Answer: Petunias are great in full sun. In order to get full, bushy plants with lots of blooms, you need to pinch out the growing tips. Everywhere you pinch the plant will produce two stems where there once was one. You don't have to pinch out a lot - just use your fingernails to pinch out the tiny little leaves developing at the very tip of each stem. Petunias will also set lots more blooms if you take the time to pinch off the spent flowers as they begin to fade. If you leave the old blossoms on they will develop seed, using energy that the plant could otherwise use to produce new flowering stems.
Best wishes with your petunias!
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