Tradescantia ‘Bilberry Ice’ – Spiderwort is a long-blooming perennial, with this cultivar producing white triangular blooms with a blush of lavender-purple in the center. These are borne from June to September (when deadheaded) over deep green, grass-like foliage. Some gardeners prune them back hard in midsummer. Grows 15″ high by 18-24″ wide. Zone 5.
Salvia x sylvestris ‘May Night’ (syn. ‘Mainacht’) – A standard perennial Salvia with upright spikes of deep violet-blue flowers from May to June, and a light repeat bloom when deadheaded. It was the 1997 Perennial Plant of the Year and the green basal foliage has a sage-like scent when bruised. Attracts butterflies. Drought tolerant. Grows 18-24″ high by 18″ wide. Zone 4.
Verbena bonariensis – Even if you live in colder regions, you can probably grow this impressive perennial as a self-seeding tender annual. This South American native produces airy, upright stems of clustered lilac-purple flowers that last from June right until frost. It is a great filler plant in mixed borders where other plants come in and out of bloom. Grows 3-4′ high by 18-24″. Zone 7.
Osteospermum barberiae var. compactum PURPLE MOUNTAIN – A perennial form of Osteospermum or African Daisy which produces a dense mat of dark green leaves. It prefers part to full sun and bears bright lavender-purple single daisies (accented with a yellow eye) from late spring through to midsummer. Grows 10-12″ high by 12-24″ wide. Zone 5.
Geranium pratense ‘Mrs Kendall Clark’ – This RHS Award of Garden Merit winner features exquisite blooms of pale lavender with pearly-grey veining that are borne from May to June. It is often cut back after flowering in order to produce fresher, more lush foliage. Drought tolerant once established and even puts up with moist soils. Grows 24-30″ high by 28″ wide. Zone 4.