There are several ways to deadhead petunias, including hand pinching and shearing. Pruning them every few weeks, or at mid-summer, will help them to fill out and bloom longer. Method 1 Pinching Petunia Blooms
Check what type of petunias you are growing. Find the seeds or the plant markers. If they are new types of petunias, like Wave or Tidal Wave, they don’t need to be deadheaded.
Many new petunias have been engineered to be low-maintenance. They will fill out without being deadheaded.
Start with pinching petunia blooms. If you have never deadheaded a flower before, you may feel more comfortable pinching off spent flowers than cutting the plant itself.
Get close to the plant, like you are weeding. Fading blooms can be hard to see after a few weeks, and you will need to get your hands dirty. Avoid wearing thick gardening gloves, because you are more likely to hurt the plant.
Find a spent bloom above a new bloom. Move 1/4 inch (0.6 cm) up, or just above the bud. Grasp it with your thumb and forefinger and pull.
It should come off easily. Discard the bloom in the compost.
Repeat with each dead bloom on the same stem. Then, move onto a new stem. Herbaceous plants like petunias can have dozens of blooms on a single stem, so plan to deadhead every few weeks during growing season.
Pinch growing tips at mid-summer. If you find your petunias are getting “leggy,” meaning each stem is long and hanging toward the ground, you should pinch off growing tips. Grasp the stem lightly and find the thickest bud at the top of a set of blooms.
Pinch it between your thumb and forefinger to pluck it off. In this case, you are plucking an actively growing part of the plant, instead of a dying part.
This type of deadheading will encourage new buds below that point to bloom. It will also make the plant look thicker and healthier.
Pruning Petunia Blooms
Allow your petunias to start blooming. You should wait to prune the plants until they are getting 6 or more hours of sun a day and they are filled with blooms. Once blooms start to die, you can start deadheading.
Get a sharp pair of pruning shears or scissors. Unlike deadheading by hand, the plant will do best with a sharp cut.
Pick up a petunia stem gently. Choose 1 that has several dying blooms on it. Find a point just below all the dead blooms.
taken from http://www.wikihow.com/Deadhead-Petunias Look at this website for some great pictures to explain how to do this.
Till next time, this is Becky Litterer, Becky's Greenhouse, Dougherty, Iowa