When plants grow too well …

Recently the UK government has announced that five varieties of pond plant are to be banned from sale next year.

Until recent years perhaps only Japanese Knotweed had been as high in public perception as a problem, but this is likely to change in the future as more and more countries get to grips with the serious damage that certain unwanted organisms can cause (e.g. Ash Dieback).

As a nation of gardeners we have imported many attractive plants over the years to brighten up our surroundings, and these have had an important role to play in making British gardens some of the most admired in the world. The vast majority of these plants have been well behaved. Many have proved to be as popular with wildlife, for shelter and food, as our local native plants.

However, a few of these introduced non-native species have been flagged up as invasive and damaging. These five pond plants were the ones that have prompted the current action:

Azolla Crassula Hydrocotyle Ludwigia Myriophyllum Image Copyright Aquapic

The five pond plants to be banned

Some outlets chose to stop selling certain of these plants back in the 1990s, and the trade association OATA worked with the government’s Be Plant Wise campaign over the past few years to highlight these problem plants and encourage responsible disposal of any unwanted plants.

The (In)Famous Five

Fairy Moss or Water Fern Azolla filiculoides (also sold as A. caroliniana). This little floating plant looks quite innocuous with its bright green fronds, turning reddish in autumn and often disappearing after hard frosts. However it spreads just as fast as our common duckweed, and can be carried on birds’ feet too.

Australian Swamp Stonecrop or New Zealand Pygmyweed Crassula helmsii (also sold as Crassula recurva and Tilloea) is a rather too versatile plant, growing anywhere from the depths of the pond to the surface and also up onto dry banks. It has proved difficult to control, and it can smother other plants.

Floating Pennywort Hydrocotyle ranunculoides is a more recent introduction from America and has made itself unwelcome in a very short time. Fragments carried on boats have also spread it throughout much of our canal system. It is quite different from our native Marsh Pennywort (H. vulgaris – a slower grower which prefers wet margins to the water, and doesn’t have the characteristic cut in the leaf of the foreign invader).

Water Primroses Ludwigia grandiflora (also known as L.uruguayensis /  Jussiaea) and L.peploides have not been too much of a problem in the UK as their invasive tendencies were realised in time, and the few escapees have been successfully controlled. However these plants have caused major problems to wetlands in France and Spain.

Parrot’s Feather Myriophyllum aquaticum (also known as M. brasiliense / M.proserpinacoides) was the most popular of the five for ponds. Although its attractive foliage would often be killed by a hard winter, it had started to spread in the milder South West of the country, and this flagged up the dangers.

All five plants have the potential to crowd out native plants if left uncontrolled, and to blanket the surface, cutting out essential light and sometimes resulting in the water becoming low in oxygen, harming fish.

Water Garden Solutions has not used any of these varieties in our own plantings, but we do find them from time to time in customers’ ponds, and can remove them if requested. If they are in a closed pond and not causing a problem they can be left well alone – especially Crassula, as pulling can spread unwanted fragments. If they are removed, they should be composted, preferably in a council green bin – the larger-scale compost heaps are more effective at breaking down plant material. However, there is some doubt as to how well Crassula will compost.

The general rule to follow is to keep all ornamental plants safely in your garden (whether invasive or not), and if you need to dispose of them, do so responsibly, and never into the wild.

Leave a Reply

Your email address will not be published. Required fields are marked *