Zinnia: Indoor Sow or Direct Sow or Potted Plant Annual
How to Sow and Plant
Zinnia may be grown from seed sown early indoors and transplanted outside after frost, or sown directly in the garden after frost, or from potted plants.
Sowing Seed Indoors:
- Sow indoors 5-7 weeks before outdoor planting date in spring using a seed starting kit
- Sow seeds ¼ inch deep in seed starting formula
- Keep the soil moist at 70-75 degrees F
- Seedlings emerge in 7-10 days
- As soon as seedlings emerge, provide plenty of light on a sunny windowsill or grow seedlings 3-4 inches beneath fluorescent plant lights turned on 16 hours per day, off for 8 hours at night. Raise the lights as the plants grow taller. Incandescent bulbs will not work for this process because they will get too hot. Most plants require a dark period to grow, do not leave lights on for 24 hours.
- Thin to one seedling per cell when they have two sets of leaves.
- Seedlings do not need much fertilizer, feed when they are 3-4 weeks old using a starter solution (half strength of a complete indoor houseplant food) according to manufacturer’s directions.
- Transplant hardened-off seedlings to the garden after the frost.
- Before planting in the garden, seedling plants need to be “hardened off”. Accustom young plants to outdoor conditions by moving them to a sheltered place outside for a week. Be sure to protect them from wind and hot sun at first. If frost threatens at night, cover or bring containers indoors, then take them out again in the morning. This hardening off process toughens the plant’s cell structure and reduces transplant shock and scalding.
Sowing Directly in the Garden:
- Direct sow seeds in average soil in full sun after all danger of frost.
- Prepare the soil by removing weeds and working organic matter into the top 6-8 inches of soil; then level and smooth.
- Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
- Sow seeds evenly 12 inches apart and cover with ¼ inch of fine soil.
- Firm soil lightly with your hand, water and keep evenly moist.
- Seedlings will emerge in 7-10 days.
- Thin seedlings to stand 18-24 inches apart, depending on the variety, when they are about 1-2 inches tall.
Planting in the Garden:
- Select a location in full sun with good rich moist organic soil.
- Prepare the bed by turning the soil under to a depth of 8 inches. Level with a rake to remove clumps of grass and stones.
- Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
- Dig a hole for each plant large enough to amply accommodate the root ball.
- Place the top of the root ball even with the level of the surrounding soil. Fill with soil to the top of the root ball. Press soil down firmly with your hand leaving a slight depression around the plant to hold water.
- Water thoroughly, so that a puddle forms in the saucer you have created. This settles the plants in, drives out air pockets and results in good root-to-soil contact.
- Use the plant tag as a location marker.
How to Grow
- Keep weeds under control during the growing season. Weeds compete with plants for water, space and nutrients, so control them by either cultivating often or use a mulch to prevent their seeds from germinating.
- Mulches also help retain soil moisture and maintain even soil temperatures. For annuals an organic mulch of shredded leaves lends a natural look to the bed and will improve the soil as it breaks down in time. Always keep mulches off a plant’s stems to prevent possible rot.
- Keep plants well-watered during the growing season, especially during dry spells. Plants need about 1 inch of rain per week during the growing season. Use a rain gauge to check to see if you need to add water. It's best to water with a drip or trickle system that delivers water at low pressure at the soil level. If you water with overhead sprinklers, water early in the day so the foliage has time to dry off before evening, to minimize disease problems. Keep the soil moist but not saturated.
- Until plants become established, some protection from extreme winds and direct, hot sunlight may be necessary. Good air movement is also important.
- After new growth appears, a light fertilizer may be applied. Keep granular fertilizers away from the plant crown and foliage to avoid burn injury. Use low rates of a slow release fertilizer such as Flower-tone, as higher rates may encourage root rots.
- Pinch young plants to encourage branching unless you are growing them exclusively for cut flowers and want long stems.
- Remove spent flower heads to keep plants flowering until fall. Zinnias make terrific cut flowers, and cutting the flowers encourages new blooms.
- Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.
- Remove plants after they are killed by frost in fall to avoid disease issues the following year.
Growing Tips
- Plant zinnias in mixed plantings with other summer-blooming annuals, or place small groups of zinnias among perennials. They are at home in cottage and children’s gardens, and they are often grown in cutting gardens.
- Shorter zinnia varieties are ideal for containers. Take care not to overcrowd them or the flowers may be significantly smaller than they should be and the plants may be taller. Always use a commercial potting mix, do not use garden soil, and make sure the containers have adequate drainage. Container grown plants will require extra water and fertilizer, look for signs of wilt or a nutrient deficiency.
- Cut stems before the flower is open for cut flowers.
- Zinnias attract hummingbirds, butterflies and beneficial insects.
Common Disease Problems
Alternaria Leaf Spot: Small, round reddish brown spots with white to gray centers form on the upper surface of the leaves and along the midrib. The lesions may encircle the stems and cause wilt. This disease is worse in warm, wet or very humid weather. Burpee Recommends: Avoid getting water on the foliage. Remove infected plant parts and do not work around wet plants. Provide plenty of air circulation. Contact your Cooperative Extension Service for fungicide recommendations.
Aster Yellows: Plants are stunted, develop witch's brooms (excessive growth), petals turn green and become deformed. This virus-like condition is spread by leafhoppers. Burpee Recommends: Remove infected plants and destroy them. Control leafhoppers. Remove weeds in the area which serve as alternate hosts to the disease.
Bacterial Leaf Spot: First signs are small translucent spots with a broad yellowish edge that slowly enlarge and become angular or irregularly circular with a reddish center. It thrives in cooler temperatures. The disease may also affect and disfigure flower heads. Burpee Recommends: Remove infected plants. Rotate crops with plants in a different family. Avoid overhead watering. Do not work around plants when they are wet.
Botrytis: This fungus causes a grey mold on flowers, leaves, stems and buds. It thrives in cool wet weather conditions. Burpee Recommends: Remove affected plant parts, avoid watering at night and getting water on the plant when watering. Make sure plants have good air circulation. Contact your Cooperative Extension Service for fungicide recommendations.
Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.
Common Pest and Cultural Problems
Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.
Leafhoppers: Leafhoppers cause injury to leaves and stunt growth. They also spread disease. Burpee Recommends: Remove plant debris. Use insecticidal soaps. Consult your Cooperative Extension Service for other insecticide recommendations.
Japanese Beetles: Burpee Recommends: Hand pick early in the morning into a bucket of soapy water.
Plants are shorter than is characteristic for the variety and the flowers are much smaller: When zinnias are overcrowded they can become stunted and produce smaller flowers. Always follow the spacing recommendations for each variety.
Spider Mites: These tiny spider-like pests are about the size of a grain of pepper. They may be red, black, brown or yellow. They suck on the plant juices removing chlorophyll and injecting toxins which cause white dots on the foliage. There is often webbing visible on the plant. They cause the foliage to turn yellow and become dry and stippled. They multiply quickly and thrive in dry conditions. Burpee Recommends: Spider mites may be controlled with a forceful spray every other day. Try hot pepper wax or insecticidal soap. Check with your Cooperative Extension Service for miticide recommendations.
Zinnia FAQs
Should I start my zinnias indoors or outdoors? Zinnias may be started indoors or directly in the garden. If you have good conditions for growing them indoors such as plant lights, a heat mat for germination, then you can get a head start by starting them indoors. But zinnias are easy to direct sow and bloom fairly quickly from seed so direct sowing may be a better option for some gardeners.
Can I grow zinnias as a houseplant? Zinnias require full sun and hot temperatures to grow well and may not perform well in many homes. Shorter varieties may be tried in a warm sunroom during the summer.
How do I condition my cut zinnia flowers? Bring a bucket of water with you to the garden when you cut your flowers and put them in the water as soon as you cut them. This will allow them to take up water through the freshly cut stem. Cut on an angle to give more surface area for the water to be taken up. Keep them out of the direct sun.
Do I need to stake my zinnias? Taller varieties may benefit from staking, especially if they are in a windy area.
Why are my zinnias tall and thin with much smaller flowers than expected? If zinnias are planted too closely together they will produce tall plants and much smaller flowers. Always follow the recommended spacing on the planting instructions.