Taking an environmentally sensitive approach to pest management
July is not a time of the year when gardeners can rely on spectacular displays of color from their shrubs and trees. The vibrant display of color from bright yellow forsythias, purple lilacs and pink magnolias are little more than a pleasant memory by midsummer. For those willing to accept a little less color, there are a few fairly durable shrubs that produce summer flowers in spite of the heat typical of July and August in Missouri.
Rose-of-Sharon (Hybiscus syriacus) is a very useful species for summer color in shrub borders or screens. Standard (tree-form) specimen are available for use as accent plants. Rose-of-Sharon is a hardy relative of tropical hibiscus and produces similar, but smaller, flowers. Flowers may be purple, violet, red, pink or white. Individual flowers last only one day, but a sequence of many flowers keeps this plant attractive during both July and August. Depending upon cultivar, Rose-of-Sharon may grow to a mature height of 10 feet.
One drawback of this plant is its prolific production of seeds which tend to invade nearby flower and vegetable gardens. Newer cultivars are available which do not produce seeds and eliminate the ability of the shrub to become weedy. 'Diana' (white), 'Aphrodite' (pink with a red eye), 'Minerva' (pale lavender), 'Helena' (white with a red eye) and 'Sugar Tip' (pale pink/variegated leaves) are examples of seedless cultivars that will not invade nearby plantings.
Japanese spirea (Spiraea japonica) and Bumald spirea (Spiraea x bumalda) are additional examples of very durable shrubs that flowers during the summer. As its scientific name indicates, Bumald is a cross between Japanese spirea and spirea species. One of the oldest and most widely planted cultivars of Bumald spirea is 'Anthony Waterer'. This cultivar produces pinkish-red flowers held atop reddish-purple foliage throughout the summer. There also are Bumald spirea cultivars with yellow foliage which contrasts dramatically with the pink flowers they bear. 'Gold Flame' and 'Gold Mound' are examples of the latter.
Attractive cultivars of Japanese spirea also are available. 'Shirobana' bears flowers that range from white to pink to deep purple on bright green, mounded plants from midsummer until fall. 'Little Princess' is another attractive cultivar with light-pink flowers over mint green foliage. 'Gold Flame', 'Gold Mound', 'Golden Princess' and 'Double-Play Gold' are examples of Japanese spirea with attractive yellow foliage.
Blue Mist spirea (Caryopteris x clandonensis) is not a spirea, but a hybrid Caryopteris. The latter is a genus of small to medium-sized shrubs prized for their blue flowers and attractive, aromatic foliage. Blue mist bears blue flowers over grey-green foliage in late summer. Plants are quite bushy and grow to a mature height of between two and three feet. Like most summer-flowering shrubs, Blue Mist bears flowers on new wood. Thus, even if a severe winter kills back its top growth, flowers still will be produced the following summer, given the roots and crown of the plant remain viable. This plant also is marketed as Bluebeard.
Another plant that bears blue flowers in late summer is Vitex (Vitex agnus-castus). Sometimes called chaste tree, we are its northern-most limit. While it frequently achieves a mature height of 15 feet in the south, at our latitude it is likely to remain shorter because of the tendency of severe winters to kill it back to the ground. It produces terminal lavender-blue spikes on current-year's growth during July or August. It thrives in hot weather and full-sun exposures in many soil conditions. However, good drainage is a must. Its unique, palmately-compound foliage is quite aromatic but is known to cause some people contact dermatitis.
Butterfly Bush (Buddleia davidii) is yet another blue, summer-flowering shrub, although additional colors are available. Sometimes referred to a 'summer liliac', the species is a vigorous shrub which can grow to a mature height of 15 feet. 'Black Knight' is a very popular cultivar that achieves a mature height of about eight feet. It produces arching branches tipped with fragrant dark blue panicles up to eight inches long from June through September. 'Royal Red' and "White Profusion' are additional popular cultivars. All Butterfly Bush cultivars enjoy abundant sunshine and a well-drained soil. Somewhat tender, Butterfly Bush often freezes back to the ground in severe winters.
The use of Butterfly Bush as a landscape plant is not without controversy, because of its ability to become invasive. Crossing B. davidii with other Buddleia species has resulted in the release of cultivars with improved horticultural merit and greatly reduced seed production. For example, 'Blue Chip' is a miniature cultivar that reaches a mature height of only two feet and bears almost no seeds. 'Asian Moon', 'Miss Molly', 'Miss Ruby' as well as all members of the Lo & Behold® and Flutterby Grande™ series are considered semi-sterile. 'Pugster® Blue' is a new hybrid that bears panicles nearly as large as the species on dwarf plants with improved cold tolerance.
For gardeners who want yellow flowers on durable plants, Bush or Shrubby Cinquefoil (Potentilla fruticosa) may be the answer. This plant boasts buttercup-yellow flowers borne on low, mounded plants which usually achieve a mature height of between two and four feet. It is an excellent choice for dry, sunny areas. Flowers normally begin to be produced in June and continue until frost. Cultivars with other than yellow flowers are available, but the yellow ones seem most characteristic of the species.
Finally, no discussion of summer-flowering shrubs would be complete without mentioning Crepe Myrtle (Lagerstroemia indica). Once relegated to the South, thanks to hybridization for cold tolerance with other Lagerstroemia species (e.g. L. fauriei), we now can grow this attractive shrub throughout plant hardiness zone six. Available in a myriad of colors, crepe myrtle produces large, showy panicles of crepe-like flowers from July through frost, depending on cultivar. It fares best in well-drained soil in a protected area that receives abundant sunlight. At temperatures of -5 degrees F, crepe myrtle often freezes back to its root system, which tends to keep this shrub shorter at our latitude than farther south. Inter-specific hybrids bred by the U. S. National Arboretum and named in honor of Native American tribes (e.g. 'Cherokee' and 'Hopi') tend to have greater cold tolerance than other cultivars.
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REVISED: February 21, 2017