The flowers appear in July through to September, and grow up to 750mm (30") high, making them a good border and cut flower plant.
Planting out in a sunny area is essential to get full benefit of their attributes.
Sowing times need to be adjusted to suit your locality, i.e. little is gained by starting them too early, to do so can mean plants sitting around awaiting the right planting out conditions.
Patience is of the essence, i.e. to plant out prematurely may check the plants and they will never grow well afterwards.
The two main problems that affect zinnias are powdery mildew and alternaria blight a fungus that causes reddish-brown lesions on leaves.
To avoid these, try and keep water off the leaves when watering, and give adequate spacing to plants.
Routine spraying with a fungicide can be an effective preventative measure.
Week 18: Sow seeds in pots / trays of seed compost and germinate at a temperature of 20°- 25°C (70°-75°F)
Germination should only take three or four days.
Week 19; Prick out seedlings when large enough to handle into 70mm (3") pots of potting compost, and grow on at 18°C (65°F)
When they have reached their second true-leaf stage, reduce the heat and harden them off at around 10°-12°C (50°-55°F).
Plants should be kept well watered as they are establishing, but avoid excessive watering to reduce the risks of fungal diseases.
Week 22: Plant out in light, rich, well-drained soil in a sunny sheltered spot 300mm (12") apart.
After planting out they may sulk until they have taken hold and become acclimatized to their new situation.
Week 24: Boost the plants by raking in a high-potash fertiliser.