Zinnias are the star of the summer flower garden, loved by novice and expert gardeners. They bloom effortlessly from summer to frost and the more you cut, the more they branch and bloom.
Zinnias are native to Mexico and as a frost-sensitive annual appreciate warm weather. Zinnias bloom heaviest when daylight is less than 12 hours.
When to sow outside: RECOMMENDED. 1 to 2 weeks after your average last frost date. The ideal soil temperature for zinnia seed germination is 70°–80°F.
When to start inside: 4 to 6 weeks before your average last frost date. Transplant outdoors after last frost. Zinnias do not benefit from being planted early; wait for warmer weather.
Use a lightweight seed starting mix/media (sterile, and lighter than potting mix), and sow seeds 1/4" deep. Sow two seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color, too. By thinning early, you minimize the negative impact of crowding, like stretching for light. If your seed starting mix does not contain nutrients, feed seedlings regularly with a balanced (equal parts nitrogen, phosphorus, and potassium), liquid fertilizer. It is better to use a weak fertilizer solution more frequently rather than to over-fertilize. Read about more indoor sowing tips.
Nearly any clean container with drainage holes will work. Often zinnias are started in 4- or 6-celled pots, and our 1½" biodegradable pots also work well. Up-pot zinnias into a larger container before they become rootbound, being careful not to disturb roots.
When 2" tall, thin to 1 every 12"
Harden-off seedlings for 7 to 10 days prior to transplanting. Transplant seedlings after the threat of frost has past, as zinnias are frost sensitive. It is best to transplant on a cloudy day or in the evening to reduce transplant stress.
How to sow
Sow a group of 3 seeds every 8"–12" apart and ¼" deep, depending on the variety (consult your seed packet).
Optimal Growing Conditions
Zinnias thrive in fertile, well-drained soil.
Keep zinnias consistently moist but not soggy; allowing the top inch of soil to dry between watering once plants are established. Water the soil, avoiding the foliage to help prevent fungal disease. Mulching zinnias can help to keep down weeds, avoid soil splashing that can spread disease, and also keep soil more consistently moist.
Choose an area with full sun (6 or more hours per day).
If soil is deficient, fertilize with a slow-release or liquid, phosphorous-rich fertilizer.
Once they have 4 sets of leaves, clip or pinch zinnia seedlings back to just above a set of leaves , to encourage them to branch out. Deadheading frequently keeps zinnias blooming because it stops them from producing seeds, encouraging them to begin the bloom cycle again.
For longest vase life, harvest before the small yellow flowers emerge from between petals.
Common Pests and Diseases
Powdery Mildew looks like a white powder on leaves and thrives in humid weather with cool nights. Reduce chances of this disease by keeping leaves dry while watering only in the morning and during the day. Proper spacing of plants will provide good air circulation, too. Do not compost diseased plants; spores may over-winter and re-infect crops the following season.
Three ways to fight powdery mildew:
Compost tea, long used as a fertilizer, can also help fight fungal diseases. Begin with a burlap or cheesecloth bag containing 1 gallon of well-aged, manure-based compost. Place in a 5-gallon bucket of water, stir well, and steep it in a warm place for 3 days, stirring regularly. Then remove the bag, put the liquid in a sprayer or watering can, and spray or sprinkle the entire plant.
Horticultural oil and baking soda has also been found to prevent powdery mildew. Mix 1 tbsp. of baking soda and the amount of horticultural oil recommended by the label in 1 gallon of water and spray plant thoroughly weekly.
Studies showed that a 20–50% milk dilution was as effective as commercial fungicides if used weekly. Dilute milk to the desired percent in water and add a couple drops of natural soap to help the spray stick to leaves, rather than rolling off.