The diversity of wetland plants in Sussex has declined significantly over the past 60 years due largely to the destruction and neglect of habitats, pollution, land management change, eutrophication, and competition from invasive alien species. Several native species have become extinct or nearly extinct in the last 40 years such as rootless duckweed (Wolffia arrhiza) and whorled water-milfoil (Myriophyllum verticillatum), and many more have become scarce and are vulnerable. Plants such as lesser water-plantain (Baldellia ranunculoides) and marsh helleborine (Epipactis palustris) are now extremely rare.
The wetland flora that survives in Sussex is still rich, and we have species of considerable conservation importance both locally and nationally. Outstanding areas for wetland plants include Pevensey Levels, the Arun Valley, and parts of the Adur and Ouse Valleys. In addition to the vascular plants of interest, parts of the Arun Valley and Thorney Island have been designated as Important Stonewort Area’s (ISA’s) of national importance for foxtail stonewort (Lamprothamnium papulosum), and great tassell-stonewort (Tolypella prolifera). The following are a few of our rarest wetland plants in Sussex :-
Sharp-leaved Pondweed(Potamogeton acultufolius)
Listed as critically endangered and a national red data book species, occurring in 15 or fewer hectads in Great Britain. Important populations of the plant occur within the Arun valley.
Cut grass(Leersia oryzoides)
A very rare perennial grass of nutrient-rich mud around the cattle-trampled margins of lakes and ponds, in ditches, on canal banks and riversides. Its British stronghold is now in West Sussex, where it is principally restricted to the Arun valley, and Amberley Wild Brooks.
True Fox sedge(Carex Vulpina)
This nationally rare native sedge of southern lowland England grows on river banks, ditch sides and damp meadows on heavy clay soils which are sometimes flooded in winter. Most of the populations are in Kent and Sussex, though it is currently confined to West Sussex. Separation from false fox-sedge Carex otrubae is difficult.
Black Poplar (Populus nigra ssp betulifolia)
Sussex is host to a huge range of lower plants including mosses, lichens, algae and liverworts, many of which are locally and nationally significant. The lichen (Lecanora jamesii - typical of mature, wet woodland growing on willow) and mosses such as (Brachythecium plumosum – found in Wealden Ghylls) are just a few of the lower plant species which are found in Sussex but which remain under-recorded and under-recognised.
Recent finds in Sussex include the flood-moss (Myrinia pulvinata) which was found on flood borne silts on trees near Billingshurst on the Arun river. The next nearest neighbours of this moss are found in Dorset, Herefordshire and Monmouthshire. Dotted hornwort (Anthoceros punctatus) which has not previously been recorded in West Sussex was also recently found in a ditch at Graffham common. There are a number of lower plants associated with some of our rarer habitats such as chalk streams and greensand springs.