To keep petunias bushy and healthy all summer, purchase the right plants from your local garden center. Look for ones with only a few, if any, open flowers and only a few buds. Then, ensure they get adequate sun, water, fertilizer, and pruning. Shear them back if they become leggy, and apply liquid fertilizer to encourage a flush of new growth and more flowers. Lastly, deadheading will help keep them blooming continuously. Proper care and maintenance will make your petunias look beautiful all season.