Causes of Basil Leaves Turning Yellow & How To Fix It

Whether you have bought a basil plant home from your local garden store or have grown it from seed, chances are you are going to feel distraught if you spot leaves that are turning yellow. 

Maybe you wanted to garnish a pizza with its leaves or make pesto! But now, you are frantically looking for solutions to fix this unwanted problem. It is always disheartening when a plant doesn’t look as loved as before. 

Thankfully, there are a few common problems that can lead to basil leaves turning yellow and I’m here to help you unravel the reasons why it might have happened to your plant and what you can do to fix it. 

Why Does My Basil Plant Have Yellow Leaves?

Pinpointing the problem of why the leaves of your basil plant have turned yellow usually comes down to one of a few common problems. These are watering issues, insufficient light, lack of nutrients, fungal infections, or pests.

Remember that just a few yellow leaves are normal for all plants and not a reason to panic. But, if the problem persists or worsens, then that’s when you need to act.

yellow leaves on basil plant

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Reason Basil Leaves Turning Yellow

You must provide their natural environmental conditions for your indoor plants to thrive. 

Basil plants are native to Asia and so their preferred climate is tropical and subtropical conditions. That means full sun, warm soil, and just enough moisture. If your plant isn’t getting enough of these, the leaves are bound to yellow. 

Basil loves full sun and so a south or west-facing window ledge is going to help it to thrive. Dark corners are a big no and will mean your plant isn’t getting enough light. 

Nutrient deficiencies are another common reason for basil leaves turning yellow – especially when observed near the leaf veins. While all herb plants need a regular dose of fertilizer, basil plants can often become lacking in potassium.

How To Fix It

Position your basil plant in a location where it will benefit from 7-8 hours of full sun per day. A south or west-facing window ledge is ideal. If you are using grow lights, they will need the same amount of exposure to artificial light as daylight each day.

If lack of sunlight is not the issue, then consider treating your basil plant for a nutrient deficiency.  Whilst basil plants are not heavy feeders; they do still need to be fertilized with a balanced fertilizer every 3-4 weeks to ensure ongoing leaf production. 

A diluted liquid fertilizer with 10-10-10 or 12-12-12 NPK compositions will get to work straight away and provide an instant boost of nutrients directly to the roots. 

Basil with Yellow Leaves and Brown Spots

Where there is evidence of brown spots as well as yellow leaves, the chances are your basil plant has been invaded by bacteria, fungi, or pests. 

A common fungal disease in plants is the Fungal Leaf Spot. This causes brown spots and holes in the leaves of basil plants.

Powdery Mildew is another common infection. In this case, the discoloration starts from the vein and extends outwards. This is closely followed by a fuzzy grey, brown color appearing on the underside of the leaf. 

How To Fix It

Leaves that have turned yellow with brown spots due to a fungal infection need to be removed as soon as the problem is spotted. Always use sterile scissors to cut back affected areas and re-sanitize your tools after every cut to avoid re-infection. 

When you are satisfied that all affected leaves have been removed, wash your basil plant with water, ensuring you include the leaves’ undersides. Dry thoroughly using a clean cloth.

Use a diluted neem oil spray to ward off any lingering organisms and repeat bi-weekly if the problem persists.

To avoid fungal infections in the first place, you should avoid getting the leaves wet when you water as this can encourage fungal growth. Also, avoid grouping newly purchased plants with your other plants until you are sure they are disease-free.

Reason Basil Leaves Yellow & Curling

If you notice basil leaves that aren’t just yellow but are also curling, then the main culprit here is usually aphids. They attach themselves to the undersides of leaves and suck the sap, which causes them to curl. You might even notice their eggs on the leaves as yellow or brown particles near the bottom. 

The most unpleasant type of fungus known to affect the Basil plant is Fusarium Wilt. According to a University of Maryland study basil plants with this disease tend to grow to 8-12 inches tall before infection takes hold. 

Thereafter, growth becomes stunted, and the plant begins to wilt. Leaf discoloration primarily starts in the lower stems before spreading throughout the entire plant. The chances of saving a basil plant with Fusarium Wilt are slim and disposing of it is the most likely outcome. 

How To Fix It

Prune away any yellow leaves using sterile equipment. Remember to re-sanitize after each snip to avoid reinfection.

Once you are certain the infected leaves are gone, isolate your basil plant and wash it well with water, including the undersides. Then, dry the leaves with a soft and clean cloth.

Next, you’ll need to remove the plant from its container and discard the existing soil. Repot the plant with fresh, well-draining soil. This will ensure that any fungus that has made its way to the soil doesn’t get a chance to linger.

Make it a practice to regularly check the leaves, including the undersides, so that you catch any sign of infection early on. 

In addition, I like to use simple diluted neem oil and water spray or diluted soapy water to spray my houseplants and herbs every 1-2 weeks. Avoid spraying during the hotter hours of the day as this can burn your foliage. 

Reason Basil Leaves Turning Yellow & Falling Off

If you notice yellow leaves that then begin to fall off, chances are you have overwatered your basil plant. 

Overwatering doesn’t just mean you have watered your plant too often. It can also be a result of soil that doesn’t allow for drainage or a pot or container that has no or insufficient drainage holes. 

Either way, an overwatered plant leads to waterlogged roots and this prevents your plant from accessing oxygen via its root system. As a result, root rot can occur and if left untreated, this can have a detrimental effect on your entire plant.  

How To Fix It

Checking for root rot is simple but needs to be done as quickly as possible after noticing the telltale signs of leaf drop and yellowing of leaves. Here’s what you need to do:

Remove the plant from its container, separate the roots from the soil, and discard this soil completely. 

Now it’s time to thoroughly inspect the roots. Healthy roots are white, firm, and odor-free, but diseased roots will be black and soft, they usually have an unpleasant odor too. These are the roots that need to be removed completely. 

As you cut away at the affected roots, be sure to re-sanitize your equipment after each cut to avoid spreading the infection to other parts of the plant.

Once you have removed all the diseased roots, leave the plant to dry out a little in the sunlight. 

After a few days, you can report your basil plant in a pot that has good-sized drainage holes and in fresh well-draining soil (adding grit or perlite to your compost mix will ensure better drainage). Water sparingly from now on!

It’s true that basil needs moist (not soggy) soil, but you should water it only when necessary. This means testing the moisture levels in the soil using your fingers. You only need to water your basil when the top 2 inches of soil are dry. If that top layer is still moist then hold off on the watering can and check back in a few days.  

Other Causes Of Yellow Basil Leaves

Here are a couple of final thoughts on the reasons why your basil leaves are turning yellow.

It could just be that your plant is at the natural end of its life cycle and has started to bolt. 

Basil is an annual herb that thrives during its growth state, where there is an abundance of vegetation. This is then followed by the production of flowers and seeds (or the reproductive state). At this stage, annual plants are focusing energy on flower growth. As a result, it is very common for leaves to begin to yellow. 

To delay the reproductive process, simply pinch off the top leaves when the plant is about 6-8 inches tall. If you do it at the central axis of the plant, you encourage the branching of the plant, which encourages more leaves. Keep doing this to encourage even more branch growth. 

Likewise, you can also pinch the flowers out. This will allow your basil plant to conserve energy for leaf production. 

One other common reason for yellow leaves is that your basil plant may have been planted too early in spring. Basils’ preference for full sun and temperatures above 60 F are not always achievable so early in the growing season. As a result, leaves will fail to grow to their full potential. If this occurs, I recommend harvesting early.

Final Thoughts On Basil Leaves Yellowing

Basil plants will reward you with a bountiful harvest of lusciously green and fragrant-smelling leaves throughout the growing season provided you invest a little time and effort in meeting their needs. 

To avoid a distressed plant and sub-optimal health, keep in mind that basil plants require adequate water, plenty of direct sunlight, consistently warm temperatures, well-draining soil, and a source of nutrients to keep them in prime condition and to ward off the onset of unsightly yellow leaves.

FAQ Yellow Leaves on Basil Plant