Start from Seed
Where spring warms up early, wait until the last frost has passed before directly sowing zinnia seeds outside. Plant the seeds only about ¼ inch deep. You'll see seedlings sprout in four to seven days. Once the seedlings reach about 3 inches tall, thin them so that they're 6 to 18 inches apart to maximize air circulation (a key to keeping zinnias looking good all season).
In cooler climates, start seeds indoors four to six weeks before your area's average last-frost date. Harden off the plants by vacationing trays outside for a few hours per day before planting them in your garden.
If you buy zinnia plants at the garden center that have already reached flowering size, ease the transition to your garden by pruning the plants back by one-third. Then sit back and watch your zinnia patch mature and flourish!
We're ready to guarantee that zinnias will not fail you. But if you live where late summer nights are cool and humid, brace yourself for a potential encounter with powdery mildew. Prevention is your best defense against this troublesome fungus, says Larry Hodgson, author of Annuals for Every Purpose. He recommends protecting zinnias from the grayish white growth by maintaining good air circulation around them, watering at the roots, and choosing mildew-resistant varieties. Last summer, Japanese beetles clustered on the zinnias growing in the OG Test Garden. However, our research editor and garden manager, Pam Ruch, observed that the beetles flocked more to the lime and white zinnias and were less attracted to bright orange, red, and purple varieties. If you live where these beetles are a pest, simply hand-pluck the marauders off the foliage and drop them into a bucket of soapy water.