How to Grow Zinnias From Seed

Zinnias love heat and hate frost, so wait until all danger of frost is past and the soil has warmed to around 70F before sowing outdoors. In most of the country that means late spring. Gardeners in cool-summer regions like the Pacific Northwest or high-altitude zones can start seeds indoors 4 to 6 weeks earlier for a head start, but zinnias grow so quickly that direct-sown plants usually catch right up.

Zinnias are one of the easiest flowers to direct-sow. Plant the seeds about a quarter inch deep in a sunny, well-drained bed, sowing a small cluster every 8 to 12 inches. In warm soil they sprout in 5 to 10 days. Thin the seedlings to 9 to 12 inches apart so air can move between the plants, which is the single best defense against the powdery mildew zinnias are prone to.

Give zinnias full sun, room to breathe, and water at the soil line rather than over the leaves. Pinch the growing tip of young plants when they are about 8 inches tall to make them branch and produce far more stems. In humid regions like the Southeast, powdery mildew is the main headache, so choose mildew-resistant varieties, space generously, and water in the morning so foliage dries fast.

Zinnias pull pollinators and butterflies into any bed, which is why they earn a spot beside vegetables and other cut flowers. These four make especially good companions, and you can add any of them in one click:

Zinnias are a cut-and-come-again flower, so harvest freely. Cut stems in the cool of the morning when the bloom is fully open and firm, using the wiggle test: a stem ready to cut stays stiff and does not flop when you shake it. Every stem you take, or every faded flower you deadhead, pushes the plant to make more, keeping it blooming until frost.

Zinnias are the workhorse of the summer cutting garden. Their long, sturdy stems and bright, long-lasting blooms fill vases for months, and the flat, open flower heads are an easy landing pad for butterflies, bees, and hummingbirds. Plant a row along the vegetable garden to bring in pollinators, or mass them in a bed for pure, low-effort color from midsummer to fall.

• Powdery mildew: the classic zinnia problem in humid air. Space plants well, water at the base, and choose resistant varieties.
• Leggy plants with few blooms: pinch young plants to force branching and keep cutting to encourage more flowers.
• Poor or slow germination: zinnias need warmth, so wait for soil around 70F before sowing.
• Weak, floppy stems: usually too little sun or overcrowding. Give full sun and thin to 9 to 12 inches apart.