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Petunias

Grandiflora Series: Storm™, Bravo™, Frost™, Ultra™

Milliflora Series: Picobella™

Multiflora Series: Hurrah™

Trailing Series: Plush®, Ramblin’™

 

1. Soil and pH Needs

Even though petunias are very adaptable and will grow in almost any kind of soil, they do best in a light but rich soil that has good drainage. Heavy clay soils will retard root growth and starve the plant of oxygen, which can lead to root diseases. Soil should be amended with fully composted organic matter prior to planting and in the case of clay soils the addition of pea gravel equal to at least 25% of the soil volume will aid in aeration. If adding organic matter, be aware that organic matter that has not been fully composted will tie up nitrogen and sulfur leading to nutrient deficiencies and poor growth.  Soil pH should be 5.5 - 6.2 and should be tested prior to planting but after adding any amendments. This can be done quite easily with a portable pH tester available for about $100. Mix one part soil with two parts distilled water, stir well and wait thirty minutes, then follow the instructions for using the pH meter. If pH is too low, the addition of lime is warranted. However, the rate will vary depending on how much you need to raise the pH and the type of soil you are dealing with. Clay soils or those with a lot of organic material have a high buffer capacity and require more lime than sandy soils, which have a low buffering capacity. A good soil testing lab can determine the lime requirement index (LRI) of your particular soil and can recommend types and rates of lime to use. The frequency of watering will depend on the type of soil, weather conditions and the amount of mulch. Mulch will not only reduce soil water evaporation but will also reduce splashing of water onto the lower leaves, moderate soil temperatures and reduce weed competition. Because they're quite drought-tolerant, petunias seldom need daily watering other than what they receive with rain; in prolonged periods of drought, however, watch that the soil doesn't get too dry.

 

2. Fertility

Petunias are moderate to high feeders in the landscape; and can have a crop life of four – six months. An application of a balanced controlled-release fertilizer (CRF) such as 10-10-10 or 14-14-14 with a 3-4 month formulation applied to the bed at planting will keep the plants well fed, depending on the amount of irrigation required and the average daily temperature. In areas of high irrigation and high temperatures it may be better to use a formulation with a slower release rate since higher temperatures will cause the fertilizer to be released quickly. Fertilizers that are not CRF are not recommended since the fertilizer will be leached out of the root zone long before the crop needs the nutrients. In high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce chlorosis (yellowing) of the foliage.  Yellow foliage on petunias may also be caused by a nitrogen deficiency within the plant. Both can be corrected with the proper supplemental fertilizers. 

 

Iron deficiency due to high pH

  

3. Disease

Today’s petunia cultivars are quite disease-resistant, but they can have a few problems you may deal with--or take precautions to avoid. Young plants are susceptible to Botrytis, a fungus that is also soil-borne and spreads quickly from infected plants to healthy ones. It thrives in cool, moist conditions and forms a powdery mold on stems, leaves and flowers. Make sure the plants are spaced correctly to provide good air circulation, avoid overhead irrigation or at least irrigate early in the morning so the flowers dry before nightfall. Petunias are also susceptible to viruses. Petunias with a virus can have foliage that is stunted and deformed, often with light-green streaks, and also discolored and deformed flowers. The best control is to remove and destroy diseased plants and keep aphids and other insects such as thrips which transmit the virus under control. 

 

Petunias showing symptoms of virus

 

More Petunias showing symptoms of virus

  

4. Insects

Petunias in the landscape can be bothered by several pests. Flea beetles may eat holes in the leaves of the plants. The Budworm Caterpillar (small green caterpillars) can attack from late June through July and feed on the flower buds. You won’t see the actual caterpillar, but you may notice small black droppings and small holes in the leaves and buds. The caterpillar will disappear by the end of July, but you could apply Bt (Bacillus thuringiensis), if it’s a major infestation. Thrips can feed on the upper surface of leaves leaving them silvery looking; they also like to feed on the pollen in the flowers which can distort their appearance. Remember that healthy, vigorous plants are less susceptible to pest damage than unhealthy plants. 

 

Thrips damage on petunia foliage

  

5. Tips for Success

  • The most important requirement for growing petunias successfully is a location with plenty of light. Petunias need at least five - six hours of good sunlight; they'll perform even better when located in full sun. The more shade they receive, the fewer flowers they'll produce. 
  • Petunias tolerate lots of heat, and are relatively undemanding when it comes to water. Leave sprinklers on long enough to soak the soil to a depth of six - eight inches every time you water. 
  • Whenever feasible, it's a good idea to remove faded flowers, including the portion below each flower where seeds will develop. This practice, called "deadheading," encourages blooming by preventing seed maturation. Although it may not be practical to deadhead masses of petunias in the garden, it's a must for flowering annuals in containers. Deadheading not only helps prolong blooming, it also keeps plants looking fresh, healthy and well-groomed. 
  • Remove weeds from the plantings that compete with the petunias for moisture, nutrients and light.

 

 Information provided by Syngenta Flowers