The biggest obstacle to growing zinnias successfully indoors is providing enough light. They require full sun which means at least six hours of direct outdoor sunlight each day. Outdoor sunlight is much brighter than natural indoor lighting, but there is a way around this. It will just take a little extra effort.
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Choose Short or Dwarf Cultivars
- Creeping zinnias (Zinnia angustifolia) do well in containers. The Cascade Beauties series (Zinnia angustifolia Cascade Beauties) and Crystal series (Zinnia angustifolia Crystal) grow to 10 inches tall. They can be grown in regular containers or hanging baskets.
- Dwarf common zinnias (Zinnia elegans) like Short Stuff series (Zinnia elegans Short Stuff) and Small World series (Zinnia elegans Small World) can be grown in containers. Short Stuff zinnias grow to 10 inches tall while Small World zinnias top out at 6 inches.
Select the Right Container
Use 6- to 8-inch containers for zinnias that only grow to a height of 6 to 8 inches. Use 10- to 12-inch-wide containers for larger cultivars. Always use containers with drainage holes in the bottom so the excess water can drain away.
Pot Them Up
Buy zinnia seedlings from the center and pot them up in more attractive containers using houseplant potting soil. Zinnias can also be grown from seed. They are easy to grow from seed and there is a much wider selection of cultivars from which to choose. Buy seeds online or at a garden center. Start the zinnia seeds directly in their permanent containers with houseplant potting soil. Set the containers in bright, indirect light and keep the soil moist. They should germinate in a week.
Pinch the growing tips off the stems when the zinnias reach 4 to 6 inches tall to encourage bushier plants with more branches and flowers.
Provide Plenty of Light
Giving zinnias enough light is key to getting plenty of flowers. While you can germinate them next to a sunny window, you'll need to give them artificial lighting if you want them to thrive indoors.
Things You'll Need
- Three- to four-bulb fluorescent light fixture
Set the seedlings in front of a south-, west- or east-facing window where they will get at least a few hours of direct natural sunlight. Move seedlings grown from seed into direct sunlight as soon as they germinate.
Set a three- or four-bulb fluorescent light fixture up so the bulbs will be 4 to 6 inches above the tops of the zinnias. Install special fluorescent high-intensity grow bulbs in the fixture. You can buy these at garden centers, hydroponics stores or online.
Plug the fixture into a 24-hour timer. Set the timer to turn on at sunrise and off 18 hours later.
Give them Water and Fertilizer
Water the zinnias when the top of the potting soil begins to dry. Empty the water catch saucers beneath the pots after the water drains through.
Remove flowers of common zinnias as they fade to encourage them to produce new flowers. There is no need to remove the flowers on creeping zinnias.
Give them a balanced 10-10-10 or 20-20-20 houseplant fertilizer every two weeks. A common dilution rate for 20-20-20 fertilizer is 2 teaspoons in 1 gallon of water, but this varies. Follow the manufacturer's dilution recommendations. Pour the fertilizer solution evenly over the soil immediately after a regular watering.