Nutritional Factsheet Petunias

Petunias propagated from both seed and vegetative cuttings are considered to have a high fertilization requirement. Seed petunias can be fertilized with 150 to 200 ppm N to maximize growth, while vegetative-types require 200 to 300 ppm N. Optimal pH values are between 5.5 and 6.2. The most common nutritional disorder is iron (Fe) yellowing (chlorosis), which occurs when the substrate pH values exceed 6.4 and thus limit Fe absorption and can lead to deficiency symptoms.

Petunias require high levels of fertility. Lower fertility at 150 to 200 ppm N is required for seed propagated cultivars while higher rates of 200 to 300 ppm N are required for cutting propagated petunias. Soluble salts [referred to as electrical conductivity (EC)] found in the substrate should be lower for seed propagated petunias, while vegetative petunias require higher EC values. In cases of excessively low EC, petunias develop symptoms of lower leaf yellowing (chlorosis). Fertility should be increased to remedy this situation. If EC becomes too high, symptoms of lower leaf browning (necrosis) occur. Leaching with clear water will help to reduce excess salts.

Substrate pH values between 5.5 and 6.2 should be maintained during petunia production. A substrate pH level below ~6.4 is required to prevent Fe deficiency symptoms. The common symptom associated with this deficiency is upper leaf interveinal chlorosis, but similar symptoms may be observed with manganese (Mn) deficiency, although Mn deficiencies are rare. In sever Fe- deficient instances, leaf tissue will become white or bleached. Tissue sampling can help to determine whether symptoms are caused by Fe, Mn, or both. Iron deficiency can be remedied with an application of Fe chelate. When substrate pH becomes too low, plants can experience significant stunting. Use flowable lime to increase the substrate pH.

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