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The reasons for your agapanthus leaves turning yellow or brown range from nature to the plant’s environment to the care you’re giving the plant. You need to do a little detective work to determine what’s wrong with your plant. To sum it up quickly before I go into more detail further down. If they are turning yellow, the first thing to consider is iron chlorosis caused by a lack of iron in the soil. I usually use soil test kit will confirm this.
If the soil pH is higher than 7 or lower than 6.5, the plant’s roots probably cannot take up the iron in the soil. Apply some sulphur in this case. Then you have sunburn, which can cause yellowing leaves, and finally, look into pests or diseases. Finally, overwatering can also cause the leaves to turn yellow as the roots suffer from root rot. Learn more below:
Natural life cycle
If your agapanthus is deciduous rather than evergreen, the leaves on the plant naturally turn yellow and fall off in the late autumn and winter months, usually after flowering.
What to do
Nothing. Your plant is in its natural life cycle.
Roots too tightly bound can make the leaves on agapanthus leaves turn yellow
In my guide, How to grow and care for agapanthus as well as in How to grow agapanthus in pots, I explain that this plant likes its roots to be contained in a smaller space than most other plants. But there’s a limit to how tightly bound the roots can become before they’re unable to take in nutrients and send them up to the rest of the plant. The leaves then become yellow or brown.
What to do
Check the roots of your agapanthus. If they’re tightly bound up together, then they’re not sending up enough water and nutrients to the leaves.
I always Divide the plant in the spring every few years to combat this before it even happens. I have instructions for this in Dividing Agapanthus – How and When to Divide Them, Step by Step guide.
Just remember not to give them too much room in the new pot as this can lead to a poor display of flowers instead.
Lack of iron leading to yellowing leaves
Agapanthus leaves turning yellow could be a lack of iron as already explained at the start if this guide. This is an essential nutrient for the plant’s health. It’s probably the cause of the problem if the yellowing is on the new growth but the leaf veins remain green. A lack of iron could mean that the soil lacks this mineral or that your plant isn’t making full use of the iron in the soil.
What to do
Use a soil testing kit to measure the pH of your soil. If it’s higher than pH7, then the plant’s roots have difficulty in absorbing the iron in the soil. Reduce the pH to 6.5 or lower with an application of sulphur by following the instructions on the packet.
If the pH of the soil is OK, then use a fertiliser with additional iron. And look at how you water your plant. Over-watering can leach the iron from the soil before the plant takes it up.
Sunlight – too much or too little – more sun the better
Agapanthus develops its best blooms when grown in full sun, but too much direct sunlight is harmful. Like us, plants can become sunburned, though leaf scald or leaf scorch are the horticultural terms. This results in the leaves first turning yellow and then a crisp brown.
If the plant is located in a wind tunnel, then windburn is much the same thing. The harsh winds, especially if they’re cold, dry out the leaves. The plant can’t replenish the water and the green changes to yellow and brown.
On the other hand, too little sunlight is also a problem. As well as preventing flowers from blooming, too little sunlight doesn’t support the amount of photosynthesis the agapanthus plant needs. This means that there isn’t enough energy to produce the green chlorophyll in the leaves and they remain at the yellow stage.
What to do
Figure out which sunlight problem your agapanthus is having. Or perhaps it’s the wind. Move your plant to a more hospitable location. It’s easy enough to do if the plant’s in a pot. But you may have to dig up a plant that’s in the garden to change where it’s planted.
Look out out sap-sucking pests
Two kinds of bugs in particular love dining on agapanthus plants. Both mealybugs and red spider mites are sap-sucking pests that suck out the juices from the plant’s leaves and stems, leaving behind a weakened plant. If there’s enough of these insects and spiders to form a large infestation, the leaves die off and turn yellow.
What to do
I discuss both mealybugs and spider mites and what to do about them in my article Orange tree pests and diseases – Identify and Treatment. Head over there to learn how to identify if these are the problems that are turning your agapanthus leaves yellow. The treatment is the same for agapanthus plants which is why I link to it here.
Agapanthus Diseases that lead to yellowing and brown leaves
Root rot and bulb rot
Root rot and bulb rot are both fungal diseases that start underground so they’re not visible until harm has been done to your agapanthus. These diseases are caused by over-watering your plant and leaving it in standing water. But if you’re shouting out right now that you’re careful about how much water you give your plant, the soil could still be waterlogged if the water doesn’t drain away properly.
And both these diseases result in brown leaves on your agapanthus.
What to do
Root rot, in particular, is such a common problem that I devote an entire article to identifying it and reducing its effect on your plant. See Phytophthora Root Rot – prevention and treatment for everything you need to know about how to deal with this disease. And follow the same instructions for bulb rot, but look at the bulbs underground instead.
Anthracnose is a fungal disease that flourishes in humid temperate climates. It’s just waiting for the summer months and you over-watering your plant and perhaps leaving moisture on the leaves. The first signs of this widespread disease are brown spots on the leaves. Look closely at the spots and see if they have a purplish ring around the edge. This is what distinguishes this disease from some others.
If the anthracnose infection turns out to be severe, the spots turn black as the disease progresses. The leaves turn yellow, die and fall of the plant.
What to do
Immediately take off the infected foliage using a sterilised cutting tool. Don’t put the diseased debris on your compost pile as the disease spreads rapidly and easily. If most of the plant is affected, you may have to get rid of the entire agapanthus.
The spores of this disease also live in the ground. Cover the soil with a layer of mulch to avoid the spores splashing up into the plant during rain storms.
You can also apply a fungicide to the plant. Those containing copper, sulphur or chlorothalonil are most effective.