Why Are My Petunias Turning Yellow?

Optimal Appearance

• Healthy petunias are annual flowers that have a long bloom time and display bell-shaped flowers in a wide array of varieties. They come in multiple colors in solid or contrasting combination hues, according to the Clemson University Extension. In bloom from spring until frost, petunias often emit a sweet scent and foliage should appear strong, not spindly.

Poor Drainage

• Whether you keep your petunias as indoor or outdoor plants, it has to have optimal drainage conditions or you will see some yellowing. Loose, loamy soil types offer excellent drainage. Waterlogged plants absorb too much water and cannot properly distribute nutrients into excessively full plant parts. Its soil needs full water saturation, so light frequent watering is not a solution because they will lead to improperly formed, shallow root systems. To improve poorly draining soil, spread a layer of compost or manure 1 to 2 inches thick on top of existing soil. Use a garden fork to combine this organic matter until it's 8 to 10 inches deep within the soil, according to the University of Minnesota Extension.

Improper Planting Depth

• Just as shallow root formation causes nutrient deficiency, excessively deep planting will keep your petunias from absorbing necessary water. The lack of water in turn leads to the yellow color. When growing petunias from seeds, sprinkle small seeds on top of soil and firmly press them past the surface with a flat object like a block of wood. The Ohio State University Extension suggests planting medium and large seeds 1/8 to 1/4 inch deep. If you're transplanting a seedling into the garden, plant the petunia with the pot and match the soil line within the pot to the outer soil. If you remove the pot, maintain the same soil level the plant experienced in the pot.

Inadequate Sun Exposure

• Petunias need at least six hours of direct sun a day to stay a healthy color according to the University of Minnesota Extension. As sun exposure decreases and shade increases, decreased flower production and spindly stems will accompany the yellowing. Petunias should be planted in open areas away from heavy cover from other plants and trees.

Proper Care Requirements

• Petunias need balanced fertilizers like 8-8-8. Apply the fertilizer in weekly distribution of 2 lbs per 100 square feet for outdoor use. According to the Clemson University Extension, the petunias need a liquid fertilizer every three weeks starting in July and should be kept in soil with medium fertility and a pH of 6.0 to 7.0. Sprinklers should be used to water them once every week.