If you end up with a yellow leaves basil plant for no apparent reason, welcome to the club. Before growing the massive basil indoor, I managed to yellow (and wilting) quite a few.

Basil leaves turn yellow for 9 reasons:

1. Excess of water causing root rot
2. Too compact soil that prevents breathing
3. Diseases like powdery mildew
4. Shortage of potassium
5. Alkaline soil
6. Low temperature or inadequate light that weakens the plant
7. Potted store basil not transplanted
8. Pests: aphids and others
9. End of life

Why do those 9 conditions lead to basil turning yellow? How to fix them?

The 9 Reasons For Yellow Basil Leaves

If your basil has yellow leaves, it is extremely likely that one of these 9 problems is the cause. Understanding these problems is paramount to first recognize them (not confusing one for the other) and fix them.

These tips can affect both indoor and outdoor gardeners, although some of them might be most common for one situation than another (I will let you know whether this is the case).

1. Excess of Water

Always wet soil can easily trigger your basil to get yellow leaves. However, the yellow leaves are just a signal of an underlying more severe problem caused by excess watering: root rot. This is by far the reason number one for yellow basil leaves among those attempting to grow it.

This problem is more common among indoor gardeners. Why? Because a potted plant in the kitchen counter is always nearby, and you might be tempted to water it on a daily basis (often not necessary). More importantly, the presence of a container does not provide the same level of drainage of an outdoor garden, so here you need to be extra careful.

Here, what is happening?

Your basil, as well any other herb and plant, breathes through its roots. If its roots are constantly submerged in water, a type of bacteria specializing in rotting. Start thriving, as discussed in this university study. When this happens, the basil roots undergo a decaying process becoming unable to extract the gas and minerals your basil needs. Essentially your plant suffocates. Among the first symptoms, you will notice your basil wilting and yellowing.

Remember

Basil needs to have moist soil, but never waterlogged. As I always say, better slightly dry if you are unsure than overwatered.

How can you verify if watering is the cause of yellow basil leaves?

If the root rot is the cause, the easier way is to check the root. In this case, you need first to let the soil dry a couple of days (place the plant under the sun to accelerate the process). Then gently remove the pot around (if it is a small one). Check the roots.

Healthy roots look like the one below, white a bit hairy. Rotten roots look brownie, slimy, and smell bad due to the broken down action of the bacteria.

Tips on how to avoid overwatering?

Tip number 1: the saucer test – Stop watering when the first droplets of water come out from the drainage holes. Do not let the saucer filled with water. Indeed, when the water leaves through the drainage holes, it means that the soil is not able to retain any more water.

Indeed, a saucer is there to avoid your floor from getting wet and dirty. However, the saucer amplifies the problems caused by excess watering, so not a great plant companion. This is because the saucer collects such water leaving the bottom part of the soil (with roots very likely if you do not have a gigantic pot) constantly wet, and so triggering rot root.

In addition, do not forget that a constant wet saucer is a perfect environment for mosquitos and many other loving insects.

Tip number 2: the finger test – as discussed more in detail in tip #11 in the 21 tips for massive basil indoor sticks a finger into the soil and checks if it is moist. That is what matters as the roots are at that level. Indeed, a soil that looks dry on the surface might still be moist (or worst soaked) a few inches below. The surface of the soil is always way drier than the bottom because of sunlight (evaporation) and temperature (higher).

This is extremely important as basil roots do not grow on the surface (some plants do, creeping roots for more), so what you should really check is the water a few inches below the surface.

How to fix it (and should you)?

If your basil roots look rot, my suggestion is to just toss the whole plant away. Also, do not reuse the soil or at least make it dry under the sun or even better sterilize it as detailed in this illustrated guide. Most of the time, it is not worth the effort to rescue a root rot basil.

Nonetheless, if you want to give it a try, however, here what I would do:

1. Remove the plant from its pot, and loose the soil surrounding the roots by gently hitting it with your hand. Make sure the soil has had time to dry a bit (this will make the operation easier)
   
2. Leave the roots dry for 1-2 hours at the open air
   
3. Cut the brownish looking roots. This will avoid the bacteria from spreading to the other roots
   
4. Place the plant in new potting soil and pot.
   
5. Avoid to expose the herb to direct sunlight for a couple of days: the plant need to focus on healing its roots after the transplant rather than producing foliage with sunlight triggered photosynthesis
   
6. Wait a week to see what’s happening

However, the chance of survival in the case is quite low, especially if more than half of the roots are infected. Indeed, this is a bacteria that, once developed, tend to spread from root to root.

2. Potting Mix Too Compact

A compact potting mix will suffocate your basil. Indeed, basil requires an airy and fluffy soil that allows the exchange of gases.

Why is your potting soil too compact?

You might have just picked up the wrong soil. Perhaps you took some soil with a high clay/silt content (for more on those materials check it how in the soil type guide). Perhaps you pick up such soil from a field/park or your outdoor garden (totally not recommended) just to save some time rather than going to the shop.

If not, even good quality potting mix gets compact with time due to the normal aging process. Indeed, if you have a good quality potting soil, some components (such as compost) tend to decay. Moreover, the watering over months compresses the soil (just gravity), reducing the air pocket within the potting mix.

How do you notice if the soil is too compact?

In this case, just press the soil with your finger. Do you feel like pressing something compact like butter just comes out from the fridge? Then, yes, your soil is too compact.

What can you do?

In this case, the safest way is to replace the soil with a good quality potting mix. There are many out there, but, to be honest, the one I normally use is the one produced by FoxFarm. It is fluffy, airy, and enriched with organic nutrients. Check it out here on Amazon.

Remember that new potting soil lasts for 1-2 years, so, hence, even if you buy more than needed for your basil plants, you can always use it later. Moreover, why not get your hands dirty and change those plants you might have hanging around growing in a small pot?

The toothpick and squeezing

In the short term, you can increase the aeration of the soil by creating pockets of air moving the soil through a toothpick or any small plastic device. However, remember, do not damage the roots and be gentle in moving the soil. If you find root resistance, just take out the toothpick and try other spots. Another method is to squeeze the planter (if plastic) to create air pockets and break the compact soil.

3. Disease: Downy Mildew

This is a disease caused by a pathogen similar to a fungus (not like the one we eat, buy way smaller) called Peronospora Belbahrii. This is one of the most common among herbs, and there are quite different types.

Which conditions trigger downy mildew?

This parasite thrives on your basil leaves in case of a highly humid environment without air circulation. One of my basil plants has been affected when I left it on a greenhouse I have at home as I did not provide any artificial air ventilation (they were in a closed bedroom).

What are the symptoms of downy mildew?

Downy mildew is relatively easy to identify. It appears always at the bottom side of the leaf (never on top) as countless black/grey spots that look dust or dirt with the color varying from light grey to black.

How to fix downy mildew?

First, remove the plants from their humid area they are and equally space them to improve aeration. Isolate the plant affected to avoid spreading.

Second, you have to manually and gently remove the leaves affected by the fungus. Remember: This fungus spreads through tiny pores that can easily float in the air or are carried by water droplets. Hence, if it gets on other leaves, it might (or might not depending on the defense of the other basil stem) affect the rest of the plant.

Another solution (in combination with manual removal) is the use of antifungal water solution spray. The good news? You can prepare them at home with ingredients you probably already have.

For the spray, you need one tablespoon of baking soda, 1 tablespoon of vegetable oil, and one dish of soap in a solution of around 4 liters of water. Spry the solution on affected leaves once a day until the trace of powdery disappears. There are also easier recipes based on garlic and even milk. For more check this article on WikiHow.

Downy Mildew on Basil - Common Plant Diseases in the Landscape and Garden

4. Potassium Shortage

The lack of potassium makes your basil leaves yellow among veins. Basil, as any other herbs need minerals to survive. There are many of them. The most famous (due to also the fact that basil as any other plants needs them in large quantities) are potassium, phosphorus, and nitrogen.

How do you know if it is potassium deficiencies?

The secure way is to test your plant soil. There are many ways to go for it. My favorite ones are the do it yourself test from Luster Leaf. I bought this one on Amazon and for more on how to use it and what it checks the full guide on soil testing for indoor herbs.

The lack of potassium is one of the hardest to spot. In general, a good rule is to observe where the yellow area appears. Do they appear between veins or on the edges, as also discussed in this study? If so, good chances are that there is a potassium shortage.

Another way is to go for exclusion. Indeed, the majority of the other causes are easier to test out, although this is not certain as nutrition deficiency problems are, in general, tough to identify.

How to fix potassium deficiency?

In this case, you can add a potassium-rich fertilizer. They are available in a nursery or online retailers as Down to Earth here on Amazon. Perhaps easier, cheaper, in the long run, is just to change the potting soil. If the potting soil does not contain too much fertilizer, your basil would be fine. In my case, I usually use FoxFarm. It never disappointed me. You can have a look here on Amazon (it also has an organic version here).

5. Wrong Soil pH

High pH levels (above 8) are known to make your basil leaves turn yellow. This is because many nutrients become less available. One of them is nitrogen that, among many other functions, is responsible (indirectly) for the green color of your herbs. To know more how a wrong pH affects your herbs, check the full guide in the article below.

What is the right pH?

Although basil can live (but not thrive) with a pH level up to 8.5 pH, the ideal is around neutral 7-7.5.

How to understand if the soil pH is the problem?

The easiest way is to check the pH of your soil. The good news?

You do not need a lab, just a few bucks for inexpensive kits like the Luster leaf (my favorite) that you can check here on Amazon.

Tip

Another action is to check where the yellowing happens. In case the lack of nitrogen is responsible, you will notice the leaves at the bottom getting yellow while the ones at the top will still be green. This is because the little nitrogen is remaining will move to the youngest leaves, letting the older ones die off.

What are the reasons for alkaline soil (high pH)?

A soil with high pH can be caused by several reasons:

1. Do you use old soil? Where does the soil come from? In this case, perhaps you “recycled” some old soil (not recommended) that was used for other plants that are alkaline loving (such as lavender, marjoram etc…)
   
2. Are you adding alkaline substances to your soil? For instance, a few gardeners used ashes or lime on their indoor herbs following practices used for outdoor. These two materials are great for alkaline loving plants (or increase the soil with a low pH) but not all plants can withstand high pH levels.
   
   If you add something to your soil, just check the label (and if not), just google: “pH of the name of your substance.” You will very likely find out!
   
3. The tap water might be harmful? It does not happen often, but your tap water can be very alkaline (above 8). Indeed, the drinking water pH is not regulated in the USA (only guidance of a pH between 7 to 8.5 is provided). Is your water alkaline?
   
   Here is a trick: even better than checking your tap water pH, it can be more useful to check the water dripping from the drainage holes of the pot when watering. Indeed, this is the water that arrives at your basil roots (and can be slightly different from the one you pour in due reaction with the minerals in the soil).

How? You can check it quickly with a tester, a cheap option is the HoneForest test (the price on Amazon here)

How to fix it?

In this case, my best opinion is to change the soil. If you look for a good quality potting soil, have a look at the FoxFarm. I bought it here on Amazon. I reviewed it in the article below if interested. If you have the occasion, also change for a bigger plant container.

6. Potted Basil From The Store

Have you planted small potted basil from the store? Then you just watered it, believing it was enough. However, after 1-2 weeks, it starts yellowing. That’s normal

Why? Potted plants from the store have been grown in extremely favorable conditions in order to have many of them (up to 20) in a tiny space (the pot they come with).

Hence, not surprising that all those basil plants are fiercely competing for space and nutrients. The youngest ones are likely to die first as they do not have enough root system to suck the (almost zero) nutrient in the soil. While the others will die soon after.

How to fix the problem?

With a couple of pots (check this post for the ideal container size) and following the process in this photo guide, you can provide long life to your stored potted basil. Remember to give a good quality potting mix (like the FoxFarm here) and at least 6 hours of direct sunlight.

7. Temperature and Light

Basil is quite sensitive to temperature and light conditions. Basil is a truly late spring-summer herb mainly widespread in the warm Mediterranean area. Indeed, as detailed in this study from the University of Nottingham, based on experimental observation, any ambient temperature around or below 15°C will stunt the growth of the plant, making also its leaves pale green or yellow.

basil grown at 15°C had paler (more yellow) leaves after only 2–3 d compared with plants grown at 25°C or 30°C –

University of Nottingham

So check the temperature of your room. If it gets below 20C, you know the culprit of your yellow basil leaves.

Lack of sunlight can cause similar problems. Provide less than 6 hours of direct sunlight, and your basil might start wilting (floppy stem) and yellowing. Remember also that an excess of sunlight (10 hours of more under scorching sun) can cause black leaves (sunburn) as discussed in this detailed black spot guide on basil.

How to fix a temperature issue?

Check the warmest spot of your house. Probably the windowsill is the best if it receives some direct sunlight. Perhaps in a small room with a heater that stays warm most of the time. If not, you can create a small greenhouse for the plant (check this article for some idea).

8. Insects: Aphids and “Company”

Aphids can be one of the most dangerous basil companions. These insatiable insects, as also discussed in this article, can appear from nowhere and multiply extremely rapidly. This insect is also loved by ants that tend to farm (yes, like cows) them (check one of my articles for more).

Aphids’ most significant power is their size: they are extremely tiny. With a millimeter or less in total size, even a dozen individuals can easily pass unnoticed if you are not carefully checking your basil. Bad news here. Aphids also like other herbs, so be careful that they do not spread in the surroundings.

Why are there aphids on your basil? They simply love eating plants. They have a kind of proboscis through which they suck the basil juice. This, of course, leaves a tiny (because the aphid “mouth” is extremely small).

One individual or a small colony does not cause any significant damage to a basil plant. However, hundreds of them can easily make a leave totally patched with such spots. Moreover, with a massive lack of nutrients, the leaves will yellow, wilt, and die. If your basil is a small plant, chances are that it will die if the infestation is severe.

How to be sure that aphids are the problem?

First, look for tiny yellow/brown spot. The aphids such nutrients from your leaves in spots. Hence, you should be able to see yellow spots. These are tiny (a millimeter in size or less), yellowish/silver, more or less round. Their location is totally random. The fact that whole leaves get yellow is just a consequence of your leaves being eaten too much.

Second, check for some sticky, light yellow, dense liquid. This is a sugar produced by aphids (their feces) after digesting the leaves nutrients. This is called honeydew. If you find it, the chances you have aphids are virtually 100%.

Third, inspect leaves and stem. If your basil is yellowing, you should already have a decent number of aphids. Aphids are tiny and, at first, might appear dust. They are often in clusters. If you find pear-shaped clumps, just move them with your finger. If they react, well, you have aphids.

How to get rid of aphids?

Such a problem is so well discussed by many other gardeners that I will just redirect you to the excellent article written by Kevin.

Aphids are not the only insects that can affect basil, although the most common. Others that you might want to check our spider mites, gnats, thrips. The good news is that they are easier to spot as way larger.

9. End of Life

The majority of basil varieties (including the well loved Genovese type) are annual herbs, as discussed in this guide. This means that they last only one year, from mid-spring (depending on the temperature and watering) to the beginning/mid-winter. They can last a bit more if temperature, watering, and pruning.

When times come, and they start flowering, the plant will slowly start to dry out. This process will cause, among other effects, the basil to become yellow and die.

What can you do?

Basil is annual, and so nothing can you do, right? Almost! There is still something you can do!

As discussed in these 21 tips to grow massive basil prune, often your basil to stimulate growth and, more importantly, remove any flower even before they appear (when you see the bud forming).

Once basil starts flowering, its “task” is accomplished, and its last effort will be focused on producing seeds. However, if you pinch the flowers, you are “delaying” the death of the plant. Of course, this does not work forever, and after a while (delayed compared to normal), the plant will eventually die.

Further Questions

Can You Eat Yellow Basil Leaves? Yellow basil leaves are generally safe to eat although their taste (bitter or way less strong) is affected by the cause of the yellowing.

Basil Turning Yellow with Black Spot? The presence of black spots might be triggered by pests, fungi, or diseases like thrips, fungi (Cercospora Leaf Spot), Parasites (Downy Mildew), and more. For more check this article.

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