Canadian pondweed.  Eloda canadensis.

 Image courtesy of Christain Fischer  CC BY-SA 3.0 License.ElodeaCanadensis.jpg


The Canadian Pondweed,as the name suggests , is a native to North America. However, it has been introduced to many regions of the world and in particular Continental Europe where it is now established throughout  the continent. 

In the Uk it was introduced in the 1800's and by 1836, it spread with alarming rapidity choking up canals ponds and ditches all over Britain. Suddenly in 1860's the plants stopped their all conquering expansion. In the UK, they are placed on Schedule 9 of the Wildlife and Countryside Act 1981 {as amended}. This covers the species that are alien but have become established in the wild. Schedule 9 of the Act makes it illegal to plant or otherwise cause to grow any plant listed on the Schedule.

Components of Canadian  Pondweed 

Public domain courtesy of Wikicommons.Elodea canadensis nf.jpg


The Canadian pondweed, sometimes referred to as the Canadian waterweed, begins its life with a seedling stem with roots growing in mud at the bottom of the water.  Further smaller roots become established along the stem, which either hang free or root at intervals along the stem. It grows indefinately to reach a length of up to 3 m or even more.

The foliage produced is translucent ,oblong, 6-17 mm long and about 1.4 mm broad. They are usally borne in whorls of three ,rarely these may be produced in twos or even fours, around the stem. it lives entirely underwater with the exception of the white or pale  purple flowers which are attached to the plant by delicate stalks and float on the surface of the water.

The flowers are of one type only on the individual plants ie, all male or all female, the botanical name for this is 'dioecious'.  The flowers have three,small, white petals. The male flowers have 4.5 to 5 mm petals and nine stamens. The female flowers have 2-3 mm  petals and three fused carpels. The fruit is an ovoid capsule about 6 mm long and the seeds ripen under water.

This plant requires summer water temperatures of  10-25 degrees Celsius and bright to moderate light.

Flowering plant.

Image courtesy of Christian Fischer  CC BY-SA 3.0 license.

Relatives and look-alike plants.

There are four that look-alike, and frequently pass for, Canadian pondweed in Britain. They are Nutall's waterweed, Elodia nuttallii, Elodea canadensis {our subject under review} , Curly Waterweed,Lagarosiphon major, and the large -flowered waterweeed, Egeria densa. All of them are alien and invasive species which originated as aquarium or pond plants which have escaped into the countryside. Three of these four plants have now become widely established across Britain.

If they have become established in a pond is it wise to remove them?. Canadian pondweed and their allies have a tendency to survive in ponds that are polluted and although they take over from native plants, in some ponds  especially those that are polluted,  the native plants would not grow anyway. once these plants are in a pond it is difficult to clear them.

The arguments against clearing them  are as follows. The Feshwater Habitat Trust----

1- When aquatic plants are completely removed from a pond the risk that the pond flips over into a pea-soup state is considerable.

2- They can be difficult to move by mechanical means, because they can grow back very quickly and often thicker than they previously were.

3- It can offer habitat for wildlife, because this habitat is better than no habitat at all. 



Elodia nutellallii. This is a very similar plant to Canadian pondweed.

Courtesy of Christain Fischer.  CC BY-SA 3.0 License.ElodeaNuttallii2.jpg

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