Reasons for control
- To prevent siltation, especially of small ponds and artificial drainage ditches. Aquatic plants trap silt, creating banks which are then colonised by emergent species. Unless replacement pond habitats can be created nearby, the only way to maintain structural diversity is to periodically clear sections of the habitat back to bare mud, and then allow the succession to proceed.Artificial drainage ditches need periodic management to prevent them silting and to maintain their very valuable range of habitats. The value of ponds and ditches is greatest where they are part of a closely-linked network of similar habitats which can be managed in rotation.
- To help prevent flooding, by reducing obstructions to the flow of water in rivers, dykes and drains, and at the same time increasing the current to maintain channels and reduce silting. Where drainage or flood control is the main priority, it may not be possible to clear areas in rotation, and ‘little and often’ may be the best method of management. Regular cutting also reduces the need for dredging.
- To promote diversity. Where one or a few plant species have established complete dominance in an aquatic or marginal habitat, other species are excluded. It may be beneficial for wildlife to encourage diversity. In some areas, a few introduced species have become dominant, to the detriment of the habitat.
- To improve conditions for invertebrates and fish. Where large volumes of aquatic vegetation die, either from natural causes or man’s intervention, the water becomes deoxygenated, resulting in invertebrate and fish mortality. However, moderate aquatic and marginal vegetation growth is essential for providing shelter and feeding areas for invertebrates and fish.
- To improve conditions for wildfowl. Completely overgrown or shaded water bodies have little appeal for most wildfowl, including diving birds.
- To improve waterways for navigation and amenity. A balance of clear water and aquatic and fringing vegetation provides an attractive environment for boating and other pursuits.
Management methods
The following methods can be used for managing and controlling aquatic and marginal vegetation.
- Digging, cutting, pulling or raking by hand, and cutting and dredging by machine.
- Chemical control. This includes herbicides, and the use of barley straw in water which releases chemicals that control growth of algae.
- Environmental control. This includes planting trees to shade out aquatic and emergent growth, altering channels to increase water flow, grazing marginal vegetation and managing fish and waterfowl populations.
Repeated application of the same management methods on a site year after year is not always advisable, as this is likely to reduce species diversity and may make any weed problem more difficult to deal with. Using a combination of different techniques as part of a management plan can encourage diversity and the development of a self- balancing system.
The information which follows concentrates on methods of digging, cutting and raking vegetation, as carried out by voluntary groups.
Factors to consider
- Is any action necessary? Excessive growth may be seasonal or temporary, and given time, equilibrium may naturally re-establish. Small ponds or watercourses that have little or no open water in mid-summer due to aquatic vegetation growth do not necessarily need any management. The water margins and vegetated shallows are a far more valuable habitat than is open water.
- What level of control is necessary? Complete removal of a particular species, or clearance of a certain area is rarely possible and often harmful. Complete clearance causes major physical disturbance which may release nutrients into the water, resulting in further rampant growth. Other species, possibly ones that are less desirable, may rapidly fill the space created by the initial clearance. Clearance may also cause bank erosion, and subsequent increased siltation elsewhere. In nearly all cases, rotational management of different areas over a period of years is more effective than total clearance in creating a balanced environment.
- If cutting or other clearance is being undertaken, have the logistics of disposing of the cut material been considered? Often this is more time-consuming than the digging or cutting. It is essential that cut material is disposed of properly, as dead material in the waterway will cause deoxygenation. Material dumped on the bank will smother bankside vegetation, and the nutrient runoff will adversely affect the waterway. Cut and dredged material should be removed to an area of low nature conservation interest, and composted. Ensure that the runoff does not directly enter a watercourse. Several common aquatic and marginal plants are toxic, but are either not palatable, or not usually accessible to stock when growing. However, when these plants are cut, they may become palatable, whilst still being toxic. Make sure stock do not have access to cut material and rhizomes of toxic species.
- Is the control method sufficiently selective? Although most aquatic and marginal ‘weeds’ tend to grow in pure stands, there may be interesting species intermixed or at the edges which need to be identified and retained.
- In flowing water, consider the effect that the disturbed sediments may have downstream.
