Rock gardens are popular in areas where the soil is thin and rocky and cannot be tilled for traditional gardening techniques. A rock garden is an ideal way to plant hills or sloping land where tillage is impractical. Natural rock formations make an excellent backdrop for a rock garden, but gardeners also have boulders and rocks brought to the garden site to mimic a naturalistic rocky setting.
The many species of Campanulas offer a range of flower color including white, lilac and blue. The foliage occurs in various bright green shades, so Campanulas provide a great deal of texture and color to the rockery. Campanulas bloom in the summer months. By growing several species, the bloom period can be extended from June through August. Campanulas need ample moisture to maintain their prolific blooming.
Primrose plants also need ample water. They are low growing plants that flower abundantly in a rainbow of colors. They spread easily, filling spaces between rocks. Hundreds of varieties of primroses are available, with one for nearly any rock garden condition.
Yarrow tolerates dry conditions and actually prefers poor soil. The foliage is silvery and finely toothed. Yarrow flower clusters bloom atop stalks that are held above the foliage. The blooming period is typically late spring into summer, depending on altitude, temperature and other location factors. Different varieties of yarrow have flowers that may be white, yellow, pink or red. Rabbits and deer do not bother yarrow.
Tough and woody penstemon plants thrive in rocky, dry soils. Over 400 types of penstemons are classified, and they range from small, spreading ground covers to shrubs about three feet tall and wide. Penstemons have clusters of flowers that are in the purple, violet and blue range. The individual blossoms resemble deep-throated snapdragons. Because the nectar is deep in the flowers, penstemons are pollinated by hummingbirds and insects with extremely long probosces, such as the Sphinx moth. Attracting hummingbirds and interesting pollinators is a good reason to grow penstemon. Penstemon foliage is evergreen, providing refreshing interest in the rock garden all year long.