The proper amount of light is critical for petunias to produce lush, healthy foliage and ample flowers. Lack of sunlight leads to weak plants that are more prone to disease. Without sunlight, petunias also flower poorly with few blossoms through the otherwise productive summer months.
Petunia seeds require sunlight to germinate. Sow the seeds on the soil surface and place them in an area that receives bright light to ensure they sprout. Both seed-grown and purchased seedlings must be supplied with plenty of light until you are ready to transplant them outside. Set the petunias in a sunny window where they receive at least six hours of direct light a day. If you don't have a suitable window, use fluorescent grow lights to provide enough light for the petunias to produce healthy, compact growth.
Full Sun Beds
Plant petunias in beds that receive full sunlight whenever possible. Petunias flower best and produce the most compact growth when supplied with six to eight hours daily of sunlight. Before transplanting petunias to a full-sun bed, harden them off so they don't suffer leaf scalding from the bright light. Place potted petunias in an outdoor area protected from direct sunlight. Gradually move them into direct sun over the course of seven days before finally planting them in the garden bed.
Partial Sun Beds
While not preferred, petunias can grow in partial-sun beds. When this is necessary, choose a bed that receives full morning sun and dappled sunlight or bright shade in the later afternoon. Petunias may not flower as well when planted in partial sunlight, and the plants may become leggy and weak looking. Prune the plants by up to half their height at midseason if they are weak and spindly. Pinching helps encourage a new flush of growth and makes the plants appear fuller.
Container-planted petunias also do best in full sunlight, whether you are using a planter or hanging basket. The high amount of light causes the soil to heat up and dry out more quickly in the container. Water daily to help prevent the soil from drying out too much. When possible, set the containers in an area where the petunias receive full sun but the base of the container receives shade. For example, A half barrel of petunias sitting behind a low shrub is shaded at the container level but sunlight can reach the petunias planted in the top unhindered.