Aubrieta ‘Doctor Mules’
Aubrietas are ideal creeping over a low wall, or growing in its crevices, but many are rather pale and thin in their colouring. ‘Doctor Mules’ is a bright, vivid purple, the four-petalled spring flowers showing up well against pale stone, and the whole plant is neater and less straggly than the unnamed types so often seen. Best in full sun, ‘Doctor Mules’ will also enjoy bright conditions out of full sun. 10-15cm (4-6in).
A colourful companion for the aubrietia would be this slightly more billowing plant, with masses of tiny yellow flowers in a foam of spring colour. Once known as Alyssum saxatilis, this rather woody little plant flowers in late spring and early summer and then, after trimming, its softly hairy, slightly greyish evergreen foliage creates a relaxed presence for the rest of the year. Also good in gravel. 20cm (8in).
Campanula poscharskyana ‘Stella’
This splendid, but sometimes uncomfortably vigorous, trailing plant brings vivid bright violet-blue flowers with white eyes to sunny positions over low walls, happily inserting itself into crevices as well as trailing over the edge. Distinguished from the superficially similar C. portenschlagiana by its much longer flowering stems and rather more star-shaped flowers the mass of colour produced can be very impressive. 15cm (6in).
Cytisus x kewensis
Tumbling tightly over sunny walls, the noticeably green twiggy branches carry short spikes of cream pea-like flowers which cover the plant in spring. With one bright burst of colour this hybrid between a French and a Spanish species is neat and well behaved, spreading and trailing much wider than its height. Cytisus x beanii, also an AGM plant, is similar but with bright yellow flowers. 30cm (12in).
Lathyrus latifolius ‘White Pearl’
Where space allows, using this herbaceous perennial pea climber as a trailer over a retaining wall can provide a truly dramatic display. Plant at the edge, guide it in the right direction as it develops and soon its weight will send it out and over in a great billowing cloud, producing spikes of white flowers profusely for many weeks and a continuing mass of new shoots from the base. Needs plenty of space. 1.2-1.5m (4-5ft).
Lathyrus odoratus ‘Solway Classic’
This annual sweet pea can be treated like ‘White Pearl’, tumbling over a sunny wall, but is shorter and more restrained. Its colour - white with red flecks and flakes - shows itself best on close inspection when the scent can also be enjoyed. Again, it needs encouragement for the stems to fall into place and, with new shoots coming from the base, flowering will continue for months if the plants are deadheaded. 60-90cm (2-3ft).
Lysimachia nummularia ‘Aurea’
For shade, there is no better perennial trailer than this vigorous but easy-to-accommodate, little plant. Creeping out over the soil, over low retaining walls and over logs edging shady borders, it produces roots at every leaf joint as it goes. Its bright foliage is at its most buttery yellow in spring, the shoots are lined with shining yellow flowers in summer, and it forms a colourful mat around bolder plants like hostas. 5cm (2in).
This prolific South African daisy spreads steadily across the soil, its rhizomes throwing up slender, slightly greyish leaves and vivid pink daisies, which are darker on the backs of the petals and have golden eyes. Hardier and more spreading than those grown as summer patio plants, it flowers for an unusually long season from spring well into autumn and is also good on steep slopes. Insists on sun. 30cm (12in).
Persicaria affinis ‘Superba’
Many of the plants in this selection are relatively early flowering, but this evergreen, mat-forming perennial, flowers in summer and continues well into autumn. Above the dark green leaves crowded spikes of pink flowers open on short stems, darkening as they mature, and the whole plant spreads tightly across rocks and down over retaining walls. As the flowers darken, the foliage also darkens to rich brown. 25cm (10in).
Phlox subulata ‘McDaniel’s Cushion’
Creeping phlox, spreading tightly over rocks and over retaining walls, come in a wide range of colours and bicolours but one of the most prolific and colourful, is ‘McDaniel’s Cushion’. With large, deep pink, five-petalled flowers, the narrow foliage is completely obscured for weeks in late May and June and, when the flowers are finally gone, the fresh-looking evergreen foliage remains as a dense carpet. 10cm (4in).