What causes the leaves of your mint plant to turn yellow? The most likely causes are overly moist soil, over-watering, and not enough sunlight. Let’s tackle each of these causes in order of probability.

The most common reason why the leaves of this otherwise rapidly growing herb would turn yellow involves overly moist soil. Your soil can retain quite a bit of water depending on its components. For example, if your soil is mostly clay, it’s drainage is going to be very different than if your planting medium is mostly sand.

Clay tends to hold quite a bit of moisture. This can lead to root rot. This, of course, can have a very negative effect on your mint plant’s health. If root rot is not the problem, the clay in your soil might just retain too much water so as to choke your plant. You might even say that your plant is drowning, and that’s exactly what would be happening in this case.

Please understand that plants normally breathe through their leaves. But they also respire through their roots. When there’s not enough aeration at the root level, this can cause a lot of health problems for the plant and in the case of fast-growing mint, this can lead to yellow leaves.

Moist soil can lead to fungus issues

Whenever there’s excess moisture in the soil, fungus problems are not far behind. A wide variety of fungi species prefer moist soil. They can spread easily through the soil and this leads to a wilting disease that severely restricts water flow in plants.

The mildest version of this disease produces leaf yellowing. Believe it or not, this is the least of your worries because if your mint plants get hit by the worst forms of fungi, your plants will die. They can be so virulent that they flat out choke water transport through the stem of your plant. Put in another way, fungus, regardless of type, are bad news.

Verticillium Wilt

In particular, be careful of verticillium wilt. This is one of the most common fungal infections afflicting mint plants which cause leaf yellowing and other problems.

Rust

Rust is another common fungal attack that can affect mint.  It can cause severe stunting to your mints and can even kill them.  You’ll start seeing what seems like blisters at the bottom of the leaves. Eventually, the leaves would turn yellow and start falling off.

Flame weeding is effective in killing rust.  It’s a thermal method to kill weeds with flames.  In the case of rust in a small garden, however, a propane torch is quickly passed over the infected areas of the plant.

If the infection is severe, it’s best to remove the plant quickly and destroy it.  The healthy plants should be transplanted to a new bed and monitored regularly for symptoms.  It would also be a good idea to burn the roots in the infected bed to kill the spores on the soil.

Insects

Another common cause is insects, in particular, aphids. You can tell if you have an aphid problem on your hands if your mint leaves pucker, curl, and then turn yellow. This is due to the fact that aphids are underneath the leaves feeding.

They suck nutrients and this leads to yellowing. The good news is you can easily handle aphids by using organic insecticides involving soap mixtures. If you want to get rid of your aphid problem as quickly as possible, you can use a wide variety of insecticidal soap formulations.

Powdery Mildew

Another common cause of yellow mint leaves is powdering mildew. You can tell your plant is suffering from this when you flip over the leaves and you see this powdery, grayish fungus growing underneath. This is a serious problem because not only are they unsightly and turn your mint leaves yellow, but they actually kill the leaves and as you know, plants without leaves aren’t going to remain alive for long.

So, it’s really important to get a handle on the problem by spraying fungicide on afflicted leaves and surrounding areas. You’d also do well by inoculating the soil with fungicide to prevent the problem from coming back

Underwatering

If your mint isn’t getting enough water, its automatic response would be to drop leaves to conserve water. The leaves would turn yellow and then wilt before falling off. If you see this happening and notice that the soil around the base is dry, then the most likely reason is that your mint is thirsty.

Try to remember the last time you gave it some water.  If you realize you may have been remiss in watering your mint, then make it a point that it gets the attention it needs. But if you’ve been watering it regularly, then there may be another reason why your mint is acting up.

Soil and Water Quality

Deficiency in nutrients will cause mint leaves to turn yellow. The soil may lack some nutrients your mint needs like nitrogen.  Hard water can also be the culprit because it will cause salt and calcium to build up around the roots that will deprive your mint of water.

Shade

In certain low nutrient situations, too much shade and inadequate nutrition can lead to yellow mint leaves. This problem is pretty straightforward. In an indoor setting, you’ll know this is the case if the leaves are yellowing on the side facing away from the source of light.  You only need to move the plant to where there’s a lot more light or you may want to open your shade or direct your light source so it can reach more parts of your mint plants.

Also, it’s a good idea to schedule the light cycle in your indoor space in such a way that your plant gets a longer light exposure. You probably would have to wing it and improvise until you get the right amount of light exposure to your plant.  If it’s still not enough, you might consider rigging up some artificial light.