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It’s going to take some detective work on your part to figure out why the leaves on your sunflowers are turning yellow – the plant’s way of signalling that it’s in distress or even dying. The problem could be over-watering, an imbalance of nutrients, not enough space and yet other reasons. We cover the key problems that cause this unwanted development and discuss ways in which to put things right for the continued good health of your sunflower.
Are you giving your sunflower too much water?
Although sunflowers do like moisture, there’s a point where the soil around them becomes saturated with water, especially if the soil doesn’t drain very well. Feel around the ground below your sunflower plant. If it’s water-logged, you have a problem.
Turn over the top of the soil and dig in a little sand and some organic material/compost to help drainage. This also aids the soil in holding the right amount of water and delivers nutrients to the roots of the plant. In addition, reduce how much water you give to the plant. Established sunflowers need regular watering around the root zone until the soil is moist to a few centimetres down.
Is the sunflower getting the right nutrients?
Not getting any nutrients, or getting too much or too little of the right ones can cause the plant to suffer. Even though sunflowers don’t usually need fertiliser, soil with poor integral nutrients should be supplied with the appropriate nutrients/fertiliser so that the sunflower can develop strongly and healthily.
The most important nutrient for sunflowers is nitrogen. Too little nitrogen leads the stem to weaken, the leaves to turn yellow and the plant to start dying. MiracleGro recommends its Water Soluble All Purpose Plant Food as the fertiliser to use for these plants but I have had some great success with tomato feed too. Be careful though as adding too much nitrogen to the soil results in the sunflower producing fewer blooms but many leaves.
Is the sunflower crowded in?
All plants need space to grow and to have adequate air circulation around their stems and leaves. Sunflowers grow deep roots that need to spread out easily. If rocks or other root systems are in their way, the growth of the plant may become stunted and the leaves won’t receive all the energy they need to remain green. This results in, among other signs, yellow leaves.
In addition, if your sunflower is in competition with other plants for moisture and nutrients, it may not even receive all that it needs in the first place. Yellow leaves may be the result again.
Check that there’s enough room around your sunflower above ground and that its root system isn’t being squished by the roots of other plants. If it is, either transplant it to a more open area or move some of the other plants out.
Are brown or black spots appearing on the yellowing leaves?
This is bad news as it indicates that the plant probably has a fungal infection, rot or mould. You have to do something about this immediately. The most common sunflower infection is Verticillium Wilt caused by a microscopically small fungal virus. Signs of the infection starts with the leaf surface between the veins turning yellow and then brown, the leaves wilting and drying out, and finally dying.
Remove the infected leaves and don’t put them in your compost bin. Keep an eye on the plant as if the fungus has made it into the plant stem, it can spread to the other leaves as well.
Is your sunflower old?
If you don’t find any of the clues we give you in this article in your plant, your sunflower leaves may be yellow because the plant is getting old. Most things lose colour as they age and your sunflower is no exception, it is at this point the heads start to turn brown and the seeds can be harvested which is one of my favourite things to do.
Other sunflower problems?
We have other articles that give great advice about solving common problems that get in the way of successfully growing sunflowers. Head over and read Why are my Sunflowers not Flowering? Why are my Sunflowers Drooping? Why are my Sunflower Leaves Curling? and our article about growing sunflowers from seed, How to Grow Sunflowers.