Annuals are plants that live for just one growing season, germinating from seed, flowering and producing new seeds all within a span of less than one year--sometimes, just a few months. Some annuals, like impatiens and geraniums, are actually perennials that die off due to frost or other extreme outdoor weather conditions. Both varieties can be grown indoors.
Fill cup with perlite or coarse sand and then soak with water.
Take stem cuttings from hardy annuals such as impatiens, geraniums, coleus or verbena. To make a proper stem cut, slice across one of the plant's thicker branches close to where it meets the main stem; remove any flowers or buds so they don't sap any energy from the plant as it starts to grow a new root structure.
Create a hole in the cup of perlite or coarse sand with your finger or the blunt end of a pencil, one to two inches deep. Stick the stem cutting in the hole and then gently massage the sand to fill the hole.
Allow the stem cutting to take root. The roots of most annuals should start growing after a week; water as needed to keep the soil moist.
Fill the planter box with potting soil and water thoroughly.
After the cuttings have developed healthy root structures, transplant them into the planter box and place it in the windowsill in a bright, sunny location. On cloudy days or during the day when the windowsill does not get sun, you may use a grow light.