PROBLEM PLANTS on WATERWAYS of the CANALS & RIVER TRUST - INFO
The Canals and River Trust is a voluntary organization that is charged with looking after the Rivers and Canals and which has taken over British Waterways, which no longer exists.
The plants listed in this category are all becoming, or have already become, an environmental problem or navigational nuisance on canals (and some slow-flowing rivers) owned by the Canals & River Trust (CRT) (formerly British Waterways Board (BWB) / British Waterways (BW)), usually by out-competing neighbouring plants, or by spreading prolifically. There are other aquatic plants that so entangle with the propeller of a narrow boat as to impede further progress without performing a weed-hatch clearance of the propeller every hundred yards or so.
Canals and other navigational waterways present special conditions for the over-growth of some plants. The water is usually fresh and very still, or has a very low flow rate. It is aerated (oxygenated) every so often by the passage of boats. The water has both shallow edges and deeper middles, able to support a variety of aquatic plants whether they like shallow or deeper waters. The water is usually shallow enough to become warm throughout during hot summers. The canal never dries out, it is always (usually) kept topped up automatically. Adjacent arable and cattle fields may increase the nutrients (phosphates and/or nitrates) and organic matter in the water. These conditions and factors lead to accelerated or explosive growth of certain plants, some of which are from foreign climes; throw-outs from the water-features in urban gardens. There is a strong case for banning the sale of any such non-indigenous invasive water plant in garden centres and nurseries, for instance Water Fern and Parrot's Feather. The invaders are not confined to plants, the toxic blue-green algae is a frequent coloniser of still or slow-flowing waters in extended hot dry summers.
The following are currently major problems on canals: Pennywort (Floating), Parrot's Feather, Pigmyweed (New-Zealand), Water Fern, (Floating/Creeping) Water Primrose (Ludwigia Hexapetala), Curly Waterweed, Spiked Water-Milfoil and perhaps the colourful carpets of Fringed Water-Lily (although some may see these as beautiful) and Water-Soldier (the latter may be a problem only on the watered sections of shallow and totally un-navigable canals - as soon as it becomes navigable and boats start churning it up, it may soon be gone).
The plants mentioned here usually entirely cover the surface of the water to the exclusion of both oxygen and the growth of other aquatic plants. It is these properties which make some aquatic plants on canals objectionable, whereas other aquatic plants are welcomed (in moderation). For instance, bulrushes (reedmaces) are not generally invasive on canals, preferring instead the shallower edges, as do Flowering-Rush, Arrowhead, Yellow Flag, the Bur-reeds, Purple Loosestrife, and the Water-plantains.
And then there are others that so entangle with the propeller of a narrow boat as to impede further progress without performing a weed-hatch clearance of the propeller every hundred yards or so.