Aquarists’ and water gardening specialists’ chuck-outs have long been a problem in some of our ponds, canals and waterways. Indeed, not only in Britain but widespread in many parts of the world..
The earliest to have been introduced to this country is Canadian Pondweed or Water Thyme (Elodea sp.) It has been used since the early 1800’s as a useful oxygenating plant in garden ponds and ornamental lakes. Native to North America it has happily naturalized here and elsewhere in Europe. It will root into the mud from small fragments and will quite rapidly form big clumps, taking over the available space.
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| Canadian Pondweed. |
Closely related is Lagarosiphon major....(Curled Pondweed)
Introduced from South Africa, this is a more robust and even more vigorous cousin, initially popular also as an oxygenator in still or slow-moving water, it was first reported in the wild in Britain in 1944. Its habits of choking waterways, impeding the flow and smothering less vigorous native plants and of propagating from fragments make it difficult to control have led to it being declared a Noxious Weed in New Zealand and the States.
Swamp Stonecrop or
New Zealand Pygmyweed (Crassula helmsii)
was New Zealand’s gift in exchange! It has been introduced through the
horticultural trade to the rest of the world and is now a pervasive aquatic and
semi-terrestrial pest, forming a dense mat of small closely crowded bright
green leaves and out-competing our natives in ponds where it had become rapidly
established since it was first recorded in the New Forest in 1976. It survives as a terrestrial form on the damp margins of ponds if the water level recedes.
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| New Zealand Pygmyweed or Swamp Stonecrop. |
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| It will form a choking mat and will survive periods of drying out of the water. |
Parrots Feather (Myriophyllum sp.) native to the Amazon has now spread throughout the world. In summer the submerged plant will send up a 12” stem with whorls of attractive feathery grey-green leaves. It will be cut back by frost but grows rapidly when conditions are warmer and like these other species will clog slow-moving waterways, making dense mats which impede fishermen, swimmers and boats.
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| Parrot's Feather. |
Floating Pennywort(Hydrocotyle rapunculoides) is another very aggressive alien from North and South America introduced in the 1980s It forms dense mats of floating leaves and has spread rapidly in the wild from the SE of England in ponds, lakes and slow-moving waterways with considerable impact on the various users of these waters.
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| Floating Pennywort. |
Water Fern (Azolla sp) has a long history of use in China. For 1000 years or more it has been recognized as a valuable source of biofertilizer in the rice paddies. It has a symbiotic relationship with a Cyanobacterium which fixes atmospheric oxygen. It is also capable of removing heavy metals from effluents and its growth is regulated by the availability of phosphates in the water so agricultural run-off can result in blooms of this tiny floating fern and it can double its biomass in a few days. It has been spread throughout the world as an attractive and novel little floating plant but it cannot withstand low temperatures.
Water Fern or Azolla.We first came across it in the seasonal freshwater pools in inland New South Wales where heat and receding water levels were stressing it and the anthocyanins in its tissues were turning it bright red.
The geni is out of the bottle too, with the Evening Primrose relative from America, the Water Primrose. Spreading aggressively through Northern Europe we have seen this plant taking over slow-flowing waterways and wetlands in NW France. It’s no doubt only a matter of time before it’s seen here too.
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| Water Primrose. The Flower is like a smaller version of Evening Primrose. |
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| Its dense growth smothers less robust native water plants. |
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| A backwater of the River Vilaine in Brittany is completely choked with Water Primrose. |
The last five of these plant species were banned from sale in the UK from 2014 because of their invasive habits. They are extremely difficult to control because of their vigorous growth, ease of propagation, rooting as they will from the smallest fragments, and impossible to control with herbicides because of contaminating the rest of the fauna and flora of the wetland environment. The dense mats of growth impact on fishing, leisure boating, swimming; they out-compete the native flora, and can prevent water creatures from moving freely or coming up to the surface to breathe.
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