Impatiens require about 12-16 weeks from sowing to turn into flowering-size seedlings you can plant outside. Impatiens are extremely susceptible to cold temperatures, so make certain that all danger of frost has passed before setting them out. Sow the seeds according to package instructions. Keep the flat slightly moist and under grow-lights. Use a sterile seed-starting mix in clean plastic flats or cell-paks. Keep the container covered with a clear top until the seedlings germinate. Impatiens seeds require light to germinate, so place the seeds on the surface of the starting mix and provide them with bright light.