce or other aquatic weeds, contact Council’s Biosecurity Officer for assistance and check NSW WeedWise.

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