Showy, heat-loving zinnias (Zinnia spp.) bloom all summer, flooding the garden with their vibrant color. More than a dozen zinnia species exist, but none are as widely grown as the common zinnia (Zinnia elegans). Few flowering plants match zinnias in terms of diversity and ease of growth, making them a satisfying and rewarding flower for novice gardeners. Zinnias are somewhat choosy about their growing conditions, however, and must be planted under the right conditions to keep them healthy and attractive.
Common zinnia and its cultivars are true annuals, which means they complete their life cycle in one season and must be replaced each year. They cannot tolerate frost or cold temperatures. So wait to plant zinnias until after the chance of frost has passed and the soil has warmed and dried out.
Zinnias need the right growing conditions to perform well, including:
- Full sun exposure. Most zinnias need six or more hours of direct sun exposure each day. Those grown in arid climates within U.S. Department of Agriculture plant hardiness zones 9 through 11, though, benefit from light afternoon shade. Likewise, sensitive cultivars such as ‘Envy’ (Zinnia elegans ‘Envy’) can grow in light shade.
- Moist, fast-draining soil. Zinnias are not picky about soil type, although it needs to hold moisture and drain quickly. Less-than-ideal soil can be altered to make it ideal.
Preparation and Planting
Site preparation is among the most important aspects of growing zinnias. Start about one week before planting so the soil has a chance to settle.
Things You'll Need
- Garden fork
- Slow-release, 10-10-10 fertilizer
- Lightweight mulch
Weed the site one week before planting. Remove all weeds by their roots to prevent them from coming back. Pick out and discard large pieces of debris, such as twigs and stones. Weed again on planting day to rid the bed of all emerging weeds.
Break up and turn over the soil to a depth of 8 to 10 inches by using a garden fork or tiller. Blend a 2- to 4-inch layer of compost into sandy or clay-based soil to a depth of 8 to 10 inches. Compost improves sandy or clay-based soil's structure and drainage.
Enrich the soil with slow-release, 10-10-10 fertilizer at a rate of 1 pound for every 100 square feet of soil surface just before planting. Work it into the top few inches of soil with a garden rake, and water the fertilized site. Water further distributes the fertilizer.
Space tall zinnia cultivars such as ‘Ruffles Mix’ (Zinnia elegans ‘Ruffles Mix’) 12 to 18 inches apart and dwarf varieties 6 to 8 inches apart. Set all zinnias at the same soil depth at which they grew in their nursery pots.
Water the soil to a 6-inch depth. pread a 1/2- to 1-inch layer of lightweight mulch, such as shredded bark, on the soil surface between the zinnias to keep their roots moist. Leave a gap of a few inches between the mulch and the plants' stems to prevent the stems from rotting.
Plant zinnias on a cool, cloudy day or in the evening to decrease stress to the plants.
Moisture is key to growing zinnias successfully. They need 1 to 2 inches of water each week from rainfall and/or irrigation. A good rule of thumb is to water whenever the top 2 inches of soil feels dry. Water at the base of each plant until the top 6 to 12 inches of soil feels very moist. Avoid using overhead watering because it allows moisture to build up on foliage and may leave the plants open to diseases.
Zinnias may need watering twice each day during extreme heat. Check their soil's moisture level often when temperatures rise.
Zinnias are light feeders. Amending the soil with slow-release, 10-10-10 fertilizer before planting them usually provides all the nutrients they need to perform well in an in-ground, outdoor garden. They may need a boost of nutrients midseason, however, to encourage better blooming; wet the soil, and sprinkle it with 10-10-10 or 20-20-20 fertilizer at a rate of 1 pound per 100 square feet of surface area roughly three months after planting.
Potted zinnias need the same growing conditions as those grown in the ground, but consider a few factors:
- Choose a dwarf variety. Cultivars such as ‘Dasher’ (Zinnia elegans ‘Dasher’) and ‘Thumbelina’ (Zinnia elegans ‘Thumbelina’) do well in containers due to their short stature.
- Pick the right pots. Grow zinnias in pots with multiple bottom drainage holes, and ensure the pots are large enough to accommodate the plants' mature sizes. A 5- to 7-gallon container will comfortably hold three or four 4-inch plants.
- Grow zinnias in a soilless growing medium amended with 5-10-15 fertilizer at a rate of 3.75 ounces of fertilizer per 1.25 cubic feet of potting mixture.
- Space the zinnias equal distances in their pot, leaving about 2 inches between the plants and the outer edge of the pot.
- Water the soilless growing medium often, especially during hot weather. Let the growing medium's surface dry out between waterings to avoid root problems. Then add water until it trickles from the pots' bottom drainage holes. Don’t let the plants wilt.
- Feed potted zinnias with 1/2 teaspoon of water-soluble, 15-15-15 fertilizer dissolved in 1 gallon of water. Replace one watering per month with the solution.
Granular, 20-20-20 or 15-15-15 fertilizer can be used instead of water-soluble fertilizer for potted zinnias. Sprinkle the granular fertilizer onto the pots' moist soilless growing medium at a rate of 1/2 teaspoon per 1 gallon of soil. Then water the growing medium well.