Those who have lived in the Midwest for any length of time know how brutally hot summers can be. Midsummer is not a time of year when gardeners can rely on spectacular displays of color from shrubs such as forsythia, lilac, weigela and the viburnams. However, for those willing to accept a little less in term of showiness, there are a few tough shrubs that produce summer flowers in spite of the heat or drought stress typical of July and August.
Once such shrub is rose-of-Sharon (Hibiscus syriacus). Achieving a mature height of between six and ten feet, it is a very useful plant for summer color in shrub borders or screens. A hardy member of the hibiscus family, cultivars are available with purple, violet, red, pink or white flowers. Individual flowers last only one day, but a sequence of many flowers keeps the plants colorful during both July and August. One problem with this shrub is its tendency to produce seeds and become somewhat invasive in the garden. To counter this problem, the U.S. National Arboretum has released four sterile hybrids. They include 'Diana' (white), 'Aphrodite' (pink with a red eye), 'Minerva' (pale lavender) and 'Helena' (white with a red eye).
The Japanese spirea (Spiraea japonica) and hybrids utilizing it as one of their parents are addition examples of tough, summer-blooming shrubs. Japanese spirea is a dwarf deciduous shrub with leaves that change color over the season. It has a mounded growth habit to a height of around four feet. Noteworthy cultivars include 'Shirobana' which has flowers that range from white to pink to deep rose in color. It flowers from midsummer until fall on bright green plants that develop into dense mounds. 'Little Princess' is another cultivar which is quite attractive. It produces light pink flowers over mint green foliage that also is low and mounded.
Bumald spirea (Spiraea x bumalda) is a cross between Spirea japonica and Spiraea albiflora. Having a more dwarf growth habit than its afore-mentioned parent, it grows two or three feet tall and three to five feet wide. There are more than 20 named cultivars of bumald spirea. The oldest, and perhaps most popular, is 'Anthony Waterer.' This spirea flowers throughout the summer producing pinkish-red flowers over deep reddish-purple foliage. 'Coccinea' is a cultivar derived from 'Anthony Waterer' but has deeper red flowers. Additional noteworthy cultivars of bumald spirea include 'Goldflame' and 'Gold Mound.' Both have golden yellow foliage which contrasts nicely with their pink flowers. One final cultivar that deserves attention is 'Lime Mound.' It produces mounds of lemon-yellow new leaves with a hint of russet. Leaves mature to a lime-green color which contrasts nicely with its light pink flowers.
Although not a true spirea, a shrub with the common name of Blue Mist spirea (Caryopteris x clandonensis 'Blue Mist') also produces nice color in the heat of summer. This hybrid caryopteris produces fragrant, purple-blue flower spikes over grey-green foliage. The flowers are very attractive to butterflies, bees and other beneficial insects. Plants of this hybrid are bushy and may grow two to three feet tall. Flowers are produced on new (current year's) wood. Therefore, even if a severe winter should kill much of the top back, it will come out in the spring and flower that summer.
Another shrub that produces blue flowers in late summer is vitex (Vitex agnus-castus). Vitex, grows to a height of between 10 and 15 feet in the south. Reliably winter hardy only through zone six, at our latitude it often is killed back to the ground each winter and grows only from three to five feet the following summer. New shoots are produced from the base of the plant each year which normally flower in July or August. Vitex is very tolerant of hot weather. Its flowers, which are very attractive to butterflies are fragrant, loose panicles pale-violet in color and about 12 inches in length.
Staying with the color blue, buddleia (Buddleia davidii), commonly called butterfly bush, is a fast-growing, deciduous shrub that produces masses of blue, fragrant panicles from summer through fall. The species grows six or more feet in height but does have the reputation of becoming invasive because of self-seeding. Hybrids derived from the species have the advantage of being more compact in growth habit, more floriferous and nearly sterile. For example, the Pugster® series bears large flower trusses on plants that achieve a height of only about two feet. Most hybrid buddleias come in a variety of colors including blue, lilac, pink and white. As the plant's common name implies, buddleia is very attractive to butterflies.
For gardeners who want yellow summer flowers on durable shrubs, potentilla (Potentilla fruticosa) might be the answer. This shrub bears buttercup yellow flowers on low, mounded plants which average about two to three feet in height. Potentilla is an excellent choice for fairly dry sunny areas. Flowers normally are produced in June and continue through frost. There are cultivars available in other colors, although the yellow ones (e.g., 'Goldfinger) seem the most characteristic of the plant.
Finally, for excellent late summer through autumn color, few shrubs can match crepe myrtle (Lagerstroemia indica). Once considered a southern plant, improvement efforts have resulted in the introduction of cultivars hardy through zone six. Most of the latter are hybrids between Lagerstroemia indica and Lagerstroemia faueri and achieve a mature height of between six to 12 feet, depending on cultivar. Crepe paper like flowers appear on terminal inflorescences between six and 10 inches in length. Additionally, some cultivars bear flowers that are pleasantly fragrant. In addition to red and rose found in the species, hybrid cultivars have expanded the color range to include white, pink, mauve, lavender and purple. In hardiness zone five, crepe myrtle often freezes back to the ground. However, its roots are able to survive zone five temperatures and new sprouts form each spring.
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REVISED: July 1, 2021