Basil (ocimum basilicum) has to be one of the most popular herbs for home gardeners. And most garden enthusiasts growing basil has faced the problem with basil turning yellow.
There are several different reasons why basil leaves can turn yellow. Some you can fix and others simply mean that it is time to give up on the plant and move on.
Here today we will go through 10 reasons why basil leaves are turning yellow. I will also list how to address each problem to help you grow basil successfully.
When basil leaves turn yellow we say they show signs of chlorosis (yellowing).
10 reasons basil leaves turn yellow are:
- Over-watering leading to root rot
- Soil is too compact
- Basil downy mildew
- Lack of nutrients
- Soil pH is hurting basil plant
- Wrong temperature for you basil plant
- Not enough light for your basil plant
- Pests like aphids attacking your basil plant
- Wrong type or size of pot
- Basil plant at the end of its lifespan?
1. Over-watering leading to root rot and yellow basil plants
Basil likes a well drained soil that is moist to wet but never soaking.
When you grow basil it is easy to over-water as the plant grows fast and seems to respond to all you do.
But over-watering your basil plant can lead to root rot and the yellow leaves on your basil plant is the way you find out.
Remember the general rule for watering herbs: water thoroughly but not too often.
Your basil plant will respond better to a mild dry-out than the root system sitting in a constant pool of water.
If you see basil leaves turning yellow you should first of all pay attention to your watering habits.
- Examine your pot. Does it have a drain hole for excess water? If not, replant the basil into a clay type pot with a drain hole. If your pot has a drain hole, move on to step 2.
- Insert a finger 1-2 inches deep into the soil. If the soil is soaking you have your answer. If the soil is moist to dry, move on to step 2.
- Gently remove the pot from the saucer. Is there water or signs of moisture on the saucer? If so, it is likely that your root system is sitting in constant wet. If the saucer is dry, move on to step 4.
- This step involves removing the plant for the pot and inspecting the root system. Healthy root systems are white and pleasant to the touch. If the roots are dark and slimy to the touch your basil is suffering from root rot.
You can try removing affected roots where possible and replant in a new pot. But to be honest it is often better to dispose of the affected plant and start again.
And again, do remember that most herbs prefers a mild dry out to having the root system constantly sitting in waterlogged soil.
2. Basil leaves turning yellow when Soil is too compact
Plants need oxygen as much as it needs water and nutrients.
And if your soil is too compact your basil plants can turn yellow due to lack of oxygen.
Healthy soil has a balance between growing medium (for example soil), air and water.
You want a loose well draining soil with good structure. The structure from organic materials at varying stages of decomposition create pockets of air where the roots can breathe.
But if your soil is too compact there are no pockets for air and yellow basil leaves can be a sign of the plant not getting enough oxygen.
You can usually tell if your soil is too compact by simply pressing the solid. Another tell-tale sign is that water sits in puddles on the surface of your soil when you water your plant.
The solution is to simply replant your basil plant and this time make sure that you include for example LECA pellets or perlite to help aerate your soil.
3. Basil downy mildew
Basil is particularly susceptible to a disease called Basil Downy Mildew [1].
Downy Mildew is not a true fungus or fungal disease and is very difficult to treat when symptoms become visible to the eye.
You will see white powdery spots and coatings develop on your leaves especially during mild and humid weather conditions.
Downy mildew will drain your basil plants by weakening their foliage and your basil leaves will turn yellow as a result. The disease can spread by wind, seeds as well as by other infested plants.
It is important to note that the best form of treatment is prevention.
Work to control moisture in your growing environments and if you have many plants you should consider removing infested plants to be safe.
For prevention, there are commercial pesticides but also good organic alternatives.
My favorite is Neem oil mixed with water to be sprayed on plants when weather conditions are mild and moist.
Downy mildew needs water to survive. By controlling moisture in your growing environment and watering your plants from underneath you can minimize the risk of plants being infested.
4. Yellowing leaves due to nutrient deficiency
Herbs do not need extra nutrients. Planting basil in good quality potting soil is enough. Right? Well, it is not that simple.
And the reason for your basil plants yellow leaf discoloration could be due to poor soil and the fact that the plant quite simply needs more nutrients.
When you grow herbs from seeds you know not to use fertilisers to germinate your seeds. You want a lean potting mix to avoid burning the fresh, new roots. All the energy the seed needs to grow is right there inside the seed.
And when you later on transplant your seedlings you buy or mix new potting soil that holds enough nutrients for the seedling to develop into a plant.
But this level of nutrition may not be enough. Especially not for plants like basil where we continuously harvest and watch our plants develop into larger and bushier plants.
Basil plants respond well to slow release fertilisers as they grow and develop. Liquid fertilizers are by design fast to release. Slow release fertilizers come in granules that look like pebbles or beads and release nutrients into the soil over several weeks to months.
Add a slow release fertilizer to your soil when you replant your basil plant or even when you first transplant your seedling. But do check for how long the fertilizer will release nutrients as it can vary from a couple of weeks to several months.
And also remember, the more you have watered you plant the more nutrients have been washed out of the soil.
When basil plants suffer from lack of nutrition you can often see lower leaves turning yellow first. The plant uses the available nutrition for new growth while older and established leaves are left to yellow and wither away.
5. pH level is hurting basil plant
Every herb and plant has its preferred range of pH levels. As a general rule most herbs and garden vegetables fall in the range of 5,5 – 7,5.
Basil is an interesting herb in that it prefers a pH range of 6,5 – 7,5. That is a slightly acidic to a slightly alkaline soil (neutral pH 7.0). And if your basil plant turns yellow you could have a problem with the pH level of your soil.
There are several ways to test pH levels of soil, but my preferred method is using a pH meter.
You simply push the probe into moist soil and after about 1 minute you have your reading. You can buy a pH meter for less than $10 on Amazon and you will find it useful.
If your soil tests are too acidic you simply add lime to make it more alkaline (more than 7).
Should your tests be too alkaline you add ammonium sulfate to make it more acidic (less than 7).
6. Wrong temperature for you basil plant
Basil likes to grow in a warm but not hot environment. You are looking for a growing environment of approximately 20 degrees Celsius / 70 degrees Fahrenheit.
You also do not want to subject basil to temperatures below 15 degrees Celsius / 60 degrees Fahrenheit or windy conditions.
If you do subject your plant to unfavorable conditions you may find your basil leaves turning yellow. This is again a tell tale sign that something is wrong.
On the bright side this is an easy fix. Simply move your plant to provide the conditions needed:
- approximately 20 degrees Celsius / 70 degrees Fahrenheit
- no wind
7. Not enough light for your basil plant
All plants convert light energy into chemical energy via Photosynthesis.
And if you are not giving your basil plants enough bright sunlight you will see leaves turning yellow to eventually fall off.
Basil needs a minimum of 6-8 hours of natural sunlight per day. And if you are growing basil indoors during the darker months of the year you need to increase this to 16-18 hours a day.
And yes, this may mean investing in a grow light to help your plants grow and thrive.
When I grow basil using Deep Water Culture hydroponics I set my timer to run my grow lights from 6 in the morning until midnight.
8. Pests like aphids attacking your basil plant
Aphids are the size of a pinhead and can be black, green and pink. They are most common during the warmer months of the year and can be seen in clusters on new shoots and flower buds.
Aphids excrete a sweet, sticky substance that ants love. It is actually often the presence of ants on plants that make gardeners aware of aphids or other sucking pests.
Yellow basil leaves can be a sign of aphids attacking your basil plant.
Aphids do have natural enemies like ladybirds and hoverflies and it is also possible to squash them by hand.
If the infestation remains you can spray the plant with an insecticide or use an organic alternative like Neem oil to fight the infestation.
9. Wrong type or size of pot
If your plant struggles and your basil leaves are turning yellow you may be dealing with a plant that has outgrown its home.
Basil plants grow large and bushy when we harvest our plants continuously during the season.
And this also means that the root system grows and develops.
If the plant looks poorly and the leaves turn light green or even yellow you should inspect your basil plant’s roots. Chances are that your plant has outgrown its pot.
Remove the plant from its pot and inspect the root system. If it has more roots than soil you need to transplant your basil into a larger size pot.
10. Basil plant at the end of its lifespan?
Most types of basil are grown as annuals and eventually they die.
Basil is a plant that grows fast and produces rich harvests throughout the growing season.
And eventually the plant will start to look tired and the basil plant turn yellow or sometimes just a bleak shade of green. Another sure sign of the end of the life cycle is the leaves turning brown around the edges.
Before you invest too much time into reviving your plant you should ask yourself if you can expect the plant to produce more.
If the stems are turning woody it is often a tell tale sign that the basil plant has come to the end of its lifespan.
Summary and conclusion
You now know 10 reasons why basil plants can turn yellow.
But what is the best way to approach the problem when it happens to you?
Start by examining the external factors.
- Over-watering leading to root rot
- Check for compact soil
- Wrong temperature for you basil plant
- Not enough light for your basil plant
- Wrong type or size of pot
- Plant at the end of its lifespan
- Soil pH is hurting basil plant
Next inspect your plants and check for:
- Basil downy mildew
- Fungal diseases and bacterial leaf spot
- Pests like aphids attacking your basil plant
And then finally add some energy to help that basil plant bounce back to life.
- Lack of nutrients
I sometimes get the question if you can eat yellow basil leaves. To be honest I never had but from what I have read it should be safe. But why would you? Basil is known for its vibrant green color and why would you want anything but that.
Helpful resources:
[1] https://agrilifeextension.tamu.edu/asset-external/basil-downy-mildew/
University of Minnesota Extension
University of Illinois Extension