Perennial planting ideas for flower borders

White flowers

Campanula persicifolia alba - up to 60 cm (2 feet) in height and flowering all summer. Useful for border and cutting.
Campanula glomerata alba - up to 60 cm (2 feet) in height with bell shaped flowers all summer. Doesn't like dry soil.
Centaurea Amethyst In Snow - up to 35cm (4 inches) in height. White petals with strong purple centre.
Delphinium Galahad White - up to 150 cm (5 feet) so put at the back of a sunny border.
Leucanthemum Victorian Secret - Shasta Daisy - Compact 35-40 cm (14-15 inches) in height with white double flowers. Very long flowering from early summer to first frosts.

Geranium Rozanne - Forms a low mound with blue/purple flowers from late spring to first frosts. Easy to grow.
Nepeta (Cat Mint) - Various shades of blue. About 80 cm (30 inches) in height. Flowers in June and you can cut it back to make it flower again.
Scabiosa caucasica Blue Butterfly - Blue/light purple. Grows up to 30 cm (1 foot) and flowers from early summer to December. Attracts butterflies.

Helleborus Penny's Pink - This Lenten Rose flowers late winter to spring. Prefers moist ground and ideal for growing under trees or woodland areas.
Veronicastrum Erica - Red buds open into pink flowers. 150 cm (5 feet) so plant at the back of a border.
Geranium oxonianum Old Rose - A weed smothering pink flowering geranium that is about 40 cm (15 inches) in height. Flowers from June until first frost and grows anywhere.