Cut back the drooping stems about halfway through the growing season when the petunia begins looking lanky and overgrown. Trim off the stems by about half with the pruning shears.
Discard the stems you pruned away. Do not leave the trimmed stems on the soil around the petunia plant because it may contribute to disease.
Provide a thorough watering of the petunia plant immediately after you prune.
Mix the fertilizer with water according to package instructions. Pour the fertilizer over the soil, taking care not to splash water onto the foliage of the petunia plant.
Water the petunia as much as necessary to keep the top inch of soil from drying out. Continue the careful watering over the next one to two weeks while the petunia recovers from the hard pruning.
Fertilize the petunia one or two times a month for the remainder of the growing season.