When to Plant Zinnias

Wait for warmth, not just the frost date

Zinnias are warm-weather annuals from Mexico and the Southwest, and they do not tolerate frost. Avoiding frost is not enough, though: they need heat to grow. Clemson Extension sows once soil and air temperatures are around 70°F. UF/IFAS gives 74 to 80°F as the range where seed sprouts in 5 to 7 days, and notes that development slows and can stop when daytime temperatures stay below 60°F.

That is why the dates for your zone above run from the last frost to about two weeks after it. In a cool, late spring, sow at the end of that window, or wait longer.

Starting zinnias indoors

You don’t need to start zinnias indoors, but it gives earlier flowers in short-season areas. UF/IFAS sows seed in plug trays 3 to 4 weeks before the last frost and transplants 3- to 6-week-old seedlings after frost danger has passed. Seedlings need high light; under weak light they stretch. Set transplants out over the same window as direct sowing, from the last frost to about two weeks after it.

Count back from your transplant date rather than your frost date: if the soil will not reach 70°F until two weeks after the last frost, sow the trays two weeks later too, so seedlings go out at 3 to 6 weeks old, as UF/IFAS recommends.

How day length changes the timing

Zinnias are short-day plants. In long summer days they grow leaves; as days shorten in late summer they set buds faster. Mississippi State Extension notes that flowering peaks once daylight drops below about 14 hours. The practical effect shows up in UF/IFAS trials in Florida: seed sown in June took 100 days to first flower, while seed sown in mid-August took 83 days. Clemson’s general figure is 8 to 12 weeks from sowing to bloom.

So a summer sowing is not just a backup. It reaches bloom relatively quickly and gives you new plants for fall. UF/IFAS also notes that stems and flowers get shorter as a planting is harvested again and again, and it calls sequential planting important for a steady supply of good flowers.

Summer and fall sowings

No extension source gives a fixed cutoff in weeks before the first frost, but the bloom time sets a practical limit. A late sowing needs roughly 8 to 12 weeks of warm weather to flower, plus some time to bloom before frost ends the season. Shortening days help in late summer, while cool nights below 60°F slow everything down.

• Cold north: one main spring sowing, plus perhaps a second in early summer if your first fall frost is more than 12 weeks away.
• Southeast and Gulf Coast: sow again in midsummer for fall color. Choose mildew-resistant types for late plantings in humid weather.
• Texas: Texas A&M AgriLife in Bexar County recommends sowing zinnias in July for late-summer and early-fall color. Garden centers usually stock fall zinnias in 4-inch pots in September; set these out in late summer and they bloom until the first frost.
• Arizona low desert: the University of Arizona lists planting from March through June, with blooms from April to November.

Warm-winter areas (zones 9 to 11)

Zinnias’ heat tolerance makes Florida’s planting season unusually long. The UF/IFAS planting calendar lists zinnias every month from March through September in North and Central Florida. South Florida adds February, November and December, which gives nearly year-round planting south of State Road 70. North and Central Florida get no zinnia planting from October through February, which matches UF/IFAS’s observation that zinnias slow or stop growing when daytime temperatures stay below 60°F.