Zinnias are a popular annual in the flower garden. You can grow zinnias indoors if you grow them under fluorescent grow lights or in a greenhouse. Choose one of the many shorter varieties of zinnias so the pots won't topple as the mass of the top growth surpasses the combined mass of the pot and its soil. Zinnias will last for months indoors if you remove the faded flowers.
Prepare the growing containers. Fill 6-inch to 8-inch pots to within an inch of the top with indoor potting soil, the kind that has fertilizer in it. Tap the bottoms of the pots on a hard surface to settle the soil.
Sow three to four seeds in each pot. Cover with 1/4 inch of soil.
Fill a large shallow container with an inch or two of water. Place the pots of planted seeds into the shallow container. When the surface of the soil in these pots looks damp, remove from water and let drain. Mist the soil daily to keep it moist until the seeds germinate.
Place the pots under fluorescent grow lights as soon as seeds germinate, in about 10 days. Provide a way to raise the lights as the plants grow, keeping them about 3 to 4 inches above the tops of the plants. Alternatively, you can grow zinnias in a heated greenhouse.
When plants get their third set of leaves, thin to the strongest seedling in each pot.
Keep the seedlings relatively moist until they are well established, then water when the surface of the soil looks dry. Do not let the pots sit in water that drains out of the bottom of the pots after watering.