Wet grasslands, while extremely diverse, all developed under human, generally pastoral, management, to which they owe their character. Wet grasslands are of five main types: flood meadows, washes, water meadows, grazing levels, and wet heaths and acid grasslands.
Flood meadows occur where rivers periodically spread out over their lowland plains and inundate them with fertile alluvium. They may persist on their own where flooding is regular and deep but in most cases natural succession is toward some form of marsh or scrub and wet woodland. Traditionally, flood meadows were managed for grazing and hay making, which prevents the invasion of coarser vegetation, but nearly all have been destroyed in recent years by gravel excavation or development or have had their vegetation greatly altered by fertilisers or ploughing. One such area, North Meadow in Cricklade, Wiltshire, has been preserved as a haven for such typical but now-rare ‘old meadow’ plants such as adder’s tongue (Ophioglossun vulgatum) and snakeshead fritillary (Fritillaria meleagris).
Washes are artifical flood meadows created for drainage or flood control purposes. Unlike normal flood meadows, which usually experience a gentle rise in ground water levels above the surface, washes may be subject to sudden and fairly violent inundation. The largest and best known are the Ouse Washes, about 2,000 hectares (5,000 acres) in extent, which act as a huge ‘safety valve’ for the River Great Ouse, receiving pumped water when it approaches overflow and containing it until it can be gradually released farther downstream. These washes are grazed early May to early November, when cattle must be taken off because of the high risk of winter flooding. Some haymaking is possible, although even in summer floods may occur. The main conservation interest of such washes is their value for wintering and breeding wildfowl and shorebirds. They often have a remarkably varied flora but usually few rarities.
Water meadows are a feature of certain chalk stream valleys in Wiltshire, Hampshire and Dorset. They were built during the 17th and 18th Centuries to increase the productivity of riverside grasslands. The system is ingenious. Stream water is impounded behind a barrage and released via sluices when and as desired, to be distributed through a network of raised aquaducts and overflow onto the fields. After flooding, which usually lasts 12 to 48 hours, other gates are opened and the water flows out through a set of drainage channels to empty via a main drain downstream.
The meadows are generally flooded in February to provide March grass for ewes and lambs. The grass is stimulated to quick growth because the water taken from the streams is several degrees warmer in winter than the surrounding land. Another flooding assists the July hay crop which is followed by grazing in autumn.
The success of the operation depends on how long the water is allowed to flood the meadows. A local saying has it that ‘you tets (takes) the water on at the trot and off at the gallop’.
Water meadows are laborious to build and maintain and require accurate timing of flooding for maximum benefit. With the loss of the men known as ‘drowners’ or ‘floaters’, knowledgeable in water meadow management, and the increasing cost of farm labour, the aquaducts and sluices fell into disuse and decay and most water meadows no longer function. However, a few have remained under continuous management, and two major sites, near Salisbury in Wiltshire, and at Sherborne in Gloucestershire, have recently been restored. Relic water meadows are dominated by grasses, up to thirty species occurring on one site. Where farmers have taken weed control measures, dicotyledons are poorly represented.
Grazing levels are wet grasslands produced by the drainage of meres, marshes and fens. The largest areas exist in Somerset and Norfolk, but in both places the active conversion to grazing land has halted where the peat is deepest and drainage most difficult. In Somerset the peat levels are rather poor quality, worth only about half the nearby drier fields on silt and clay, and are often used as rough grazing for beef cattle although the region as a whole is rich dairy country. Poorly drained fields a long way from paved roads are now allowed to deteriorate to scrub and wet heath or are dug up for the peat, and are often invaded by purple moorgrass (Molinia caerulea), brambles, sweet gale and birch. As with other lowland grasslands, the ditches and drains intersecting or bordering them often have an interesting aquatic flora independent of the quality of the grassland.
Wet heaths and acid grasslands often surround or grade into true wetlands, particularly acid bogs. Grasslands surrounding bogs are themselves likely to be acidic and very different from the rich grasslands of chalk, limestone and other basic or even neutral soils. There are three major acidic grassland communities: bent-fescue (Agrostis-Festuca) grassland on siliceous and sandy soils, Nardus grassland on damp siliceous soils and on disintegrated peat eroded from high moors, and Molinia grassland, dominated by purple moorgrass (Molinia caerulea), on peaty soil which is wet nearly all the time but where the water moves at least slowly and so remains aerated. All three acidic grassland communities have a very limited associated flora. Intensive grazing, or burning on grouse moors, along with the destruction of the original woodlands, has eliminated many of the most interesting flowering plants which otherwise could be expected to exist in wet ‘flushes’ and other favourable places.
