Flowering annual zinnias make for relatively maintenance-free and easy-care garden or patio colour. Zinnias have large, fast-growing seeds. The flowers perform well in the ground or in containers, offering colourful flowers from 1/2-inch to 5 inches in diameter, depending on the cultivar. The flowers attract hummingbirds and butterflies. Disease-free for the most part, zinnias suffer most often from Alternaria blight and powdery mildews. Keeping the leaves of the plants dry is the best preventive method to fight fungal diseases.
Time your planting for the best conditions. For outdoor pots, allow any frost danger to pass before planting. Seed may be started indoors up to 4 to 6 weeks before the last frost date. The National Garden Bureau suggests planting zinnias at the same time gardeners would plant peppers in your area.
Select a container with good drainage.
Select a zinnia cultivar that matches the size of your pot. Zinnia plants may reach up to 4 feet high. Taller plants may require additional support. Dwarf varieties of 8 to 15 inches are available, some of which spread to equal width.
Use a growing medium or seed-starter instead of soil. These mixtures lack weed seeds and disease-causing organisms. Such mixtures also provide good aeration and drainage because they have not compacted. Good aeration and drainage help to prevent root rot.
Water the medium until it is moist. Allow any extra water to drain away.
Place the seeds on the surface of the medium. Plant 3 or 4 seeds per pot.
Cover each seed lightly with the planting medium.
Cover the container to facilitate germination and keep moisture available within the seed's environment. A plate of glass or a strip of cling wrap may be used as a cover. Remove the cover permanently once the seedlings become visible.
Transplant excess seedlings into individual pots when they have two true leaves. Pots should be at least 2 1/4 inches in diameter.
Keep the medium moist and the temperature at approximately 23.9 degrees Celsius. Zinnias prefer full sun and warm, humid temperatures.
Allow the soil to dry between waterings, once the plants germinate. Most diseases common to zinnia can be prevented by allowing good air circulation, watering the soil instead of the plant and watering early in the day.
Fertilise zinnias once a month with a soluble fertiliser.