Wave petunias (Petunia x hybrida), touted as easy-to-grow plants with prolific blooms, can still have problems. These tropical plants are usually treated as annuals, though they're technically perennials in U.S. Department of Agriculture plant hardiness zones 10 through 12. Several factors, including water, weather conditions, sun exposure, availability of nutrients and root health can cause these colorful prolific bloomers to decline or die.
Water: Too Much or Too Little
While Wave petunias do need a decent amount of water to thrive, they will not do well if weather conditions are constantly wet, the plants are being over-watered or soil drainage is poor. If the roots are constantly saturated, bacterial and fungal diseases, such as black root rot and Botrytis, may develop and the petunia will eventually die. On other hand, if Wave petunias do not get enough water, they will also perish. Wave petunias thrive in hot weather, but need adequate water. In the heat of summer, they will need daily watering if planted in containers and every two to four days if planted in the ground.
Full Sun Only
Wave petunias need sun to flourish. They prefer to be planted outdoors in full sun and will decline if planted in shady areas or indoors. Make sure these sun-loving plants are getting at least six hours of direct outdoor sun a day. In addition, to avoid excessive stress to the plants, plant them as early in the season as possible when the danger of frost has passed, if you live in a cool climate. This gives the petunias time to develop a larger root mass. If planted in full sun on a hot day, the petunias may die from transplant shock.
Nutrients: Heavy Feeders
Wave petunias thrive when fertilized regularly and heavily. Commercial growers and nurseries who grow these plants fertilize heavily to produce plants that look healthy and flower prolifically when put up for sale. To ensure continued flowering and avoid shocking the plant from this heavy fertilization regimen, apply a granulated, slow-release fertilizer, such as a 19-6-12, in the hole or container when planting. Use 1 tablespoon for a 10-inch pot, 8 tablespoons for a 24-inch pot and 3 tablespoons per 4 square feet of garden. At 70 degrees Fahrenheit, the granulated fertilizer will last three to four months. At 90 F, the fertilizer is active for one to two months.
The Root of the Problem
The petunias' leaves and stems may tall of problems below ground. If the plants have been sitting in their original small pots for too long, they can develop girdling roots. The roots begin to circle themselves and eventually strangle the plant. To prevent this, plant your petunias as soon as possible in the ground or larger container. When planting, gently tease the roots apart and spread them out if they are showing signs of circling. Wave petunias form dense mats of roots close to the surface. This is normal and is not an indication the plants will strangle themselves.