Given the paucity of blue- flowered summer-blooming trees and shrubs, it's a wonder more people aren't planting chaste trees. These drought-tolerant, pest-free plants combine striking blue or purple, tubular summer blossoms with hand- some leaves shaped like an open hand. They grow in almost any well-drained soil, will take coastal conditions, and are not usually browsed by deer. The common name derives from the belief in medieval times that an extract from their fruit would curb the libido.
V. agnus-castus. LILAC CHASTE TREE. Deciduous shrub or small tree. Native to southern Europe, central Asia. In most areas, grows fast to make a multitrunked tree with a broad, spreading habitabout 1520 ft. high and wide. In the Upper South, growth is slower and mature size is 810 ft. high and wide. Aromatic leaves are divided into five to seven narrowly oval, pointed leaflets that are grayish green above, green beneath; each leaflet is 26 in. long. No real fall color.
Small, lightly fragrant, lilac to dark blue flowers in 6- to 12-in. spikes appear at branch ends and leaf joints in summer. Blooms come on new growth, so prune in late winter, removing twiggy growth and crowded branches. If you remove spent flowers before seeds form, plant will send out a second flush of blooms.
Unnamed seedlings vary in showiness of flowers, so buy a plant in bloom to see what you're getting. Or choose one of the following superior named selections. For blue or violet flowers, look for 'Abbeville Blue', with very showy deep blue spikes; 'Montrose Purple', a strong grower with rich violet flowers; and 'Shoal Creek', with blue-violet blossoms on spikes up to 112 ft. long. Pink-flowered choices include 'Fletcher Pink', a fast grower with lavender-pink flowers, and 'Rosea', with mauve-pink blooms. White-flowered selections include 'Alba', with particularly large flower spikes, and fast-growing 'Silver Spire'. V. a. latifolia has shorter, broader leaves and is said to be tougher and more cold hardy.
V. negundo. CHINESE CHASTE TREE. Deciduous shrub or small tree. Native to southeast Africa, eastern Asia. Similar to V. agnus-castus but a little larger and more cold hardy. The 5- to 8-in. flower spikes aren't as showy. V. n. heterophylla has delicate-looking, finely lobed leaflets.
V. trifolia. Evergreen shrub or small shrubby tree. Zones CS, TS; USDA 9-11. Native to Asia and Australia. Fast growth to 1220 ft. high and wide. Leaves are sometimes simple but are usually divided into three 4-in.-long leaflets with felted gray undersides. Lavender-blue flowers with a white spot on the lip open in summer on 4- to 10-in.-long spikes. Makes a fine-textured hedge; clip regularly. 'Purpurea', sometimes called Arabian lilac, is the most widely sold selection; leaf undersides are dusky purple. 'Variegata' has white-variegated foliage; cut out shoots that revert to plain green.