Common Name Zinnia 
Genus  Zinnia
Species Z. elegans, Z. haageana, Z. peruviana
Family Asteraceae
Life Cycle  Annual 
Tray Size  210 
Plug Care Zinnias are fast growers and will quickly become  rootbound in the plug trays if not planted out soon after  arrival.
Netting / Staking Not absolutely necessary but 1 or 2 layers of support can be helpful. 
Temperature Range Extremely frost sensitive - zinnias like it hot and sunny,  so wait until the threat of frost has passed, the soil has  warmed up and temperatures stay consistently above 5F at night. 
Spacing Minimum 8-12" depending on variety.
Soil Preference Well drained, fertile soil. Neutral to slightly acidic pH for  best results. 
Day Length Facultative short day (Zinnias can still flower under long  day conditions but are most floriferous when the day  length shortens). Plant zinnias in full sun. 
Pinching One pinch is recommended when the plants are about a foot tall - pinch down to three or four sets of leaves to promote side branching. 
When to Plant In the spring after all threat of frost has passed and the  soil has warmed. 
Harvesting Expect the first flowers approximately 60-80 days after  planting. Harvest flowers when fully open and when the  stem is stiff. If the flower wobbles from side to side when the stem is wiggled, it is not fully mature and the vase life will suffer. Zinnias are a cut and come again plant - expect many stems over a long flowering window. 
Post Harvest Care Place into cool water immediately after cutting. Cut  zinnias benefit from a cvbn tablet or a couple drops of  bleach in the water to increase vase life. Do not store  zinnias in a cooler as they are cold sensitive and the  petals will brown (store at or above 50F). 
Diseases / Insects Zinnias have a reputation for developing powdery mildew and alternaria leaf spot, especially later in the season. Increasing airflow between plants and keeping foliage dry when irrigating can help slow the spread of these diseases. Zinnias have few insect pests. 
Bailey's Notes

Zinnia plugs may bud early due to becoming rootbound.  Do not be alarmed, when you plant and pinch the plug it  encourages lots of side shoots and the stems will quickly elongate as the plant establishes. 

Deadhead all flowers as they fade to promote continuous flower production. Seed development saps  a lot of energy from the plant and productivity will decline. 

Resources

Zinna Elegans for the Farmer Florist

Zinna Elegans Benarys Giant