Zinnias are annual flowering plants prized for their ability to withstand heat and drought-like conditions that would destroy other plants. They will likely fail in humid, wet conditions but are well-suited to arid locations. Zinnia plants can grow from 6 to 36 inches in height, depending on variety, with a spread of up to 18 inches. White, yellow, orange, red and violet blooms appear throughout summer. Zinnia plants need only basic care to thrive, and they make great additions to border gardens, edgings and flower beds.
Choose a planting location for zinnia plants that receives at least 6 hours of full sun per day and has fertile, well-drained soil. Zinnias are warm weather flowers and perform best in hot, dry locations. Prepare the site by incorporating 1 to 2 inches of organic compost into the soil with a garden tiller prior to planting.
Sow seeds directly in the garden in spring after all threat of frost has passed. Space plantings 12 to 24 inches apart to allow room for growth and to promote air circulation. Taller varieties of zinnias need at least 24 inches in between plantings, but dwarf varieties are fine with 12 inches in between.
Water zinnia plants immediately after planting and at least once per week until growth emerges. Reduce watering to once every two weeks after plants become established, but only when there has been no natural rainfall. Zinnias prefer dry conditions, but supplemental watering is necessary in times of drought.
Feed zinnias once per month during the spring and summer, beginning in the second month of growth. Use a balanced flower fertilizer according to the manufacturer's instructions for proper dosage. Wait until the soil is completely dry, water the soil thoroughly at ground level, apply the fertilizer and then water again to distribute the nutrients and prevent injury to the roots.
Remove spent zinnia flowers after they begin to fade, to promote additional blooms. Pinch them off as close to the stem as possible using your fingers, or cut them back with pruning shears. Cut zinnias back for fresh flower arrangements if desired, as this has the same effect as deadheading and will encourage the plants to continue flowering while allowing you to enjoy their beauty indoors.