What to avoid
The main plants to avoid include giant hogweed (Heracleum mantegazzianum) and Japanese knotweed (Fallopia japonica).
Invasive non-native tree species such as evergreen oak (Quercus ilex) and false acacia (Robinia pseudoacacia) are very large trees and unsuitable for gardens. With climate change these species are establishing in the countryside as native oak and beech come under increased environmental stress. Evergreen oak is, however, a useful evergreen hedging plant and, kept trimmed, does not set seed.
Rhododendron ponticum has established in the countryside in areas of acid soils and in moister upland areas where it shades out native flora. Today with many more ornamental species and cultivars available to gardeners there is little reason to grow it.
Low growing cotoneasters (Cotoneaster microphyllus agg, C. horizontalis) and Gaultheria shallon are tough, reliable groundcover shrubs - ideal for difficult situations and poorer soils. However, the seeds are spread by birds. If you live in the countryside near downland or hill country try to avoid them.
Some introduced annuals and perennials also cause problems in the countryside. Himalayan balsam (Impatiens glandulifera) readily self seeds into the wild and grows so rapidly that it shades out native vegetation, especially alongside rivers and lakes in both towns and countryside. It is a serious threat to our rare wetland species and should always be avoided by gardeners. In our milder coastal areas Hottentot fig (Carpobrotus edulis) has clothed cliffs pushing out native plants. Gardeners in these areas should not grow this plant.
While Spanish bluebells (Hyacinthoides hispanica) and their hybrids (H. hispanica x H. non-scrpta) are tough bulbs ideally suited to the poorest soils in towns and cities, they hybridise readily with native bluebells and should not be grown by gardeners in areas where our native species is found. Most gardeners would also wish to avoid the weedy few-flowered leek (Allium paradoxum) that so freely self seeds in gardens that it becomes a nuisance.
Those lucky enough to have streamside gardens need to think carefully about planting. Winter floods can carry seeds and portions of roots and rhizomes downstream. In this way plants, such as skunk cabbage (Lysichiton americanus and L. camtschatcencis), are spreading along waterways where their dense leaf growth shades out native species.