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Discoloured Petunia Leaves

A look at Interveinal Chlorosis and solutions

How to cure the most common cause of yellow petunia leaves

Created by the team here at DIY Gardening

Welcome to another entry in our blog

Take a good look at the photo below:

Yellow petunia leaves

What you can see is a distinct yellowing of the leaves between the veins, which remain green.

This is Interveinal Chlorosis and is typically caused by one of two nutrient deficiencies in your growing medium – either too little iron or magnesium.

If left untreated, the leaves will turn a pale yellow, maybe even a whitish colour, growth will be stunted, and the plant will probably die.

Below is a photo showing the leaves in an earlier stage of chlorosis:

Early stage interveinal chlorosis

How to Determine Which Nutrient is Causing the Petunia Leaves to Yellow

If you have an extensive petunia collection, it’s certainly worth having a soil test conducted to confirm whether it’s a lack of iron or magnesium. Still, there is another way if you’re in a hurry:

Iron deficiency causes Interveinal Chloroisis in new leaves and growth, while magnesium deficiency causes it in older leaves.

So check your yellow petunia leaves to see where the chlorosis is forming. If it’s new shoots and leaves, assume it’s iron.

Petunias are quite susceptible to iron deficiency, so my hunch is that in most cases, a lack of iron will be the leading cause.

It certainly was in our case.

Yellow leaves and green leaves on petunia

Causes of Iron Deficiency in Petunias

There are many causes of iron deficiency; I’ll list them here, perhaps some of these will apply to you?

  • Using coconut coir as a growing medium – we love using coconut coir as it helps with drainage, aeration and moisture control. Unfortunately, it also deprives many plants of the iron they need to thrive via a high cation exchange rate (source).
  • Overwatering – this will flush out the iron, leaving the plant deficient. Petunias grown in hanging baskets often dry out quickly in the summer and need lots of water so take note!
  • Over fertilisation – excess amounts of some minerals, such as manganese, copper, zinc, calcium and phosphorus can lock out the iron.
  • Waterlogged soil – not usually a problem if you have holes in the bottom of your baskets or containers but waterlogged soil reduces the uptake of iron.
  • Soil pH levels are too high – the optimum pH level for petunias is between 5.5 and 6. Many still claim that between 6-7 is best, but petunias can start to grow yellow leaves at around 6.5pH. We’ve listed the best home soil testing kits here should wish to test the soil yourself.

Your listing of 6.0 – 7.5 for Petunias is incorrect. Petunias have a high Fe (iron) need and so should have a lower pH. Try 4.5 – 6.

I am the retired Director of Horticulture at the Mass. Horticultural Society.

Bruce Roberts. April 22nd 2020. Source: Almanac.com website

How to Stop Petunia Leaves Turning Yellow

Assuming your petunia leaves are turning yellow between the green veins and it’s mostly affecting new shoots and leaves, you should try adding iron chelate (aka sequestered iron) to the root area as a quick fix.

Assuming you haven’t completed a soil test, it would be prudent to feed the iron to a small number of petunias first, perhaps one hanging basket or container, before deploying the iron to all your plants.

The good news is that in most cases, the yellow petunia leaves will start to turn green within a week (see photos below).

Repeated applications may be needed but don’t overdo it, keep an eye on the leaves for yellowing and also conduct a soil pH test and lower the pH level if required.

We used this Doff Plant Tonic product on our petunia baskets which were filled with over 50% coconut coir and were clearly showing signs of iron deficiency:

Doff Plant Tonic

Doff plant tonic contains sequestered iron and magnesium and is used by growers as a solution to yellowing leaves.

Use this product as a feed and apply between March and September. 

You should see results quickly, some greening up of the leaves should be evident within a week.

We recommend watering on an overcast day where you should avoid splashing the mixture onto the leaves.

A weaker leaf wash can be applied directly to the leaves as they will absorb the iron faster, but that procedure is outside the scope of this guide as we’ve never tried it on our yellowing petunia leaves. There is a basic guide on this site should you wish to try it yourself.

Did We Resolve the Issue of Our Petunia Leaves Turning Yellow?

Yes, we did.

The Doff Plant Tonic did the trick for us.

It’s worth noting that the petunias we grew in the flowerbeds never turned yellow, but the ones grown in baskets and troughs did.

So what was the difference between the two?

The flowerbed soil was just a mixture of manure, compost and garden soil while our hanging baskets and containers had at least 50% coconut coir in them.

Coconut coir is well known for limiting iron uptake in many iron-sensitive plants. (Read more: 7 things every gardener should know about coco coir)

We were also watering them far more than the petunias grown in the ground, so iron washout was possibly a factor too.

Also, as we were keen to get some lovely photos for our guide to growing trailing petunias, we were deliberately over fertilising the baskets and containers. We eased off on the fertiliser until the yellow leaves turned green.

Now, after six weeks, our basket and container petunias are growing far better than the ones in the flowerbed; a sign that coconut coir can help with plant growth provided the grower maintains the correct nutrient balance.

A lesson for us at DIY Gardening!

Yellow Petunia Leaves – Before and After Photos

Click to see high resolution images:

Before:

Yellow Petunia leaves before treatment

Before:

Yellow petunia leaves

After 5 Days:

Petunia leaves after treatment

After 5 Days:

petunia leaves after treatment with iron chelate

Conclusion

There are many reasons why leaves may turn yellow, from overwatering, over fertilisation and lack of sunlight to more complex issues such as the growing medium blocking the uptake of iron.

Leaves may also turn yellow in different ways; for example; yellow veins within green leaves are often a sign of nitrogen deficiency.

Disease and pests can also yellow leaves; some may fall off while others may curl up or turn white.

High pH levels are also something to keep an eye out for – try one of our preferred soil pH testing kits if you don’t already own one.

We hope you found our insights into and photos of yellow petunia leaves to be helpful.

Be sure to check out Our Blog and also the Our Plants section of our site where you can see how we’re getting on with the plants currently in our gardens.

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This guide to stopping petunia leaves turning yellow was created by the team here at DIY Gardening

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