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May 2013 - Aquatic aliens

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Dick Warner outlines the threat from invasive aquatic plants

In spring the water temperature in a garden pond, particularly in a fairly deep pond, lags behind the air temperature in the garden by several weeks. This means that underwater plants will start into growth long after the grass in the lawn. It also means that this is an ideal time of year to plant new oxygenating plants in a fish pond or to propagate from existing ones.

But every gardener knows that some land plants have to be treated with caution because they have the potential to become invasive weeds. A mistake I made with horseradish in a herb garden early in my gardening life took me over ten years to sort out. The same is true of water plants. And they not only have the potential to take over your pond, they also pose a very serious threat if they escape into waterways. A number of species have already done so and the efforts to control them have been very costly in effort and money.

One fairly easy one to recognise is the red water fern, Azolla filiculoides. It is a free-floating plant, like duckweed, only a bit larger with fronds that are a little like maidenhair fern and which usually have a red or orange tinge at the margins. It can spread and multiply with amazing speed and was a major environmental hazard on the River Barrow. But it is not tolerant of very cold winters and seems to have declined in recent years. New Zealand pygmyweed, Crassula helmsii, is a succulent that looks rather like stonecrop, in fact, an alternative common name is swamp stonecrop. It can grow under water or on damp pond margins but its four-petalled white flowers on long stalks always appear above the water surface. It also has the ability to spread with extraordinary rapidity.

Nuttall's pondweed, Elodea nuttallii, is not easy to tell from Canadian pondweed without help from a book or information online. Canadian pondweed is also an alien species but it was introduced into our waterways a long time ago, at least a century, and is not regarded as invasive. Nuttall's pondweed, on the other hand, has choked navigation channels on the Grand Canal and Barrow Navigation and caused considerable ecological damage to a number of ponds and small lakes, principally in the east of the country.

Curly-leaved waterweed, Lagarosiphon major, a native of South Africa, has escaped into Lough Corrib where there is a major programme aimed at containing its spread. It grows very densely, even in quite deep water, and has effected fish stocks and boating on the lake. It is somewhat similar to both Nuttall's pondweed and Canadian pondweed, to which it is related, though it has a characteristic dense cluster of leaves at the tip of each frond. The frond stems are hollow and rather brittle. They can easily be broken by wind, waves or boat traffic and each broken piece tends to become a viable new plant, making it very invasive.

Each of these plants was originally introduced into this country for oxygenating ponds but they have now become major problems. They should avoided, if possible, and any weeds removed from the pond should be disposed of with great care. Next year these plants will be banned in the Britain, but they are available in this country and are widespread in garden ponds and in the wild. More on these and other species at




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