By Nannette Richford

Zinnias (Zinnia elegans) range in height from 6-inch dwarf varieties to towering specimens up to 4 feet tall. Their daisylike blooms emerge atop foliage on branched stems and range in color from pure white, pink or red to sizzling shades of yellow and orange. These annual plants bloom from late spring until fall, depending on the cultivar, and attract hummingbirds and butterflies. Plant zinnias along fences or the back border of your garden, where they will sway in the summer breeze, adding movement as well as color to the flowerbed.

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Hot pink zinnias growing in garden.

Light and Soil

Zinnias perform best when in direct sunlight for six to eight hours each day. They survive in less sunlight, but it inhibits their blooming and stem strength. They tolerate a range of soils, from sandy to loamy, but thrive in fertile, well-drained soil with a pH level of 5.5 to 7.5. Mix a 2- to 3-inch-thick layer of well-rotted manure or finished compost into the top 6 inches of soil you prepare for zinnias.

Timing and Placement

Because they are tender plants, zinnias must be planted after all danger of frost passes. Although their seeds can be sown directly in prepared soil in spring, planting seedlings in spring produces earlier blooms. Start zinnia seeds indoors four to six weeks before your area's last average annual frost date, or purchase zinnia seedlings at planting time. If you wish to sow seeds indoors in individual pots or cells or directly in an outdoor garden, then plant them at a soil depth of 1/8 to 1/4 inch, and cover them with soil. Seeds and seedlings of dwarf varieties should be spaced 6 to 8 inches apart and those of taller varieties 12 to 24 inches apart when put in an outdoor garden. Position each seedling in the soil so that the top of the plant's crown -- where the roots meet the stem -- rests at the level of surrounding soil. Plant tall zinnias in front of a fence or along a building to provide them with support.

Water and Fertilizer

Zinnias grow best in evenly moist soil and typically need watering once or twice each week, depending on the weather. Water them deeply to moisten their soil to the root level, and water them again when the soil is dry 1 inch below its surface. Fertilizer promotes healthy zinnia growth. Feed your zinnias by evenly sprinkling on their soil surface a balanced fertilizer such as a granular, 10-10-10 blend at a rate of 1 pound per 100 square feet about four weeks after planting and again in midsummer. Water the soil well after each fertilizer application.

Removal of Old Blooms

Removing old or faded blooms on plants is referred to as deadheading. It sends a message to the plants that they have not produced enough seeds to reproduce, triggering the plants to produce more blooms. Regular deadheading will keep your zinnias blooming well into fall, until they are killed by frost.