Zinnias are fast, warm-season annuals that bloom until frost. University of Minnesota Extension says to give them full sun in a warm spot with well-drained soil, and warns that they are highly susceptible to powdery mildew.
Zinnia at a glance
| Type | Flower |
|---|---|
| Season | Warm-season |
| Sun | Full sun |
| Plant spacing | 12 in |
| Row spacing | 18 in |
| Per square foot | 1 |
| Seed depth | ¼ in |
| Days to maturity | Seeds germinate in 4–8 days |
| How to start | Direct-seed when the soil is at least 70°F; transplants can go out once soil reaches 60°F. |
| Water | 1 in per week |
These are the same numbers our plant spacing, square foot and harvest date calculators use. See every crop in the plant database.
When to plant zinnia
Direct-seed when the soil is at least 70°F; transplants can go out once soil reaches 60°F. To start indoors, sow about 6 weeks before your outdoor planting date. Seeds germinate in 4 to 8 days at 70 to 75°F. Check your frost dates and the planting calendar.
How to plant zinnia
- Sow seeds about ¼ inch deep.
- Space plants 8 to 24 inches apart; single-stalk types can go closer than bushier types. See the plant spacing calculator.
- Give full sun, 8 or more hours a day, in a warm spot, with well-drained soil at pH 5.5 to 7.5. Add compost or sand to heavy soils.
Caring for zinnia
Water regularly but avoid overwatering: water only when the top 1 to 2 inches of soil are dry, and aim water at the base rather than the foliage. See the garden watering calculator.
Work a general-purpose fertilizer with equal nitrogen, phosphorus and potassium into the soil at planting, then fertilize monthly with a formula higher in phosphorus than nitrogen to promote blooms. Remove old blooms to encourage new flowers, and pinch plants to make them bushier if flower stalks cannot support the heads.
Companion plants
| Grow nearby | Other sun-loving annuals and vegetables, since zinnias attract pollinators |
|---|---|
| Keep apart | Crowded, shady plantings with poor airflow |
Good air circulation helps prevent powdery mildew. Check other pairings with the companion plant finder.
Common problems
| Problem | What you see | What to do |
|---|---|---|
| Powdery mildew | White powdery coating on leaves | UMN says zinnias are highly susceptible. Prevent it by keeping foliage dry and providing good air circulation; a fungicide can treat it. Keep the soil surface clean to reduce disease. |
| Leggy or floppy stems | Tall stalks that cannot hold the flower heads | Pinch plants to make them bushier. |
Not sure what you are looking at? The plant problem identifier ranks likely causes from the symptoms you see.
When and how to harvest
Zinnias are mainly grown for color and cutting. Removing old blooms (deadheading) keeps new flowers coming through the season.
Plan your zinnia
- Plant spacing calculator: how many plants fit your bed
- Planting calendar: sowing and transplant dates for your frost dates
- Frost dates by ZIP code: your last spring and first fall frost
- Harvest date calculator: when your crop will be ready
Frequently asked questions
How far apart should I plant zinnias?
Space zinnias 12 inches apart in rows 18 inches apart. In a square foot garden, plant 1 per square foot.
How deep do I plant zinnia seed?
Sow zinnia seed about 1/4 inch deep.
How long do zinnias take to grow?
Seeds germinate in 4 to 8 days at 70 to 75°F.
How much water do zinnias need?
About 1 inch of water a week, from rain and watering combined. Water only when the top 1 to 2 inches of soil are dry, at the base of the plant.
When do I plant zinnia seeds outside?
When soil is at least 70°F; transplants can go out at 60°F.
How far apart do I plant zinnias?
8 to 24 inches apart, closer for single-stalk types.
How do I stop powdery mildew?
Keep foliage dry, water at the base and ensure good airflow.