When to Sow Zinnias Outdoors: A Zone-by-Zone Guide
Zinnias germinate quickly in warm soil but sulk or rot in cool conditions, making timing crucial for direct sowing success. Gardening expert Madison Moulton breaks down the best sowing windows by USDA hardiness zone.
Contents
Zinnias are one of the easiest flowers to grow from seed. But even so, it’s important to sow them at the right time for the best possible results. If you get the timing wrong, you might see spotty germination or far fewer flowers later in the season.
The right sowing window depends entirely on where you live. Zinnias need soil temperatures around 70°F to germinate well, which happens at different times across the country, depending on your zone.
Generally, it’s best to sow zinnia seeds outdoors around one to two weeks after your last frost date. Since the timing to sow zinnias differs by zone, let’s take a look at the specifics for each region.
Zones 11-12
Warmer zone gardeners are lucky when it comes to zinnias. You can sow almost any time from late winter through early spring in these frost-free zones. The soil warms up early and stays warm, so germination isn’t an issue.
That said, fall and winter in zones 11 and 12 can actually be your better planting season. Sowing in late winter gives you spring blooms before intense summer heat sets in (not that zinnias mind the heat much). In these zones, you can also treat zinnias as cool-season annuals, growing them from fall through spring.
If you do sow zinnias in these zones in late winter, aim for late January planting. This gives plants time to establish and bloom heavily. You can succession plant every few weeks through early spring for continuous color.
Zone 10
To sow zinnias in zone 10, late January through early February works well. Wait until after your last expected frost date (if your area gets any frost at all), which in zone 10 typically falls sometime in January, depending on your specific location.
The soil should be warming up but not yet hot. Zinnias germinate best when days are pleasant rather than scorching. If you’re in southern Florida or coastal Southern California where frost is rare to nonexistent, you have more flexibility with timing and can sow earlier.
Check soil temperature before sowing rather than just going by calendar dates. Stick a soil thermometer a couple inches deep in the morning. If it reads 65°F or higher, you’re good to go. Below that, give it another week or two.
Zones 8-9
Late February through early March is your window to sow zinnias if you live in these zones. Last frost dates in these zones vary quite a bit depending on whether you’re in the warmer or cooler parts of the zone, but you’re generally looking at March for frost-free conditions.
Sowing a couple of weeks before your last frost date is not the end of the world for zinnias since the seeds won’t germinate until soil warms anyway. By the time they sprout, frost risk has usually passed in these areas. This strategy gets seeds in the ground early without the actual seedlings being vulnerable to cold.
If you’re in a coastal area with minimal freezes, you can push the sowing date back to February. Inland areas with more pronounced winter cold should wait closer to March. When in doubt, wait until soil feels noticeably warm when you stick your hand in it rather than cold and clammy.
Zones 5-7
For cooler zones, April is your month. The exact timing within April depends on which end of this zone range you’re in and whether you’re in a warm microclimate or a cold pocket, but mid to late April works for most gardeners to sow zinnias in these zones.
Wait until soil temperature hits at least 60°F, ideally closer to 70°F. Cold, wet spring soil in these zones can rot zinnia seedlings before they have a chance to establish. If April stays unusually cold and wet, push your sowing date back rather than gambling on conditions improving after you’ve planted.
For zone 5, early May might be safer than late April unless you’re confident spring has truly arrived. A warm spell in April followed by another cold snap is common in zone 5, and it’s frustrating to sow seeds only to watch them sit there for three weeks waiting for warmth.
Zones 3-4
Finally, zones 3 and 4 should wait for May or early June, depending on when your soil actually warms up. Last frost dates in these zones can stretch, and soil takes time to warm even after air temperatures rise.
If you want earlier blooms, start seeds indoors in late April or early May and transplant after the last frost. Direct sowing is simpler, but it does push bloom time later into summer. For these zones, that’s often fine since zinnias will bloom straight through until hard frost in fall anyway.
