Polythene sheeting
Submerged rooted plants can be controlled by the use of black polythene sheeting or fertiliser sacks, laid on the bottom to shade out vegetation. Sheeting should be opened out on the bank and then spread on the unwanted growth. Weight the corners with stones or bricks. If using sacks, make sure they are clean, otherwise the nutrient balance may be upset by adding fertiliser to the water. With non-rhizomatous species the polythene sinks to the bottom as the plants die. Leave it in place for about a month, but remove it before silt starts to accumulate. For rhizomatous species it should be left for a full growing season. The bare patch should stay relatively weed-free for at least a year.
Hydrological management
Fluctuating water levels provide a natural check on aquatic and marginal plant growth. Ponds which dry out during most summers have greater long-term stability than ponds with permanent water. This is because the growth of floating, submerged and emergent plants is controlled during the dry period, thereby holding back long-term vegetation spread, siltation and succession. Underground rhizomes normally survive in the damp mud, and regrow when the area is flooded again.
Rooted, floating-leaved plants are sensitive to rapid increases in the water level, but they are able to adapt to gradual increases by growing towards the surface. Emergent plants can be controlled by rapid, extensive changes in level leading either to drying out and freezing, or due to prevention of normal metabolic processes by flooding. To make flooding practical, this is best done in spring when the plants are still short. Alternatively, growth can be cut down and then flooded during summer. Flooding of damp mud during May and early June, on which emergent plants germinate, is another method of control.
Native fish
Some species of native fish have significant effects on aquatic vegetation, and careful management of fish populations can be used as a method of vegetation management. Carp and bream are bottom feeders and stir up the sediments, uprooting small plants. They make the water turbid, thereby reducing light penetration and suppressing the growth of submerged plants. However, overstocking can rapidly denude a pond or lake of most plant types, leaving the water dominated by unicellular algae. Roach feed on filamentous algae, and in some situations may help to reduce growth. Stocking of fish in large ponds, lakes or watercourses should only be done in consultation with EA and MAFF (or equivalents).
Grass carp
These introduced fish are effective biological control agents against many submerged and floating plants, and will also eat filamentous algae and some emergent plants when the food supply is limited. Their use is limited to enclosed waters, where they cannot escape into river systems. A MAFF (or equivalent) licence is required for their use, and the Environment Agency (or equivalent) should also be consulted.
Invertebrates
Water fleas (Daphnia spp) feed on unicellular algae and can be used in small water bodies to clear the water, provided that fish, which predate on water fleas, are absent or few in number.
Grazing
Low-intensity grazing and trampling by stock, normally cattle, can maintain open conditions at the edges of ponds and watercourses. This not only keeps vegetation in check, but helps maintain the conditions of bare mud and gravel which are favoured by many invertebrates. Where grazing occurs on surrounding land, fencing which allows limited access for stock to a pond or stream can be beneficial. Only allow access for cattle where the banks are very gentle, and completely fence off all steep-sided drainage ditches, to prevent cattle becoming trapped. Note that several common marginal plants are poisonous but unpalatable to stock when growing, but become palatable once cut. When clearing marginal vegetation, ensure stock do not have access to the cut material.
Grazing by geese, ducks and other waterfowl has a strong influence on the amount of aquatic, marginal and bankside vegetation. Even a few ducks can quickly decimate the vegetation in and around a small pond, and they should not normally be encouraged. The feeding of bread scraps and other food to ducks and geese on urban ponds is a major factor in maintaining imbalances in population and in lowering water quality, and should be discouraged. On larger ponds, scrapes, gravel pits and coastal lagoons, provision for waterfowl is often a primary aim, and the balance between population and food supply is an important factor in management.
Summary of common aquatic ‘weeds’
The following section lists the most commonly encountered water ‘weeds’, and recommends control methods acceptable from a wildlife conservation viewpoint.
Free-floating plants
Algae and duckweed (Lemna spp) multiply rapidly in highly nutrified water, and any reduction of nutrients entering the water will reduce their dominance. Algae is best controlled by barley straw.
Duckweed can form a green carpet of vegetation on stagnant water, made up of countless individual plants, each plant a single ‘leaf’, or group of two or three, with short trailing roots. Each plant can rapidly reproduce by budding. Thorough removal by mechanical means is almost impossible, and its method of reproduction allows it to rapidly recolonise. Duckweed can be reduced by increasing the amount of shade on the water, either by planting trees and shrubs, or by planting water lilies and other floating-leaved plants. Disturbance of the water surface with aerators or fountains also suppresses the growth of duckweed. In many situations it is best left to stabilise itself. It is only worth raking off if it is causing problems by blocking pipes or sluices, or to prevent deoxygenation during warm summer periods. The removed weed can be composted.
Rigid hornwort (Ceratophyllum demersum) and frogbit (Hydrocharis morsus-ranae) sometimes densely populate still waters, hornwort remaining completely submerged, while frog-bit has floating leaves like little water lilies. Both can be controlled by raking out in autumn. Rigid hornwort is suppressed by shade. The spread of rigid hornwort in static water is indicative of deteriorating water quality due to eutrophication.
Submerged rooted plants
Canadian pondweed or water thyme (Elodea canadensis), a species introduced to Britain in the 1800s, is a widespread and invasive plant of lowland ponds and sluggish rivers. It grows upright just below the surface, and flourishes in sunlit areas. It regrows rapidly from broken stems and fragments, and can only be controlled by thorough uprooting. Canadian pondweed can seldom be eradicated from a site, but as much as possible should be cleared to postpone its inevitable reinvasion. In flowing water, the best way to clear it is by hoeing, which also gets rid of much of the mud trapped by the plant. Use Sedimat to trap the released silt, to prevent it smothering other areas. Remove cut weed from the water to prevent deoxygenation, and compost away from the site. Large amounts should be taken to an authorised disposal site.
In still water, Canadian pondweed is best cleared using muck rakes. Work in early spring when the weed is covered with small shoots and matted just below the surface of the mud. At this stage the stem is much stronger than in the summer, so much more root is pulled out with the stem. Grass carp can be used to control further growth, and native bottom feeding fish will also help prevent regrowth. Shade from other floating-leaved plants or from trees will help suppress growth.
Mare’s tail (Hippurus vulgaris) is managed similarly to Canadian pondweed, as are the other submerged rooted species which only occasionally need control.
Australian swamp stonecrop (Crassula helmsii) is an introduced plant which has spread widely due to its availability at garden centres and aquatic nurseries. It is sometimes mislabelled as Tillaea recurva. This plant can grow in a wide range of aquatic environments from damp margins down to water 3m (10′) deep. It rapidly forms a dense mat of vegetation which out-competes all other aquatic vegetation and harms invertebrates and fish. Severe oxygen depletion occurs below the mat. The plant grows throughout the year, and is tolerant of shade, frost and dessication.
No form of mechanical control should be used onAustralian swamp stonecrop, as cutting or pulling will produce fragments that can regrow and spread the infestation downstream or elsewhere. The only effective form of control is a carefully managed spraying programme by professional operators using approved herbicides. For details refer to Information Sheet 14 (Centre for Aquatic Plant Management, 1994) and contact the Environment Agency (or equivalent) for further advice.
Floating-leaved plants
The floating-leaved plant which most often needs clearance is the yellow water lily (Nuphar lutea). In recent years, the fringed water lily (Nymphoides peltata) has also become a problem in some static or very slow flowing waters, while the white lily (Nymphaea alba) has become rarer and should normally be conserved. Water lily stems have little effect on water flow, and the shading effect of the leaves is usually beneficial in suppressing growth of submerged plants. However, they may need clearance where the water surface is densely covered. Lilies sometimes grow too deep to be cut out easily with a muck rake or pulled by hand, in which case they can be cut apart at the roots using a long-handled pruning saw or a bow saw attached to a long handle. The advantage of hand pulling is that rhizomes are easily gathered at the same time, as other methods tend to break off bits of rhizome which reroot unless raked in and collected.
Emergent plants
For small areas, digging by hand is the main method. Flooding is another option where water levels can be controlled. Large areas are best cleared by machine dredging, or by flooding.
Bur-reed (Sparganium erectum), while a valuable wildfowl food plant, can cause problems when it extends across still or gently flowing water. Its rhizomes reach out into the water while its seeds settle in mudflats and on emergent islands of silt and water plants. To control bur-reed, dig it out in August or September when the plant is fully grown but before it begins to wither and go brown.
Common or Norfolk reed (Phragmites communis) has intertwined rhizomes holding so much silt that they don’t float easily. If you can pull it loose in big rafts you can get it ashore with the aid of a winch or grappling hooks and ropes. See also the section on reedbed management.
Reedmace (Typha latifolia) prefers shallow water, but if the water level rises abnormally, may float and extend over deep water. In very soft silt you can pull out the rhizomes by tugging on the stems, but any pieces that remain will resprout. Pulling out the stems alone is not effective, and thorough digging is the only reliable method.
Reed grass or reed sweet-grass (Glyceria maxima) sometimes forms pure stands, and if control is required, digging is the usual option. It can extend over open water by binding a layer of silt on the surface, from which it can be cut and removed in chunks. On solid silt, dig it out in turfs.
Rushes (Juncus spp) grow in very shallow water and extend well into marsh and terrestrial zones. Dig out as necessary.
