How to Grow Impatiens in a Greenhouse

Fill a plug flat with a soilless seed starting potting mix. For best results, use a 406-plug flat, because it's big enough to host the impatiens until you can transplant them outdoors.

Place one impatiens seed on the surface of each plug section in the plug flat. Because impatiens seeds are very small -- approximately 60,000 seeds in a single ounce -- you may find it easier to use tweezers to pick out individual seeds. Alternatively, dip a toothpick in water and then stick it in your collection of seeds; seeds will cling to the wet surface of the toothpick.

Spread a thin layer of fine grade No. 3 vermiculite or perlite over the surface of each plug section in the plug flat. This helps trap moisture around the seed while also allowing light to penetrate, which impatiens seeds need for optimal germination.

Place the plug flat on the southern, sunny side of your greenhouse. If your area doesn't have full sun yet, use fluorescent lights suspended 6 inches above your plug flat; leave lights on for 12 hours every day.

Mist the plug flat surface with water twice a day or as necessary to keep the flat consistently moist. Once the seeds sprout -- germination takes approximately 10 days -- reduce watering to once per day.

Adjust your greenhouse temperature to 72 to 75 degrees Fahrenheit until the impatiens seeds sprout, then lower the temperature to 68 to 72 degrees in the evening and 70 to 75 degrees during the day. If your greenhouse has a powered heater, adjust the thermostat. Otherwise, open or close your windows or greenhouse vents as necessary to achieve the right temperature.

Fertilize your plug flat once a week. For the best impatiens development, use a standard fertilizer labeled for use on houseplants, but applied at 25 percent of the recommended strength.

Harden off your impatiens seedlings once the last frost date has passed in your region. Remove the plug flat from the greenhouse and place it in a shaded, wind-shielded outdoor location for two days. Gradually increase the amount of sun the flat gets each day.