Petunia Leaves Turning Yellow? | Causes and How To Fix

Petunias are annual plants that produce colorful masses of flowers that may be plain or patterned. They are a popular summer flower that can be planted in beds, pots, or hanging baskets. 

If you have noticed the foliage of your petunia looking less green and want to know why you are in the right place. I have compiled a list of the most common causes of petunia leaves turning yellow and how to fix them.

Why Do Petunia Leaves Turn Yellow?

Chlorophyll is a fundamental pigment present in all photosynthetic plants. Without chlorophyll, plants, including petunias, cannot photosynthesize and will die as a result. 

The green color that is synonymous with plant foliage and stems is caused by Chlorophyll. Without it, leaves will appear yellow due to the lack of this pigmentation. This is known as chlorosis and it can occur for a variety of reasons. 

Read on to find out more about how to identify the symptoms for each cause and ways to remedy or prevent them. 

What Causes Petunia Leaves to Turn Yellow?

Petunias are annuals meaning they have a life cycle of one growing season, during which they continuously bloom. In warmer climates, they may last around 3 years. 

Yellow petunia leaves are an indication the environmental conditions are inadequate. It’s important to identify the cause so you can resolve the issue promptly. 

Overwatering

Petunias favor full sunlight but do not like to be dry. During particularly hot weather they should be watered daily, otherwise, a deep and thorough watering weekly to keep their soil moist should suffice. 

However, good drainage is important to avoid petunia plants becoming overly saturated and waterlogged. Signs you have overwatered your petunia include yellow leaves, wilting flowers, and overall drooping of the plant. 

The latter symptom is likely a result of root rot, which is when fungi that thrive in damp conditions attack the roots, slowly killing the plant. 

To revive an overwatered petunia, I recommend removing it from the soil and pruning off any black or brown roots. 

Re-pot using fresh potting soil that contains coco coir, and grit to aid drainage and compost to provide nutrition and replant your petunia. 

Make sure the topsoil is dry before watering your petunia, to avoid overwatering.

Iron Deficiency

Iron deficiency is another key cause of chlorosis in petunias. Iron is a mineral essential for the formation of chlorophyll and so photosynthesis. Iron is taken up from the soil through the roots. However, the poor availability of iron impedes the plant’s ability to photosynthesize. 

Symptoms of an iron deficiency include yellow leaves with green veins that may turn brown at the edges as the cells die. The youngest leaves at the shoot tips are normally affected first but can spread throughout the plant. 

If you suspect an iron deficiency, you should test the pH of your petunias soil, as this affects iron uptake. Petunia with an alkaline pH should be treated for iron deficiency. A chelated iron spray can be sprayed directly onto the leaves, whilst an iron-rich fertilizer can be mixed into the soil. 

Over-Fertilizing

The saying “too much of a good thing” is certainly true when it comes to fertilizer. Excessive fertilizer can alter the pH of the soil, hindering nutrient uptake. Additionally, over-fertilizing can also cause chemical burns to the roots, potentially killing the plant.

Symptoms of overfertilization include yellow leaves, wilting, stunted growth, and burned leaf margins. Overfertilized petunias are also more vulnerable to disease and insect pests.

It is possible to save an overfertilized petunia if you act at the first signs. You should stop fertilizing them immediately and water thoroughly to help disperse and flush fertilizer from the soil. If liquid fertilizer was used as a foliar spray, you should rinse the leaves with water. 

Soil pH

The pH of soil directly impacts the plant’s ability to absorb important nutrients from the soil including manganese, iron, sulfur, magnesium, potassium, and nitrogen. Petunias grow best in acidic soil with a pH of around 6.0.

Chlorosis is a common symptom of a pH that is too alkaline causing plants to be unable to absorb essential nutrients, particularly iron. Other notable symptoms of an incorrect soil pH include stunted growth, brown spots on leaves, and reduced blooming. 

Before attempting to alter the balance of your soil, you should test the pH. If the pH confirms alkaline soil, you can add compost, mulch, or a sulfur treatment to the soil to make it more acidic. 

Aphid Pests

Aphids are the most common insect pests in petunias. Aphids are normally found in clusters on the underside of leaves. They suck out the leaf sap causing discolored, curled, and wilted leaves.

Aphids are also responsible for spreading the tobacco mosaic virus (TMV), which petunias are susceptible to. TMV causes a brown and yellow mottled pattern, which are parts of the leaf the virus has killed. 

TMV may not kill your petunia, but it can seriously stunt its growth and damage the flowers. Unfortunately, there is no cure for TMV and you should discard any infected plants and disinfect anything that has come into contact with the plant to prevent the virus from spreading.

The best way to treat an aphid infestation is to dislodge them from the leaves using a strong stream of water. You can also spray them with insecticidal soap to kill any present aphids and prevent future infestations. 

If you enjoyed this article, why not read Why are my African Violet Leaves Turning Yellow?

FAQ Petunia Leaves Turning Yellow

Can Petunias Receive Too Much Sun?

Petunias are very heat tolerant and thrive in the full sun. They will produce the best and biggest blooms when located in direct sunlight all day. The more shaded they are, the fewer flowers you will see. 
Petunias do not need protection from the sun and favor temperatures between 61oF and 80oF during the day.

Citations

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.