Why Do Basil Leaves Turn Black? (UNCOVER THE SCIENCE!)

Are you a home chef? Do you have a basil plant on your windowsill? If so, you may have noticed some of the leaves turning black.

You might be wondering why this is happening.

Don’t worry, we have the answers! In this article, we will uncover the science behind why basil leaves turn black and how to prevent and treat it.

We’ll explore the effects of cold temperatures, pests, inadequate sunlight, and over-fertilizing on basil plants.

By the end, you’ll have the knowledge and tips needed to keep your basil healthy and green!

Short Answer

Basil leaves can turn black due to a variety of reasons.

One of the most common causes is due to a fungal infection, which is usually caused by overwatering or high humidity.

Additionally, the leaves can turn black due to environmental stress, such as too much direct sunlight, cold temperatures, or nutrient deficiencies.

Finally, the leaves can also turn black if they are bruised or damaged due to rough handling.

What Causes Basil Leaves to Turn Black?

Basil leaves can turn black for a variety of reasons, including cold temperatures, pests, inadequate sunlight, and over-fertilizing.

Cold temperatures can cause basil leaves to turn black due to frost damage.

When temperatures drop to freezing or below, the water inside the basil leaves can freeze and expand, causing the cells to rupture and turn black.

This is known as frost damage and is one of the most common causes of black basil leaves.

Pests such as aphids, spider mites, and whiteflies can cause black spots on basil leaves as well.

These pests can suck the sap from the leaves, causing them to turn black and eventually fall off.

Not getting enough sunlight or too much fertilizer can also lead to blackened leaves.

Basil plants need six to eight hours of sunlight each day to thrive, and too much fertilizer can burn the roots and cause the leaves to turn black.

If you notice your basil leaves turning black, its important to take action quickly.

Check the basil plants for pests and adjust the amount of sunlight and fertilizer accordingly to prevent further damage.

Pruning off the affected leaves can also help your plants bounce back.

Finally, make sure to mulch your basil plants to insulate the roots and protect them from extreme temperatures.

Cold Temperatures

When basil leaves are exposed to cold temperatures, they can easily become damaged and turn black.

This is because basil is a tropical plant and its leaves are not adapted to handle cold weather.

When the temperatures drop below 50F (10C), the leaves can become damaged and will start to blacken and die.

Additionally, if frost forms on the leaves, this can also cause them to blacken and die.

As a result, its important to take measures to protect basil plants from cold temperatures during the winter months.

This can include covering the plants with blankets or sheets, or bringing them indoors.

If the basil plants are already starting to blacken, it may be too late to save them, but taking precautions to protect them from further damage can help.


When it comes to basil leaves turning black, pests can be one of the major culprits.

Common culprits include aphids, spider mites, and whiteflies.

These pests feed on the basil leaves, leaving behind black spots in their wake.

Aphids are small, pear-shaped insects that feed on the leaves of a plant by sucking out the sap.

Spider mites are tiny, eight-legged mites that spin webs on the underside of leaves to protect them from the elements.

Whiteflies are small, white flying insects that feed on the underside of leaves.

In addition to leaving behind black spots, pests can also cause the leaves to curl or distort.

If you notice any of these symptoms, it is important to inspect your basil plants for signs of pests.

To protect your plants from these pests, it is important to practice good gardening habits such as avoiding overcrowding your plants and providing adequate drainage.

You can also use natural pest control methods such as neem oil and insecticidal soap to get rid of the pests.

Inadequate Sunlight

Basil is an herb that loves a good dose of sunshine, and not getting enough of it can cause its leaves to turn black.

Basil is a Mediterranean plant, so its used to hot and sunny days.

But if youre growing it in a cooler climate, you may need to give it some extra help.

If your basil is planted in a spot that gets less than six hours of sunlight a day, its likely that the leaves will start to turn black.

To combat this, make sure your basil is planted in a spot that gets plenty of sun.

If youre growing it indoors, put it in a bright spot near a window.

If youre growing it outdoors, make sure its in a sunny spot and not in the shade.

Its also a good idea to rotate the plant every few weeks to ensure that its getting an even amount of sunlight.

Another way to give your basil some extra sun is to use a grow light.

These lights will mimic the suns rays and keep your basil healthy and vibrant.

Many gardeners also use grow lights to extend the growing season and give their plants a head start in the spring.

Finally, make sure youre watering your basil correctly.

Too much or too little water can cause the leaves to blacken.

Water your basil deeply once a week and make sure the soil drains well.

If youre growing your basil in a pot, make sure youre using a potting mix that drains quickly.

With the right combination of sunlight, water, and soil, your basil should stay healthy and green.


Over-fertilizing can lead to blackened basil leaves, and it is a common cause of discoloration.

When fertilizing your basil plants, it is important to make sure you are not using too much fertilizer.

Too much fertilizer can cause a buildup of salts in the soil, which can lead to a decrease in the plants ability to absorb nutrients and cause the leaves to turn black.

It is best to use a fertilizer that is specifically designed for basil, and to follow the instructions on the package.

Additionally, try to avoid fertilizing too often, as this can also lead to an accumulation of salts in the soil.

If you do notice blackened basil leaves, check the soil to make sure it is not too salty, and adjust the amount of fertilizer you are using.

Prevention and Treatment

When it comes to preventing and treating basil leaves that have turned black, it is important to investigate what caused the blackening in the first place.

Is it due to cold temperatures, pests, inadequate sunlight, or over-fertilizing? Once the cause has been identified, it is possible to take steps to prevent further damage.

For example, if cold temperatures are causing the blackening, it may be necessary to provide more insulation for the basil plants to protect them from frost damage.

If pests are the cause, then it may be necessary to use an appropriate pesticide to eliminate the pests.

If sunlight or fertilizer is the cause, then it may be necessary to adjust the amount of sunlight or fertilizer to prevent further damage.

In addition to prevention, it is also important to treat basil leaves that have already turned black.

If the leaves are only lightly blackened, then it may be possible to prune the affected leaves and allow the plant to continue to grow normally.

If the leaves are heavily blackened, then it may be necessary to remove the entire plant and start again from scratch.

In either case, it is important to take steps to prevent further damage, such as providing more insulation from cold temperatures, using an appropriate pesticide for pests, and adjusting the amount of sunlight or fertilizer.

Tips for Healthy Basil Plants

Healthy basil plants require the right mix of sunlight, water, and nutrients. To ensure your basil plants stay healthy and strong, here are some tips you should follow:

1. Sunlight: Basil plants prefer full sun, so make sure to give them at least six hours of direct sunlight each day. If your area gets too hot in the summer, you can provide some shade during the hottest part of the day.

2. Water: Basil plants should be watered regularly, but not too much. Too much water can cause the leaves to turn black, while not enough water can cause the plant to become stressed. Water your basil plants when the soil surface is dry to the touch.

3. Fertilizer: Fertilizer should be used sparingly on basil plants. Too much fertilizer can cause the leaves to turn black. Use a balanced fertilizer with a ratio of 10-10-10 or 20-20-20 and mix it into the soil at half strength.

4. Temperature: Basil plants prefer warm temperatures, but can be damaged by cold. If temperatures drop below 55 degrees, the leaves can turn black. If you live in an area with cold winters, make sure to bring your basil plants inside or provide a cold frame for protection.

These tips should help you keep your basil plants healthy and free from blackened leaves.

If you follow these guidelines, you should have a thriving basil garden that’s perfect for adding flavor to your favorite dishes!

Final Thoughts

Basil leaves turning black is a common problem that can be caused by a variety of factors.

By being aware of the causes, such as cold temperatures, pests, inadequate sunlight, and over-fertilizing, you can take steps to prevent and treat this issue.

With the right care, you can enjoy a healthy and vibrant basil plant! So next time you see your basil leaves turning black, follow these tips to help bring your plant back to life.

Peter Kirsch

Peter is an avid gardener and herbalist. He loves learning about the healing and medicinal properties of herbs and enjoys writing about them. He’s been passionate about herbs since he was a child and has learned a lot about them over the years. He’s written several articles for various publications, all about herbs and their uses. He’s also spoken at several conferences and workshops about the topic.

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