Do your borders seems to flower at all one level? By introducing some height with taller perennials and shrubs you'll add another dimension. A drift of Veronicastrum, which comes in pinks, lilacs and white, will add bulk as well as height, or consider Eupatororium purpureum, a late summer staple which can grow to a magnificent 2m in height. Thalicturms, which are lighter and airier, have delicate flowers that will contrast against denser leaved shrubs and are perfect at the back of a border. Verbena bonanriensis is a another favourite for adding height and movement, with indestructible, everlasting purple flowers that are a magnet for butterflies. If it's a shrub you are looking for consider Viburnum plicatum ‘Kilamanjaro’, a pretty, deciduous shrub with lace-cap like flowers on tiered branches, can grow up to 3m in height in the right conditions but will only spread to about 1.5m, great for a tight spot.
