Plants
The distribution of vascular plants across the reserve has been well documented. Due to past cultivation and forestry practices across much of the site the flora is not exceptional. None-the-less, over 400 species have been observed since 1982.
Perhaps the two most notable plants are the national rarity, Loddon Lily and the Nationally Scarce Greater Dodder. Both occur along the riverbank associated with other typical riverside plants. Both the grassland areas and Little Wittenham Wood contain some remnants of semi-natural vegetation.
Riverside Plants
A typical Thames-side community occurs, with riparian plants such as Flowering Rush (Butomus umbellatus), Sweet Flag (Acorus calamus) and Yellow Waterlily (Nuphar lutea). Although often hidden amongst dense stands of Nettle, there are many water loving wildflowers, such as Marsh Marigold (Caltha palustris), Meadowsweet (Filipendula ulmaria), Yellow Flag (Iris pseudoacorus) and Skullcap (Scutellaria galericulata). These fare best along sheltered parts of the riverbank where boat wash is reduced and low lying damp areas exist.
Loddon Lily (Leucojum aestivum): This plant which looks rather like an overgrown Snowdrop is of particular note. Although very rare in Britain (it is listed in the British Red Data Book which documents rare and vulnerable species) it is actually quite common in parts of the Thames Valley. On the reserve it occurs amongst Sedges and Nettles in damp hollows beside the river. It is encouraging that populations of this rare plant seem to be increasing.
Greater Dodder (Cuscuta europaea): This is another British rarity. The plant is entirely parasitic on Nettles and feeds on their sap using especially adapted 'suckers'. It used to be quite widespread along the Thames Valley, but today it has disappeared from many sites and very few plants have been seen on the reserve in recent years.
Grassland Areas
The grassland areas across the reserve are highly variable. This mainly reflects past agricultural management before the area was designated as a nature reserve, but also variations in aspect, soil and hydrology. Four distinct NVC vegetation communities are recognised, although sub-communities are not always clear.
MG7 Rye Grass Ley: This species poor sown agricultural sward, dominated by Rye Grass (Lolium perenne), would have once dominated the reserve's grasslands. Today it persists only where trampling by the public has prevented or slowed the colonisation of many of the wild flowers found in the more diverse MG6 community described below. It is mainly restricted to the tops of the Hills and along grassland paths.
MG6c Rye Grass/Crested Dogstail grassland - Golden Oat sub-community: Most of the reserve's grasslands conform to this community. It is derived by the slow colonisation of plants into a MG7 Rye Grass Ley. In addition to a multitude of grasses, such as Quaking Grass (Briza media) and Golden Oat (Trisetum flavescens), there are many wild flowers. Cowslip (Primula veris) occurs throughout, whereas Lady's Smock (Cardamine pratensis) only grows in damp areas. Short turf areas maintained by rabbit grazing hold an intimate mix of plants, including Fairy Flax (Linum catharticum), Devilsbit Scabious (Succisa pratense) and Dwarf Thistle (Cirsium acaulon). It is apparent that many of these species are increasing their range across the reserve.
MG1 False Oat-grass grassland: This rank species poor grassland has developed on areas, such as Castle Hill and parts of Church Meadow, where the soils are fertile and grazing has been minimal. They are dominated by False Oat-grass (Arrhenatherum elatius) which thrives under these conditions. It is associated with coarse herbs such as Cow Parsley (Anthriscus sylvestris), Stinging Nettle (Urtica dioica), Cleavers (Galium aparine) and Wild Parsnip (Pastinaca sativa).
CG3 Bromus erectus grassland: A small remnant of this characteristic chalk grassland community occurs on the east facing ramparts of Castle Hill. Once this community would have been more widespread on the hills, but now only survives here because the ramparts were too steep to be ploughed. The vegetation is mainly dominated by aan impressive display of tall herbs, including Greater Knapweed (Centaurea scabiosa) and Field Scabious (Knautia arvensis). Although sparse in their distribution plants characteristic of shorter swards, such as Horseshoe Vetch (Hippocrepis comosa), Clustered Bellflower (Campanula glomerata) and Carline Thistle (Carlina vulgaris), also occur.
Little Wittenham Wood
Little Wittenham Wood is probably only about 300 years old, having developed from ornamental parkland established early in the 18th century (see land use history). This relatively recent origin and past land management for forestry and shooting have had a major impact on the vegetation. It is also apparent that underlying hydrology has a strong influence on ground flora.
Trees and Shrubs: The wood classifies as a W8: Ash/Field Maple/Dogs Mercury woodland. This is the typical woodland community of heavy, calcareous soils in lowland Britain. In areas that have escaped replanting the canopy is dominated by mature Ash (Fraxinus excelsior) and Sycamore (Acer pseudoplatanus) mainly developed from abandoned coppice stools. There are large numbers of mature Oak (Quercus robur) trees even within areas that have been planted with dense stands of conifer. The great variety of trees includes a large clone of Aspen (Populus tremula) and several very large Grey Poplars (Populus x canescens). In the 1950's and 1960's large tracts of woodland were re-planted with conifer, Oak or Beech plantation.
The shrub layer is generally sparse throughout. Regeneration is possibly slowed by the shady conditions and heavy deer browsing. Hazel (Corylus avellana) is the most widespread species, but all species typical of W8, including Wild Privet (Ligustrum vulgaris) and Spindle (Euonymus europaeus) are present.
Ground Flora: Much of the woodland is botanically poor and dominated by Rough Meadow Grass (Poa trivialis) with patches of Dogs Mercury (Mercurialis perennis) and Ground Ivy (Glechoma hederacea). In more open areas Nettles (Urtica dioica) and Brambles (Rubus fruticosa agg.) become locally abundant.
In many areas where drainage is impeded by outcrops of Gault clay the ground flora is much more diverse than seen elsewhere in the wood. Wild flowers include Bugle (Ajuga repens), Creeping Jenny (Lysimachia nummularia), Goldilocks Buttercup (Ranunculus auricormis), Common Gromwell (Lithospermum officinalis), Twayblade Orchid (Listera ovata), Greater Butterfly Orchid (Platanthera chlorantha) and plenty of Early Wood Violet (Viola reichenbachiana). Small glades allow species such as Marsh Thistle (Cirsium palustre) and Wild Angelica (Angelica sylvestris) to flower.
