When summer's heat sets in, it can be challenging to find flowering plants that handle high temperatures and still put on a colorful show, but garden zinnias (Zinnia elegans) are undeterred by heat. Mexican natives and members of the daisy family, zinnias do well grown as annuals in all climates.
Zinnias have compound flowers made up of a central disc of tiny, fertile flowers, surrounded by a ring of infertile, highly colored, petal-shaped florets that attract insect pollinators. Zinnias have been cultivated since the late 1800s and many different varieties in all possible colors are available for planting. The plants make good additions to a butterfly garden, because their colorful flowers attract these garden visitors, who use their outer petals as a landing platform, while the inner, flowers provide nectar.
The zinnia grows as a bushy, leafy annual with upright, hairy stems that branch naturally. Depending on the variety, the plant grows between 4 and 48 inches. The plant thrives in intense summer heat and often blooms until early fall. Pinching the stems back in early summer helps promote more profuse blooming, because each new shoot develops flower buds. When starting with nursery-grown zinnia seedlings, cutting each plant back by about one-third at planting can also stimulate new branching and produce healthy, bushy plants. You can also start zinnias indoors three to four weeks before the last spring frost, or direct-seed zinnias in the garden when soil stays above about 65 degrees Fahrenheit.
The zinnia plant is easy to grow when given a spot that gets full sun for most of the day. It can tolerate a little shade, but needs at least six hours of sun for best blooming. It also does well in any garden soil, but prefers an organically rich loam. Adding compost to your soil at planting can increase your soil's organic content and boost its fertility. The zinnia can develop fungal diseases, such as leaf spot or root rot, when grown in conditions that are too moist. Choosing a site with good air circulation, clearing debris from under plants regularly and watering with a soaker hose or drip irrigation to minimize wetting of foliage help prevent these problems.
Zinnias are divided into groups by their plant size and the type of flower they produce, and are available in dozens of varieties. A mixture called "Benary's Giant" includes 3- to 4-foot-tall plants with thickly doubled, flowers in 13 different colors. Some of these plants are available grouped by color, such as the "Hot Crayon" series with red, yellow or orange flowers. Zinnias also come in a Cactus-Flowered mix, with double- or semi-double flowers that have curved, pointed and slightly twisted petals on 3- to 4-foot-tall plants. The "Candy Cane" mixture grows 18 to 24 inches tall with 3-inch-wide, double flowers in white streaked with red or pink, like a candy cane, while Thumbelina mix zinnias have semi-double, 1-inch-wide flowers in many colors on 6- to 10-inch-tall plants.