Sunflowers are quite simple to grow from seed and can be found flourishing in gardens around the world.
Despite the ease of cultivating them, like any plant, growing a sunflower isn’t without its problems.
Discoloration of leaves is a common concern amongst gardeners, with the question ‘why are my sunflower leaves turning yellow?’ (and often also ‘why are the leaves on my sunflowers turning brown?’ which this article also addresses) being asked quite regularly.
The good news is often the answer is quite straightforward, and can be easily rectified once you have identified the cause of the problem.
So without further ado, let’s dive in.
Why Are My Sunflower Leaves Turning Yellow?
If your sunflower leaves are turning yellow (or brown) it is most likely that, one way or another, your sunflower is not getting enough nutrients. Make sure it has sufficient room to grow, is not overwatered and gets enough sun. Keep an eye on any bugs frequently visiting the plant too, as they can stop vital nutrients from getting to its leaves and causing them to discolor.
1: Your Sunflower Isn’t Getting Enough Nutrients
When I was researching this article someone stated, “sunflowers are absolute gluttons”, and that is not an over-exaggeration.
They are heavy feeders and consume nutrients at a high rate and if they aren’t getting enough, expect to see changes in the color of their leaves.
The most common deficiencies are in nitrogen and phosphorus. Yellow leaves that remain green near the veins indicate a lack of nitrogen, while yellow leaves with a purple or reddish tinge indicate a lack of phosphorus.
You can solve this by using a fertilizer with added nitrogen or phosphorus.
2: The Pot Your Sunflower Is Planted In Is Too Small
If you are growing your sunflower in a pot, you need to make sure the pot gives the plant’s roots enough room to grow otherwise it will become root-bound.
In my experience, if you are going to use a pot I would look for at least a five-gallon planter to provide enough soil and nutrients for the plant.
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You will also want to make sure the pot allows for proper drainage, so you are not flooding the plant when you water it.
If you do decide to repot your sunflower, make sure you do it gently and efficiently taking care not to disturb the roots.
Water it once you have repotted it and then wait for the top to be dry before you water it for a second time.
3: Your Sunflower Is Getting Too Much Water
If the leaves on your sunflower are turning yellow, it could be because it is getting too much water.
During the germination phase sunflowers do require a lot of water. However once they are growing, around 1 inch to 1.5 inches per week should be enough for a healthy plant.
Sunflowers do need a consistent supply of water, but too much water can leave them waterlogged.
A good rule of thumb is to give your sunflower or sunflowers enough water to moisten the top six inches of the soil. When the topsoil is dry, water again.
4: Your Sunflower Is Not Getting Enough Sun
Unsurprisingly sunflowers like A LOT of sun! They require at least six to eight hours of unfiltered sunlight per day.
A word of warning though, if you are asking why the leaves on your sunflower are turning brown, it could be because it is dehydrated.
It is rare, but if a sunflower is exposed to excessive heat above and beyond 32C or 33C for long periods of time it could wilt.
If you live in an area where your summers are extremely hot and dry, keep an eye on its water intake to make sure it does not get dehydrated.
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5: Your Sunflower May Have Fungal Rot
Fungal rots can affect various parts of sunflowers, including their root, stem and crown, and are also a common cause of leaves on sunflowers turning brown or yellow and wilting.
Fungi living in the soil attack the plant, moving upwards, with often the first sign being dark brown spots on the leaves and stem or foliage that appears dull or yellows without warning.
Initially, you may think this is a sign your sunflower isn’t receiving sufficient water, but if your plant appears to be wilting after being watered, and has brown spots on its leaves then fungus is most probably responsible.
The only way to deal with this issue is to remove your sunflowers as they could cause infection in other plants.
If you decide to plant sunflowers again, make sure you plant them in a different area of your garden and not in the previously affected soil.
6: Your Sunflower May Have Fungal Spots On Its Leaves
Yellow or brown spots on a sunflower leaf can also be a sign of a smaller fungal infection.
Whereas fungal rot will also affect the roots of a sunflower, a fungal infection will affect just the leaves.
This tends to be identified by yellow discolored spots on leaves with brownish or purple fuzz underneath.
To treat this remove any leaves that are badly affected, and spray the other leaves with a mixture made up of a tablespoon of baking soda, half a teaspoon of liquid soap, a teaspoon of vegetable oil and a gallon of water.
7: It Could Be Sunflower Rust
If the leaves on your sunflower are covered in what looks like a reddish, brown powder with yellow halos on top this is most likely sunflower rust.
This is caused by a fungus called Puccinia Helianthi. It needs water on the surface of leaves to prompt growth, so a good way to reduce the risk of this is to make sure you water at the base of the plant.
If left untreated sunflower rust can kill your sunflower, but if you deal with it early it should not cause any significant damage.
Simply remove any infected leaves and then burn them or put them in a bag and dispose of them.
8: Insects Could Be To Blame
Unfortunately, many bugs find your sunflower just as appealing as you.
In fact, your sunflower could be home to a variety of bugs that are feeding on the plant’s sap, and depriving its leaves of nutrients causing them to turn yellow or brown. Culprits include:
- Beetles: they chiefly attack young plants in the spring and summer.
- Aphids: these tiny insects can be found in clusters on the underside of leaves sucking the juice from plant tissue.
- Spider mites: almost invisible to the naked eye, spider mites weave tiny webs on the plant.
The best way to deal with any of these is to use an organic insecticide or horticultural oil to kill them.
9: It Could Be Nature Taking Its Course
If it is leaves towards the bottom of the plant turning yellow or brown this is natural.
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It means these leaves have done their job getting the plant started, and the sunflower is now shedding them so it can concentrate on its blooms.
In this case, you don’t need to do anything, they will die naturally.
Final Thoughts
As you can see if your sunflower leaves are turning yellow (or brown), it could be for a number of reasons.
The most important thing to remember is that, in most cases, once you identify the cause of the problem there is a way to solve it and encourage healthy growth.
In the worst cases, for instance, severe occurrences of fungal rot, it can be fatal and you might have to chalk it down to experience going forward.
Overall though sunflowers are easy to grow, require minimal maintenance and are pretty adaptable plants.
They are perfect for first-time gardeners and will add a huge amount of color to your yard.