British woodland gardens are often extensive and largely feature flowering shrubs beneath a canopy of native trees, with oak (Quercus robur, Q. petraea) being favoured for its compatability with a rich ground and shrub layer. The ground layer can become quite rich and sometimes quite visually exciting, but almost by default, as management practices such as strimming to remove brambles (Rubus fruticosus) tend to favour its development. Bluebells (Endymion non-scriptus) are particularly favoured by this practice and are much appreciated when they develop into large colonies. In areas with a rich natural flora, a diverse wildflower community can result, with different combinations of moisture and light resulting in a patchwork of different wild plant combinations (Kingsbury 1994:104).
With shade and additional stresses, such as competition for moisture and nutrients from the trees, aggressive weedy species and pasture grasses are largely eliminated from woodland, making it much easier to naturalise non-native species than in the open. Colourful drifts of geophytes and clumps of slow-growing woodland forbs are a feature of many British woodland gardens. The dormancy period of geophytes is a particular boon for management as it allows the chemical control of weeds during this time. A glyphosate-based herbicide applied during mid to late summer eliminates weeds, and also has the side-effect of stimulating an attractive growth of moss, which then acts as an increasing deterrent to the germination of unwanted seedlings (Hickson 1994).
The reduction of the competitive weed flora should also theoretically allow for greater reproduction through seed of shade-tolerant perennials. This may happen but is very dependent upon the rate of seed production and seedling growth, which can be limited for many woodland perennials. Species with a more ruderal character are most successful at naturalising under these conditions, with some Himalayan primula species (for example Primula florindae, P. denticulata, P. japonica, P. bulleyana, P. pulverulenta) forming spectacular colonies in moist shade in some gardens, with extensive hybrid swarms also occurring (Kingsbury 2000). The patchy growth of grass in light shade may also result in the naturalising of a colourful flora. A number of large historic gardens feature combinations of spring-flowering Cyclamen coum, Galanthus nivalis and Primula vulgaris (the latter two native), along with autumn flowering Cyclamen hederifolium and C. repandum growing in mown turf, for example Dartingon Hall and Greenways Garden, both in Devon, and Painswick Roccoco Garden, Gloucestershire.
In truth, the spread of native wildflowers or of non-native herbaceous species in woodland gardens is nearly always incidental to the main function of the garden; the cultivation of showy non-native shrubs, with rhododendrons being the centrepiece of many such gardens.
The Pacific northwest, with a similar maritime climate to that of the western British Isles and the northwest of the Iberian peninsula, is an area where there is considerable potential for integrating native and non-native woodland vegetation, despite the fact that there are well-founded concerns over the naturalisation of aggressive species of alien plants. The area is characterised by a very dynamic interest in horticulture and a strong horticultural industry, the climate lending itself to the cultivation of plants from an exceptionally wide range of origins. Lovejoy, in a book aimed at the amateur garden market, notes several woodland gardens where natives and non-natives are grown side by side, with some naturalisation of the latter, and she observes the process of ‘editing’ that goes on where native woodland and garden vegetation meet. She echoes many others when she states that ‘most (native-only gardens) are more earnest than beautiful, and for many years, artful garden makers looked askance at the natural movement, finding it limited in palette and intention’ (Lovejoy 1998). However, gardens that blend native and non-native species are more likely to be driven by a strong design ethic, and she states that ‘ (northwest naturalistic gardens) owe a strong debt to Zen tea and sand gardens, both of which emphasise the spare and the sculptural’ (Lovejoy 1998).