Late summer on into autumn can be a difficult time to find vibrant blooms in your landscape. Spring flowers have long faded and summer blossoms have dwindled to a few lingering yellow composite blooms. Fortunately, a number of late-blooming shrubs that can liven up your autumn floral display, many with rich lavender to deep purple blooms that present a perfect counterpoint to gold and burgundy mums and autumn leaves.
The butterfly bush (Buddleia davidii) is a large shrub, growing about 4 to 10 feet wide and 6 to 10 feet tall. The Clemson University Extension Service advises that butterfly bushes are cold-hardy to Zone 5, and come in numerous cultivars with various color blossoms and different blooming times. The shrubs are highly attractive to butterflies, and fall-blooming varieties provide invaluable late-season nourishment for these attractive insects.
Purple flowered cultivars include 'Lochinch,' which has rich lavender blossoms and silvery-gray foliage, and 'Black Knight,' with emerald leaves and deep gemstone-purple blooms. The Clemson Extension advises that with deadheading, these shrubs will continue blooming well into October.
Bluebeard, Blue Mist, or Caryopteris (Caryopteris x clandonensis), is a small shrub growing to 2 or 3 feet high, hardy as far north as Zone 4 (although the University of Illinois Extension advises that in Zones 4 and 5, Caryopteris is likely to die back to the ground each winter).
Purple-flowering Caryopteris cultivars include 'Dark Knight' and 'Longwood Blue,' each of which will bloom from mid-August into late September, or later in warmer climates. Caryopteris prefers full sun but will also tolerate light or dappled shade.
The hydrangea bush (Hydrangea macrophylla) can bloom either pink or a lovely periwinkle blue, depending on soil acidity. According to the Clemson University Extension Service, acidity in the soil makes aluminum available to the plant, which functions to turn the flowers their distinct shade of bluish-purple. There are dozens of hydrangea cultivars available, and many are hardy to Zone 5.
Purple-flowering cultivars like 'Endless Summer' and 'Nikko Blue' as well as several of the white-and-blue 'lace cap' types will bloom well into September if they are fertilized annually with a well-balanced all-purpose fertilizer. Be sure to maintain soil acidity by turning in or mulching with peat moss or oak leaves to enhance the deeper purple tones of the hydrangea blossoms.