Petunias are a self-seeding annual. In other words, they propagate seeds on their own, spreading throughout your garden. Eventually, the spread will slow or stop as available soil nutrients are used. To accommodate a healthy, self-seeding petunia plant, you must leave enough space on each side of the plant.
Petunias will typically cover a 3-foot diameter after they begin to seed. This means you need to leave about 1 foot, 6 inches on each side of the petunia plant.
To ensure healthier petunia growth, you should prune dead flowers. So-called "deadheading" will help ensure that the healthy flowers get enough nutrients, even as your petunias are expanding to fill their ideal 3-foot radius.
To ensure your petunias grow healthily in your garden, you must keep the soil moist to the touch. Petunias also require full sun; even moderate shade can hinder petunia growth.