Nothing shouts “Summer!” more emphatically than hanging baskets brimming with masses of petunias (Petunia spp.) Where they don't bloom year-round in U.S. Department of Agriculture plant hardiness zones 10 through 11,
Feed for Flowers
The cascades of color that make petunia hanging baskets so appealing won't materialize without at least six hours of daily sun and regular doses of fertilizer. Producing all those trailing stems of flowers takes lots of energy.
To give them the best start, mix slow-release 5-10-5 granulated plant food into the soil before planting. One manufacturer recommends using 2 teaspoons for each 1 square foot of soil. After adding the petunias, fill the container with soil and water well.
Rain and maintenance watering both leach nutrients from hanging baskets. Yellowing leaves are a sign of nutrient deficiency. When they appear, begin fertilizing the petunias with liquid, 5-10-5 plant food every 10 days to two weeks for the remainder of the growing season. Mix a solution of 4 tablespoons, or the label's specified amount, of the plant food and 1 gallon of water.
Hanging baskets dry out quickly. In hot, dry weather, petunias may need watering twice a day. Overwatering, however, may rot their roots. When you do water, stop only when water begins draining from the bottom of the baskets.
Groom for Growth
Many petunia varieties need regular grooming to look their best for an entire growing season. If their spent flowers are allowed to go to seed, they'll stop blooming. Deadheading, the practice of pinching old blooms off below the thickened base where seeds form, keeps new ones coming.
If the plants become overgrown, prune them back by one-third. Using clean, sharp stem cutters, make each pruning cut just above a set of leaves. Healthy new growth and flowers soon follow.
Petunias' occasional diseases include root rot from excessively wet soil. As long as their baskets have at least four drainage holes, root rot isn't likely.
Gray mold surfaces as fuzzy, grayish to brown blotches on the stems and leaves. The fungus loves humidity, so don't overcrowd baskets. Water the petunias at their bases so the leaves stay dry .
Late blight causes water-soaked, irregular spots on young leaves. Affected leaves eventually turn brown and die. Spraying weekly with ready-to-use chlorothalonil protects the plants when warm days, cool nights and wet weather favor the disease. Spray until the fungicide drips from their leaves.
Yellow mottling on petunia leaves indicates tobacco mosaic virus. Other symptoms include stunted or distorted growth and leaf lesions. No cure exists; dispose of infected plants and their potting mix and baskets in sealed plastic bags, and disinfect any gardening tools they may have contaminated in a solution of 1 part household bleach to 9 parts alcohol.
Organic, ready-to-use pyrethrin insecticide kills both insects. Spray the plants until they drip, being sure to cover the backs of the leaves where the insects usually feed.