This includes the management of waterside trees, shrubs and herbaceous plants and grasses. Many types of wildlife, and not just those dependent on being close to water, will benefit from bankside vegetation which is varied in structure and species. Bankside trees, shrubs and tall vegetation which create shade and help make a diverse bank structure are also important in the overall balance of the aquatic environment. Shade suppresses aquatic vegetation, which in turn lessens the rate at which watercourses silt up.
In intensively farmed or urban areas, the strip of bankside vegetation along watercourses may be the only remaining semi-natural habitat in the area, and thus has even greater value.
Factors to consider
- Trees and overhanging branches shade the water and reduce growth of aquatic plants, lessening the rate at which ponds and watercourses silt up. Trees also create a relatively stable environment in terms of light, shelter and temperature, which is important for many organisms. Excessive shade tends to reduce the wildlife value of water habitats however, so a balance needs to be made between open and shaded areas. Where removal is necessary, it’s usually best to clear growth on the southern side, to give maximum light improvement for minimum loss of trees. Also select for removal trees which are recent, or those of introduced species not typical of the habitat.
- Mature trees or branches that grow over or fall into the water may be a hazard to navigation, and possibly create a blockage in flowing water. However, mature and dying trees have great value for invertebrates, birds and fungi in any situation. At the water ’s edge they also create sheltered nesting conditions for waterfowl, shelter for fish fry and other aquatic organisms and potential breeding sites for otters and other mammals. Clear only what is necessary for navigation or flood control.
- Tree roots stabilise banks against slippage, and allow undercut features to develop which are valuable for wildlife.
- Old pollarded willows and other bankside trees are especially valuable for the continuity of genotype which they represent. Riparian habitats may be the only remaining piece of semi-natural habitat in the area, and contain a valuable reserve of local genotypes. For new plantings, use cuttings or seed from existing trees. Maintain pollards in rotation over 20 years, so there are always some mature pollards to provide habitat for invertebrates and other organisms.
- Scrub at the waterside is an effective deterrent to access, so preventing disturbance to bankside habitats.
- Trees and shrubs at the water ’s edge provide shelter for mayflies and other weak-flying invertebrates, and provide vital niches in the life-cycles of many other organisms.
- Dams and other artificial banks should be kept clear of scrub and trees, and are normally maintained as short grass. This allows easy inspection of the dam or bank, and helps keep the slope stable as the grass roots bind the soil. Although the roots of scrub and trees help stabilise banks, they can cause slippage when they fall, or if their shelter encourages burrowing animals.
- The banks of artificial drainage, flood control and irrigation channels may be mown for similar reasons as given above. Short grass helps maximise water carrying capacity, maintains bank stability, and protects adjacent property from slippage. Straight, artificial channels with uniform banks are economic to maintain by mowing. Invertebrates, birds and small mammals benefit from mowing regimes which allow longer grass and herbaceous plants to develop.
Grasses and herbaceous plants
Banks of grasses and herbaceous plants are usual along artificial drainage channels. Along streams and rivers, and around ponds and lakes, sections maintained as rough grassland intermixed with shrubs and trees give a varied habitat structure and an attractive landscape. Grasslands can be maintained by grazing, machine mowing or cutting by hand. Herbicide application can be useful, especially to control troublesome weeds such as creeping thistle, docks and bracken (MAFF, 1995).
The timing of any cutting will depend on the balance of management aims between nature conservation and water control, and on the characteristics of the site.
An early cut, up until the end of June, will encourage grass to tiller, making a dense sward which is resistant to erosion. Thistles and docks will be cut before they seed, so reducing their spread. However, depending on summer rainfall, an early cut may stimulate growth so that another cut is needed in late summer. An early cut may also disturb breeding birds.
A late cut, after August, is usually more beneficial to wildlife. Tussocky growth with a more varied structure will result, favouring small mammals, invertebrates and birds. Plants have time to set seed, providing food for birds and small mammals. Overall grass production will be lower, giving reduced maintenance costs. Spot herbicide treatment or digging, cutting or pulling of thistles, docks or ragwort may be necessary if these become numerous. All these treatments are most effectively done when the plant is in full growth, but before they have begun to set seed.
Where machines are being used, a complete cut of an entire bank should be avoided, but plan instead to cut different sections in rotation on an annual basis. This promotes a variety of habitat structure and species diversity. On most sites, sections should be left uncut for no more than three years, or rank grasses and scrub will dominate. A similar rotation can be used for hand-cut banks.
The strip at the base of the bank, along the margins of the watercourse, should always be left uncut to prevent disturbance to the marginal habitat and aquatic organisms. Leave occasional strips uncut from the top of the bank to the base, to retain the range of habitats.
Hand cutting
A scythe or grass hook are the traditional tools. Keep a safe distance from other workers. Tools should be sharp and properly maintained, and honed frequently for most efficient use. Cut material should be raked up, to prevent it suppressing the growth beneath. Pile up the cut material on areas of low conservation interest, or remove from the site for composting.
Machine cutting
Strimmers and brush cutters are suitable for small teams of workers, who should be properly trained in their use. These machines are noisy and tiring to use, and are not suitable for use with groups of volunteers.
Where access is possible, a tractor mounted flail mower on a long arm is the usual choice for banks.
Problem weeds
Particular problem weeds of waterside habitats include Himalayan balsam, giant hogweed and Japanese knotweed. The prevention of spread of the latter two species is required under the Wildlife and Countryside Act 1981. Most of the following details are from Information Sheets 5, 6 and 7, published by the Centre for Aquatic Plant Management.
Himalayan balsam
Himalayan balsam (Impatiens glandulifera) is an introduced plant which has escaped from gardens, and can rapidly colonise river banks and other areas of damp ground. It is an annual plant, which grows to about 2m (6′), with purplish-pink slipper shaped flowers from June to August. The mature seed pods explode when touched, scattering the seed, which is further dispersed by water. Himalayan balsam grows in dense stands which suppress the growth of grasses and other plants, leaving bare ground in winter which is liable to erode. Any control must be carried out before the seed pods have formed, to give long-term benefit.
Himalayan balsam is easy to cut by hand or machine, provided there is access and the plant can be cut at ground level. A cut above the lowest node will only cause the plant to regrow and flower later in the season. Frequent cutting will prevent the plant flowering. The plant is shallow rooted, so hand pulling is suitable for limited areas. Remove all the cut or pulled growth for composting or burning. If erosion is not a problem, it may be a good idea to leave the site fallow and allows seeds already in the ground to germinate, and then repeat the cut the following year. This should get rid of most of the viable seed source, and the area can then be sown with a suitable wildflower mix. On erodible slopes it may be best to get a grass sward established immediately after the first cut of balsam, and then keep the grass mown or grazed for a few years, until all remaining balsam seeds have germinated.
Balsam can be effectively controlled with glyphosate, applied in late spring when the plant is nearing maximum height, but before flowering.
Giant hogweed
Giant hogweed (Heracleum mantegazzianum) was introduced to Britain as a garden plant, and has colonised many river banks and areas of waste land. It can grow to 4m (13′) height, and produces umbels of white flowers followed by large quantities of viable seed, which can be spread by water. The plant is biennial, flowering and seeding in its second year. However, if it is cut down before it produces seed, it will survive into a third or subsequent season, attempting to flower each year. It can form dense stands which suppress the growth of native plants and grasses, and leave banks bare in winter and liable to erode.
The sap of giant hogweed contains a toxic chemical which sensitises the skin and leads to severe blistering when exposed to sunlight. This reaction can recur for many years.
Hand or machine cutting should only be undertaken by operators wearing full protective clothing, to prevent skin contamination by the sap. Tools, machines and clothing should be thoroughly washed after use. Cutting before flowering produces only temporary control, and results in regrowth the following season. Cutting after flowering has no benefit once the seeds have been formed, except to clear away the dying vegetation.
The only effective control method is to spray glyphosate when the plants are about 1m (36″) high, in April or May. Seeds remain viable in the ground for seven years or more, so it is advisable to establish a dense grass sward as soon as possible to suppress germination. Sow the seed a couple of days after spraying, scattering the seed as evenly as possible amongst the dying hogweed. In the late summer, cut down the dead stems and mow the grass to encourage a thick sward. Check for hogweed seedlings at intervals the following spring, and spray individual plants with glyphosate, taking care not to spray grasses and other plants.
Japanese knotweed
Japanese knotweed (Polygonum cuspidatum) is an introduced plant which rapidly colonises river banks and areas of wasteground. It is a perennial, which grows from rhizomes to reach a height of about 3m (10′) in mid-summer, with stiff, bamboo-like stems that remain erect over winter. The plant produces masses of small white flowers, but is not thought to produce viable seed in this country. The rhizomes are extremely tough and can grow through concrete and tarmac. Tiny fragments of rhizome as small as one gram can produce new plants, and plant spread can easily occur by water, or by movement of contaminated topsoil. The plant forms dense, impenetrable stands in summer, and leaves bare, easily eroded ground in winter.
Cutting the shoots in spring or summer has no long-term control, and may encourage spread unless the shoots are collected and burnt. Cutting plants on the banks of watercourses is not recommended, as pieces of the plant are very likely to be spread by the water current. Rhizomes can survive for many years even if the shoots are cut regularly.
One method of control is to spray with glyphosate when the plant is about 1m (3′) high, usually in May. This does not normally kill the plant completely, and the dead growth should be cut and burnt about three weeks later, and then the regrowth sprayed in July or August. Alternatively, a single application of spray can be made in July or August when the plant is fully grown, but because of the height of the plant, thorough application is difficult. In the following years, spot treat any surviving growth in May and August.
A different approach is to treat the cut stems, using a similar technique to that of treating stumps of scrub and tree species. The easiest method is to inject the diluted glyphosate into the hollow cut stem using a large plastic syringe, with the needle removed. Trials on National Trust properties have proved successful. Contact the manufacturer of the herbicide for advice on dilution and application rates.
Horsetails
Horsetails (Equisetum spp) are an ancient plant family, native to Britain, with a simple form of growth. Water horsetail (Equisetum fluviatile) and marsh horsetail (Equisetum palustre) can form dense stands in water margins, marshes and waste ground. They reproduce by means of spores, not seeds, and have an extensive root system which is difficult to dig out. Horsetails are poisonous even when dead, and stock should not have access to treated plants or to cut material, which they may eat. Living horsetails are unpalatable. Regular mowing can control growth, but will not kill it, and plants will reappear quickly once mowing stops. If you are cutting plants on a site not owned by you, under the Environmental Protection Act 1991, you must dispose of the cut material at a registered disposal site.
Horsetails are susceptible to herbicides containing dichlobenil. Casoron G is a granular formulation designed for use by non-professionals, and suitable for use near water. Herbicide treatment of horsetails in water should only be done by professionals.
Herbicides
Although use of herbicides is not generally to be encouraged in conservation management, herbicides are very useful and effective in some situations and for some species, such as those described above. All herbicides are subject to regulation, and the use of herbicides in or near water is subject to further control. The Environment Agency, Scottish Environmental Protection Agency or the Department of the Environment for Northern Ireland should be consulted on the use of herbicides in or near water. Work on Sites of Special Scientific Interest or Areas of Special Scientific Interest (Northern Ireland) require consultation with the statutory nature conservation body.
All herbicides must be approved under the Control of Pesticide Regulations 1986. Employers are responsible for ensuring that all employees involved in the storage, sale, supply or use of a herbicide have received adequate instruction and guidance and are competent for the duties they are required to perform. All users of agricultural herbicides are encouraged to obtain National Proficiency Tests Council (NPTC) certificates of competence, even if they are exempt through age or experience. Any user born after 31.12.64 or providing a commercial service is required to hold the certificate of competence.
For full details on herbicides, and useful advice on selection of weed control methods other than herbicides, refer to the Guidelines for the use of herbicides on weeds in or near watercourses and lakes (MAFF, 1995).
The NPTC certification applies to agricultural herbicides, but not to herbicides approved for garden use. Garden herbicides are widely available through shops and garden centres, and can be used by any person on their own land. The active ingredients of some agricultural herbicides are also used in garden herbicides, but in a formulation which is suitable for amateur use. Notable is glyphosate, which is effective on several problem weeds (see above), and is available as a garden herbicide under various brand names including ‘Roundup Biactive’.
Dichlobenil, effective on horsetails, is available under the brand name ‘Casoron G’. Note that these garden herbicides are only suitable for application to weeds in non-aquatic situations. Agricultural herbicides approved for aquatic plant control are only available for use by professional operators. Garden herbicides are suitable for small areas of weed control, but are not economic for larger scale applications. The Environment Agency (or equivalent) should be consulted over all use of herbicides near watercourses. Garden herbicides should be used in accordance with the manufacturer ’s instructions.
Bankside trees
Any maintenance required on bankside trees should be carried out between October and May, to avoid disturbance to breeding birds and invertebrates. Pollarding, coppicing or removal of large branches must only be done when the sap is down, to avoid seriously weakening or killing the tree.
Most broadleaved species respond to cutting by producing more growth, a feature which is utilised in coppicing and pollarding. Cutting of young or established trees to reduce obstruction to flood flows, open up banks or reduce the amount of shade is likely to produce the opposite effect, with thick, bushy growth resulting. In general, most trees are best left to grow naturally, with control concentrating on removal of seedling trees as necessary. Where clearance of young or established trees is required, stump treatment or removal will be necessary.
Where watercourses need to be kept clear of overhanging branches for reasons of navigation or flood capacity, it may be possible to do the clearance work from punts. Occasional routes can be made up the bank as necessary to remove cut material. This method avoids unnecessary clearance, and avoids disturbance to the upper parts of the banks.
Pollarding and coppicing
Pollarding is a traditional way of managing trees on grazed banks and in fields and woods, as the regrowth is out of reach of cattle, deer and other animals. In the past, the pollarded poles were used for fuel, fencing and other purposes. Willow is a common species on riverbanks, because of its tolerance of wet soils. Willow grows fast, and responds well to pollarding, which should be done at intervals of about twenty years. Depending on the amount of growth, pollarding up to about this interval is possible using hand-tools, but if left longer than this, the size and height of the poles may require specialist attention.
To pollard growth up to about 100mm (4″) diameter, use a pruning saw or triangular bowsaw. Ensure that the ladder is securely positioned. If right-handed, work in a clockwise direction, moving round so that the cut branches fall clear of the ladder.
Young willow trees can be pollarded from about ten years of age. Cut to a height which is typical of pollards in the area, normally between 2.5m-3m (8 -10′)in height. Try to cut at a point above some side branches, which are then left in place for at least two growing seasons. Make the cut on a slant to shed water.
Follow standard coppicing procedures for coppicing hardwood species. To coppice existing stools, cut away thin, whippy growth with loppers or a billhook, and then use a triangular bowsaw or billhook to cut the main stems. Work in a spiral pattern round the stool, sloping each cut up towards the centre, to promote runoff of rainwater. Normally the cut should be made to the existing height of the stool, but where browsing by rabbits or muntjac deer is a problem, a cut up to about 500mm (20″) height can be made. Protect coppice stools against grazing stock.
Disposal of cuttings
Pollarded willow poles can be reused for bank stabilisation, revetment and other work (pp81-83). Even quite large poles will re-sprout if inserted in the ground. If this is not desirable, insert the poles ‘upside-down’. To avoid confusion, the poles can be marked when cut, for example by making a slanting cut at the upper end if re-sprouting is not wanted. Thinner poles and ‘withies’ can be used for willow spiling, hurdles, planting baskets and a variety of other purposes.
Where reuse on site or nearby is not possible, consider the options. Material over about 100mm (4″) can be cut up for fuel, and smaller material put through a woodchipping machine to produce woodchips for use as mulch or surfacing material for woodland paths and play areas. Where access and a suitable machine with trained operator is available, this is a good option. Many local authorities now run recycling stations to which green and woody material can be taken for chipping and composting, although transport requirements may mean this is not viable.
If woody material has to be burnt on site, limit the size and number of fire sites, and only burn on ground of low conservation interest. Rake up and remove ashes.
Other methods
For other methods of trees and scrub control, see Wetland vegetation management.
