Managing wet scrub
Wet scrub occurs across a range of damp soils. If unmanaged, however, scrub encroachment into these habitats will damage their wildlife interest.
About wet scrub
Lowland wet scrub is generally dominated by grey willow, along with other native or introduced willow varieties (for example, goat willow, crack willow and osier).
Depending on soil and climatic conditions, downy birch, alder, bay willow, alder buckthorn, elder, hawthorn, bramble and ash also make up the other woody elements of wet scrub communities.
Wet scrub occurs across a range of damp soils. It is an important component of mires and fens, the edges of ponds, lakes, rivers and streams, damp hollows and dune slacks, as well as disused mineral workings.
This page describes managing wet scrub as a part of those habitat mosaics without compromising the important wildlife value of the scrub and the habitat in which it occurs.
Why is it important?
Wet scrub is an important part of both wetland and woodland habitats.
As well as being important to a range of invertebrates, such as weevils, craneflies and moths, its structure can provide shelter and warmth within more open habitats.
Many birds benefit from wet scrub, including Cetti’s, grasshopper and marsh warblers, along with reed buntings, willow tits and, in mature areas, lesser spotted woodpeckers.
Wet scrub can also provide otters with lying-up sites or holts. The high humidity of wet scrub favours the growth of mosses, lichens and fungi. Young scrub of four to seven years old supports high numbers of insects and is of particular value, especially to warblers.
Older scrub supports a wider range of species and is especially favoured by the scarcer invertebrates and lower plants.
Why manage wet scrub?
If left unmanaged, wet scrub will continue through its natural successional stages and develop into woodland.
The rate of succession will, however, depend on the wetness and other factors. Scrub may encroach into and dry out open habitats such as mires, sedge and reed fen.
In order to retain its wildlife value and prevent damage or loss of the other important habitats, rotational management should be considered. Old wet scrub should be carefully retained, with management targeted at the younger, expanding scrub.
- Wet scrub is an important wildlife habitat, particularly for many insects, lower plants and birds. Its value is enhanced if it is open in structure and within wetland mosaic.
- If left unmanaged, it may encroach into and dry out important open wetland habitats. Careful management ensures these open habitats remain and the scrub itself retains its wildlife value.
- Use appropriate techniques to help meet your objectives. Old wet scrub should be valued. Early removal of unwanted scrub is beneficial. Eradication of established scrub is difficult, especially on archaeological sites.
- Consult relevant statutory organisations and specialists where appropriate.
Wet scrub benefits from rotational management to create a diverse age structure. Planned ‘movement’ of stands around a site helps maintain the mosaic of young and old growth as well as retain the desired amount of open habitat.
Management can be through grazing or cutting, or a combination of the two, sometimes with the use of herbicides. Use of large machinery is often necessary to tackle severe cases of scrub encroachment. In some circumstances, wet scrub may be killed or suppressed by raising water levels.
Mechanical management of scrub should take place in the late autumn and winter. Carefully select areas for access and as burn sites where required, avoiding any sensitive wildlife or archaeological areas.
When cutting or clearing wet scrub it may be necessary to use marsh mats to protect sensitive vegetation and prevent sinking into the soft ground.
Maintaining and restoring scrub
Grazing is an important tool for managing wet scrub.
Cattle and ponies find young shoots palatable and will break up blocks of scrub and open up access for further management by trampling and browsing. Start with a low stocking rate of around 0.25 Livestock Units per 0.01 sq km, monitor the impact and adjust accordingly.
Increase the diversity of large blocks of even aged scrub by cutting adjacent areas in sequence. As a guide, cut about 1/15th of the stand annually or 1/5th every third year. This helps wildlife that cannot easily disperse by providing successive areas of favoured habitat. Retain patches of open wetland, such as pools or marsh, where possible.
Where mosses, lichens and fungi occur, selective thinning of bushes ensures that the structure, continuity of age, shade and humidity are maintained. Again, care should be taken with old scrub.
Where management has been neglected, scrub will encroach into and threaten other important open habitats.
In severe cases and where access is possible, scrub can be removed quickly and efficiently with a digger. This removes the scrub and roots, as well as the nutrient-rich topsoil, to re-expose the mineral layer and restart natural succession. It has the added advantage of creating excellent small pools in wetland habitats.
Avoid burying the excavated scrub – ideally burn it within a trailer and remove the ash. Alternatively, pile and burn it on lower value habitat within a wet site.
Do not use a digger where it will damage wildlife or archaeology. In such situations, and where access is difficult, scrub will need to be cut. This often requires follow up treatment with herbicide, repeated cutting of regrowth, browsing by livestock or usually a combination of all of these.
Prolonged submergence in water as shallow as 30cm can kill low-cut stumps of some species. This can create new wetland habitats such as bare mud, pools and reed beds. This method has limited application, however, and water levels should not be raised to kill stumps where it will affect soil structure, sensitive flora, invertebrate communities or archaeological interest.
Preventing scrub establishment:
Grazing and mowing will suppress encroachment into important open habitats. For livestock, start with a stocking rate of about 0.25 LU per 0.01 sq km, monitor the effects and adjust to suit.
Wet scrub may also be suppressed by raising water levels where appropriate. This should not be done where it will affect sensitive flora and invertebrate communities or any archaeological interest.
Natural colonisation of scrub:
Once grazing and mowing of open habitat ceases, wet scrub will regenerate rapidly from small cuttings and seed if gaps appear in the sward. When encouraging natural succession of scrub it is important to maintain the correct balance with other priority habitats to meet objectives.
Always be prepared for follow-up treatment with herbicide (knapsack spraying or handheld weed wipe), especially on peat soils.