Why Are My Mint Leaves Turning Black: Everything You Need to Know to Prevent Black Spots on Mint Leaves

Although the mint plant grows tenaciously in the USDA’s (United States Department of Agriculture) zone 4 to 9 and is an extremely sturdy plant, regardless, it is not immune to problems. Do you ever wonder why the leaves of your mint plant turn witty and black? or why black spots on mint leaves? Have you tried numerous options available out there but still failed to maintain its health?

Why mint leaves turning black? Mint leaves turning black because of the 7 reasons like 1- Overwatering & Poor Drainage, 2- Humidity and Temperature, 3- Mint Rust, 4- Over Fertilization/Fertilizer Burn, 5- Leaf Blight, 6- Verticillium Wilt, 7- Mint Aphid

Luckily, my friend, you’ve come to the right place! The key to preventing block spots on mint leaves is by identifying and understanding the essentials and ideal conditions of growing mint plants.

mint leaves turning black
mint plant

In this article, we will go through the various reasons for mint leaves turning black and discuss how to fix the situation with appropriate care and caution.  

What Do Black Mint Leaves Indicate?

Black mint leaves or black spots on a mint plant indicate signs of distress. There are many reasons and ways to put your mint plant in distress. Do not worry; we’ll be covering each reason with its solution shortly ahead.

Just bear in mind that as easy it is to grow a mint plant; however, the key to keeping it healthy requires adequate attention and maintenance. So, as soon as you notice any color change in your mint leaves like mint plant leaves turning black or black spots on mint leaves, it is critical that you act swiftly to prevent it from spreading to the rest of the plant.

7 Causes of Mint Leaves Turning Black & Their Solutions

Let us look at some common reasons for mint leaves turning black and how to prevent it from happening black spots on mint leaves:

black spots on mint leaves
causes of mint leaves turning black

1- Overwatering & Poor Drainage

You may have heard that mint plants require regular watering, but keep in mind that overdoing it can lead to black or brown leaves on the bottom layer of the plant and overgrown roots. In this case, overwatering refers to watering the mint plant once every 1 or 2 days, even when it is not summer.

Additionally, mint plants have very sensitive roots; thus, insufficient drainage systems can further add stress to the plant. As they are already a tenacious and fast-growing herb, you must suspect and check for an overgrown root when you find the leaves nice and full.

Remember, mint thrives best in light soil with adequate drainage to allow easy water flow to pass through it and keep the roots healthy to sustain its growth.

Pro tip: to identify if it is just an overwatering/drainage issue, check for healthy leaves on the top of your mint plant; if they are and block spots are present only near the roots, then it is a drainage problem. 

How to Prevent Mint Leaves Turning Black from Poor Drainage and Overwatering?

The following are few ways to avoid poor drainage and overwatering of your mint plant, so that you can prevent black spots on mint leaves:

Tip #1: Water Your Mint Plant Less

  • The first step is to pick and remove any bad leaves that have turned black or brown from the plant.
  • Once that is done, start a habit of watering your plant less, especially during cool or moderate months, when the water does not evaporate easily and sits around the plant’s roots.
  • You must decrease both the amount and the frequency of watering.
  • Start by watering every three to four days that too, if the environment is humid enough.

Tip #2: Repot or Divide Your Mint Plant into a Bigger Pot 

  • If you can see any roots poking out from the drainage hole, it shows that you’ll either have to repot the plant in a bigger container or divide the plant into two separate pots.
  • To divide the plant into two, you can either take a cutting or layer the plant. Both options are suitable for keeping your mint plant healthy and natural.
  • On the other hand, if you don’t require two or more mint plants, you can transfer the existing plant into a larger container or pot. The extra space will allow the roots to spread and grow well.

Tip #3: Create a Good Drainage System 

  • First, you must ensure that you’re using the right kind of soil. Mint thrives in a well-drained, moist, loamy soil with organic matter.
  • The next step is to create two to three holes at the bottom of your plant container.
  • These holes will ensure that only the required water for your plant’s growth stays in the soil.
  • It will also prevent adequate drainage in case you accidentally overwater the mint.
over-fertilization leading to black mint leaves
high-salt content leading to black mint leaves

2- Humidity and Temperature

Mint is a rapidly growing herb that thrives best in full sun to partial shade. Direct exposure to high temperatures can dry up or burn your mint plant resulting in black leaves. And this does not necessarily occur by keeping the plant out all day long. Instead, this can also happen within a few hours of extreme heat exposure.

Therefore, it is vital to regulate your mint plant within its required temperature i.e., 69-84°F (21-29°C), but always make sure to shield it from high sun exposure. Another consequence of excessive heat is the faster evaporation of the water from the soil that will lead to dry roots and hamper your herb’s growth.

Similarly, mint plants do not grow very well in high humid environments, although they require warm temperatures. So, ensure you find the balance between both.

How to Prevent Mint Leaves Turning Black from Excessive Heat and Humidity?

The following are few ways to prevent your mint leaves turn black from high temperatures and humidity:

Tip #1: Relocate the Plant to a Cooler or Shaded Space

  • If excess heat or sunlight is bothering your mint plant, the first thing you must do is to relocate the plant into a cooler or shadier place.
  • Bear in mind that your mint plant would still require sunlight to grow and thrive. Hence, place it in a spot where it can receive the morning sun for a few hours and make sure that the noon sunlight’s heat doesn’t hit it.

Tip #2: Increase Watering Frequency 

  • If you move your mint plant to a cooler spot, the chances are that the humidity would still be high there.
  • Due to this, the water will evaporate quickly and will leave the roots dry.
  • Therefore, in colder temperatures, we suggest that you water the plant once or twice a day.
  • However, do not increase the amount of water! Split it into half and pour on the roots as routine.

3- Mint Rust

Another common reason for having black spots on mint leaves is mint rust. Mint rust occurs because of fungal spore infestation, and apart from black leaves, you’ll notice mint leaves turning yellow or bright orange during late summer. Eventually, with time these colors would turn black.

To make sure that this disease doesn’t spread to the rest of your herb or nearing plants, you should remove the infected part from the plant or isolate it from other plants.

Read on to learn how you can prevent this disease from spreading to the rest of your plants:

How to Prevent Mint Leaves Turning Black from Mint Rust?

Follow the given instructions to prevent further damage caused by mint rust:

Tip #1: Separate Infected Leaves 

  • First and foremost, if you think that the disease is at a mild stage, you must separate and remove the infected leaves from the rest of the plant.
  • Make sure you dispose of the affected leaves by either burning them or throwing them away. In either case, just make sure they are not near the potting soil.

Tip #2: Provide Better Aeration 

  • The next step is to see if your mint plant is too congested or has developed some humidity among the leaves. 
  • If that is the case, then it is time that you free up some space for better aeration.
  • To do that, you can trim the excess or withered mint leaves from the plant. 
  • Additionally, if you’ve placed your mint pot in a corner or near a wall, it is better to change its place and keep it in an airier spot to boost air circulation around leaves. 

Tip #3: Modify the Watering Technique 

  • Drops of water stacked on the mint leaves are the main cause of this fungal disease.
  • Thus, start watering your mint plant from the bottom and then move upwards.
  • Additionally, try to water your mint plant in the morning so that it gets ample time to evaporate the water.

Tip #4: Apply Fungicide

If the fungal invasion is severe and none of the three tips mentioned above work right, the last way to save your mint plant is through applying some fungicide.

One of the best fungicides to apply is Azoxystrobin, as it helps to undo the chemical effects of fungus on the mint plant and allows you to harvest faster. Here are a few recommended Azoxystrobin for mint plants: 

Pro tip: Regularly disinfect the potting container of your mint plant to avoid future fungal attacks. 

4- Over Fertilization/Fertilizer Burn

Over-fertilizing your mint plant can hamper its growth and can invite weeds to grow on it. Eventually, this makes the mint vulnerable to diseases and allow the mint leaves to turn black. This phenomenon is also commonly known as fertilizer burn.

To make things worse, the soil can dry out of essential nutrients at the same time. Thus, it is best that you fertilize your mint plant’s soil once or twice in a year to provide sufficient nutrients for healthy growth.

The fertilizer dose can easily be determined through the instructions printed on the fertilizer packet, or you can also ask the seller. Moreover, it would be best if you use a fertilizer that has organic matter in it.

How to Know If Your Mint Plant is Over Fertilized?

To determine if your plant is over-fertilized, you can do either of the following two things:

  • Look for salt builds or white matter on top of the soil as high-salt indicates over-fertilization.
  • Another easier way to check over-fertilization is by using a pH tester that is readily available at garden and plant centers.
  • If the pH tester shows a value below 7, it indicates that your soil is high in salt and needs to be fixed.
  • You can minimize the high salt content by mixing plain soil with it and create a potting mix.

5- Leaf Blight

Leaf blight distresses mint plants that are already in a debilitated state, typically due to mint rust. Although it is not a common disease, leaf blight spreads swiftly and leads to entire mint leaves turn black and fall prematurely.

This fungus can strike any time of the year, but you are most likely to witness it during cool, wet spring or fall weather and damp winters; when your mint plant goes into a dormant state and turns black. So Leaf Blight is another reason why mint leaves turn black.

How to Prevent Mint Leaves Turning Black from Leaf Blight?

With the help of the following tips and tricks, you can prevent your mint plant from becoming a prey of leaf blight:

Tip #1: Before winter approaches, try moving your mint plant into a warmer environment or keep it in a place where it can easily receive sunlight as much as possible

Tip #2: If you’ve grown your mint plant outdoors, try to transfer it indoors to provide a warmer atmosphere.

Tip #3: If you see any sign of fungal infection, the first thing you must do is to separate it from nearby plants to curb its spread.

Tip #4: Remove the affected areas by cutting them off from the stem to make sure the rest of the mint stem grows healthy.

Tip #5: To guarantee that there’s no fungal transfer, wash the rest of the healthy mint leaves with fresh water.

6- Verticillium Wilt

Verticillium wilt is the result of a toxic fungus that occurs in the soil. Symptoms of this fungus involve twisting, curling, and stunting of the upper mint leaves. The infected mint leave starts changing their color from green to yellow and eventually turn black.

Once fungi enter the soil, it stays there for several years. There’s no fungicide treatment for verticillium wilt; thus, it is best to completely eliminate the mint plant.

7- Mint Aphid

Mint Aphid is one insect that can turn the leaves of your mint plant black or black spots on mint leaves. It is usually green, yellow, brown, or black in color. It forces mint leaves to turn black and die. To save your plant from this insect, you can either apply neem oil or insecticidal soap on it.

Pro tip: If there’s a massive presence of mint aphid, spraying some insecticide on it can help you resolve the issue. 

Tips to Prevent Your Mint Plant From Turning Black 

why mint leaves turn black?
tips to prevent your mint leaves from turning black

Keeping the following points in mind can help you keep your mint plant feel less stressed and achieve healthy growth:

  • Keep your mint plant far away from driveways or pathways as it will ensure it stays safe from dust and any other physical damage.
  • Try to grow the mint plant in a particularly designed pot for herb plants as they provide adequate room for optimal growth.
  • Make sure to provide adequate lighting and sufficient water.
  • Water the mint plant only when the soil appears dry to touch. Overwatering can hamper the growth of your mint plant and lead to overgrown roots.
  • Mint plant requires more water in hot weather, and be sure that you cultivate the roots of your plants at least three inches deep and also leave additional space (3 inches or more) for the plant to reach its optimal growth at the root level.
  • Young mint leaves are more prone and vulnerable to pests and diseases; this always keeps a close look at them.
  • Never keep your mint plant in full or direct sunlight; instead, keep it in a partially shaded area.
  • Mint plants do not thrive in high-salt content, therefore leach the soil at least three times to flush out any residues.

Frequently Asked Questions (FAQ) About Black Mint Leaves

Are Black Mint Leaves Bad?

Black mint leaves are nowhere close to good, and you should not consider eating them. The first priority must be to treat the plant correctly to help it get better.

Can I Eat Mint Leaves with Black Spots?

If it is a fungal disease that has caused the black or brown spots on the mint leaves, you cannot and should not eat them! If it is due to heat or irrigation issues, they can be eaten, but only once they are properly treated. 

Can Black Mint Leaves Come Back?

If you separate and remove all the affected leaves appropriately, there is only a slight chance of it returning.

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Bottom Line

The issue of mint leaves turning black or black spots on mint leaves can certainly be solved through the right diagnosis and appropriate treatment. Just make sure to keep a careful eye on your mint plant to avoid any preventable issues. We hope this article helps you sustain your mint plant’s health and allows it to reach its maximum potential!