When to Prune Blue Spirea?

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When to Prune Blue Spirea?
Cut Blue Spirea back hard as routine early spring gardening. (Jupiterimages/liquidlibrary/Getty Images)

Plants known as Blue Spirea or Blue Mist Spirea are not true spiraeas, or members of the genus Spiraea, though the common name can cause confusion. Blue Spirea, also popularly known as Blue Beard or Blue Misty Shrub (Caryopteris x clandonensis), is a woody perennial in the verbena family. Arthur Simmonds, once secretary of England's Royal Horticultural Society, developed the Blue Spirea hybrid in 1933. Newer, bluer and more compact cultivars are also available. Cutting the plants back hard annually and some pinching back are the only pruning they require.

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Uses for Blue Spirea

Blue Spirea is an excellent bee-and-butterfly shrub, providing steady bloom and nectar from summer into fall. Most cultivars of blue spiraea are fairly low-growing with a mounding form, sometimes spreading wider than they grow tall. Plants are attractive at the front borders, with fine-textured foliage. Blue Spirea is heat-, humidity-, drought- and deer-resistant. Because of their hardiness and self-sufficiency, they are great for planting in just about any sunny or partly shady spot where plants get neglected.

Choosing Blue Spirea

Taller cultivars, growing to 3 feet in height, include deep-green-coloured, dark-blue-flowered Dark Knight and lighter-blue Azure. Similarly sized light-blue-flowered Blue Mist has grey-blue foliage; Worcester Gold has medium-blue flowers and yellow-gold foliage that starts out as bright chartreuse. Smaller cultivars include Kew Blue, with dark-blue flowers and grey-green foliage; and Summer Sorbet, a Kew Blue mutant with golden variegated foliage; Heavenly Blue has dark-blue-violet flowers. One of the smaller cultivars is Longwood Blue, a silver-grey plant that grows only to 18 inches.

Growing Blue Spirea

These tough, drought-tolerant plants thrive in sunny sites with reasonably good well-drained soil, and they also tolerate partially shady locations. Blue Spirea plants need moderate to heavy fertilisation support for heavy midsummer to fall flowering. Plants are hardy from Zone 6 through Zone 9, growing as far north as Chicago. Woody stems at the base of Blue Spirea and underground roots are cold-hardy; but in cold climates, tender top foliage dies back in winter. These shrublike perennials live only four or five years before they need replacing, but gardeners can easily propagate new plants by separating rooted side shoots and transplanting those. You can also start new plants from softwood cuttings.

Pruning Blue Spirea

These plants flower on new growth each spring, so encourage busy, compact plants and ensure good flowering by cutting plants back to woody stubs in late winter, just before new growth begins. Pinch back all growing tips in late spring to encourage fuller, bushier and compact plants when blooming begins. Also tip-prune Blue Spirea in late summer, cutting stems back individually by several inches, to encourage new growth.

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