Zinnias are frost-tender annuals that like heat. Sow seed outdoors after your last spring frost, once the soil is about 70°F (University of Minnesota, Clemson), which GardenPal estimates at about 2 to 4 weeks after the last frost. To get earlier flowers, start seed indoors about 4 to 6 weeks before planting out (University of Minnesota, Wisconsin) and set transplants out once frost danger has passed.
Zinnias are native to Mexico and Central America and grow quickly from seed, which is why most gardeners sow them straight into the bed (University of Minnesota). They range from 6-inch edging plants to 4-foot cut-flower types, and they flower from summer until frost. They are also one of the best flowers for bringing butterflies and bees into a vegetable garden.
Planting dates by last frost
The table counts from your average last spring frost. Indoor dates follow University of Minnesota (about 6 weeks ahead) and Wisconsin (4 to 6 weeks). Transplants go out once the danger of frost has passed (Clemson, NC State). The direct-sowing column waits for warm soil, at least 70°F per University of Minnesota; the 2-to-4-week offset is GardenPal's estimate, so check with a soil thermometer if you have one.
| Average last spring frost | Start seed indoors | Plant transplants out | Sow seed outdoors (soil about 70°F) |
|---|---|---|---|
| Mar 1 | Jan 18 – Feb 1 | Mar 1 – Mar 15 | Mar 15 – Mar 29 |
| Mar 15 | Feb 1 – Feb 15 | Mar 15 – Mar 29 | Mar 29 – Apr 12 |
| Apr 1 | Feb 18 – Mar 4 | Apr 1 – Apr 15 | Apr 15 – Apr 29 |
| Apr 15 | Mar 4 – Mar 18 | Apr 15 – Apr 29 | Apr 29 – May 13 |
| May 1 | Mar 20 – Apr 3 | May 1 – May 15 | May 15 – May 29 |
| May 15 | Apr 3 – Apr 17 | May 15 – May 29 | May 29 – Jun 12 |
| Jun 1 | Apr 20 – May 4 | Jun 1 – Jun 15 | Jun 15 – Jun 29 |
Starting zinnias from seed
Sow seed about ¼ inch deep (University of Minnesota, Clemson). University of Minnesota says zinnia seeds need darkness to germinate, so cover the tray (black plastic works) until they sprout, then move them under bright light. Wisconsin germinates them at 65 to 70°F. Zinnias don't like their roots disturbed for long, so start them in individual cells and plant out while they are still young. Clemson says not to plant transplants any deeper than the top of the root ball.
Hot, humid summers
NC State notes that zinnias may slow down at the peak of summer and recommends disease-resistant varieties in hot, humid areas. Clemson lists series bred for better resistance to powdery mildew or leaf spots, including 'Profusion', 'Zahara', 'Oklahoma', 'State Fair' and 'Dreamland', plus narrow-leaf zinnia (Z. angustifolia). In the South, a second sowing in early summer can give fresh plants for fall, after the first planting has tired; that's GardenPal's suggestion, not an extension rule.
Planting and spacing
| Step | What extension sources say |
|---|---|
| Sun | Full sun, 8 or more hours a day, in a warm spot (University of Minnesota) |
| Soil | Well-drained; pH 5.5 to 7.5 (University of Minnesota); fertile and rich in organic matter (Clemson) |
| Spacing | 8 to 24 inches depending on habit (University of Minnesota); Clemson spaces dwarf types 8 to 9 inches and 2- to 3-foot types a foot apart, with rows 12 to 18 inches apart |
| Thinning | Thin direct-sown seedlings when they are about 3 inches tall (Clemson) |
| Water | Water at the base when the top 1 to 2 inches of soil are dry, about 1 inch a week (Clemson, University of Minnesota) |
| Feeding | Balanced fertilizer at planting, then monthly with a higher-phosphorus fertilizer (University of Minnesota) |
Powdery mildew and other problems
University of Minnesota calls zinnias highly susceptible to powdery mildew. The best prevention is spacing for airflow, keeping leaves dry, and choosing resistant varieties (University of Minnesota, Wisconsin, Clemson). Crowding raises disease, so don't skip thinning. Clemson's pest factsheet covers leaf spots and insect pests such as aphids and mites.
Zinnias in the vegetable garden
Zinnias are a good fit for the edges of a vegetable bed. Their open flowers feed pollinators and beneficial insects through the summer, which helps squash, cucumbers and other crops that need pollinating. Pair them with marigolds, which go in around the same time; see when to plant marigolds and flowers that attract beneficial insects.
Frequently asked questions
When should I plant zinnias outside?
After your last spring frost. Sow seed once the soil is about 70°F (University of Minnesota, Clemson), and set out transplants once the danger of frost has passed.
When should I start zinnia seeds indoors?
About 6 weeks before planting out (University of Minnesota), or 4 to 6 weeks (Wisconsin). Start Rocky Mountain zinnia earlier, 6 to 8 weeks before the last frost (Wisconsin).
Is it better to direct sow zinnias?
Usually. They grow so fast that direct sowing works well (University of Minnesota). Starting indoors just gives earlier flowers.
How long do zinnia seeds take to germinate?
About 4 to 8 days at 70 to 75°F (University of Minnesota). Zinnia seeds need darkness to germinate, so cover them.
How far apart should I plant zinnias?
8 to 24 inches, depending on the variety (University of Minnesota). Clemson uses 8 to 9 inches for dwarf types and a foot for 2- to 3-foot types.
How do I stop powdery mildew on zinnias?
Space plants for airflow, water at the base, and grow resistant series such as 'Profusion' or 'Zahara' (University of Minnesota, Clemson).
Can I plant zinnias in summer?
Yes, as long as there is time to bloom before frost. In hot, humid areas they may slow at the peak of summer, so disease-resistant varieties do best (NC State).
Check which vegetables you're planting near your zinnia and marigold border, and spot any poor pairings before you plant.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- University of Minnesota Extension, "Zinnia"
- Clemson HGIC, "How to Grow Zinnias" (HGIC 1149)
- Clemson HGIC, "Zinnia Insect Pests & Diseases" (HGIC 2110)
- University of Wisconsin–Madison Extension, "Zinnias"
- NC State Extension Gardener Plant Toolbox, "Zinnia elegans"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.