A quick guide to five invasive aquatic plants

The Wildlife (Northern Ireland) Order 1985 was updated so that it is now illegal to introduce a range of aquatic plants into the wild. This includes local ponds and waterways. Below are five examples of such invasive aquatic plants that you might find in your pond. You can enlarge the image of each plant by selecting it. If you do discover an invasive species, don’t panic – just 'Be Plant Wise' and remember to dispose of the plant by composting it.

Invasive aquatic plants

New Zealand Pigmyweed, also known as Australian Swamp Stonecrop

(Crassula helmsii - also sold incorrectly as Crassula recurva, Tillaea recurva and Tillaea helmsii).

Recognisable when growing at the water’s edge by its narrow, fleshy leaves. However, it grows both in and under the water, as well as on nearby land. It can regenerate from tiny fragments and, as such, easily spreads to new areas.The plant grows on the muddy margins of ponds and it does not die back in winter.

Water Fern, also known as Fairy Fern

(Azolla filiculoides)

This plant has tiny scale-like leaves no bigger than 2.5mm – forming small plants around 2.5cm long which can cluster to form a dense mat. It is green in summer but usually turns a distinctive red in autumn and winter. This is a small, free-floating water fern that grows and thrives in canals, ponds and sheltered aquatic habitats.

Always take care when buying new plants for your pond and make sure that water fern isn’t already present on the plant.

Floating Pennywort

(Hydrocotyle ranunculoides – may also be sold as water pennywort or simply pennywort).

Identified by its shiny, kidney-shaped leaves with crinkled edges. This weed was first brought into Northern Ireland as a plant for tropical aquariums and ponds. It has since escaped into the wild.

Curly Leaved Waterweed

(Lagarosiphon major – may be sold incorrectly as Bunched Elodea Crispa Oxygenating Plants).

Identified by a long stem that is brittle and easily broken (aiding dispersal) and its leaves that are strongly recurved and are arranged in whorls or in a spiral arrangement. Surprisingly, most active growth occurs during the winter.

Parrot’s Feather

(Myriophyllum aquaticum - may also be sold as Myriophyllum brasiliense, Myriophyllum proserpinacoides, Brazilian water-milfoil, or simply as ‘oxygenator’).

It has bright green leaves (sometimes with a blue-grey sheen) that have a characteristic feathery appearance. It is a perennial plant that grows submerged but it also produces emergent, feathery shoots.

Further details of other invasive plants and the ‘Be Plant Wise Campaign’ can be found at the Invasive Species Ireland: website: