6 reasons your indoor basil plant might be turning brown or yellow

Basil plants are a favorite of gardeners everywhere, and they can easily be grown in containers on your windowsill or on a coffee table in the mudroom.

But one day you walk in and the leaves are starting to turn brown or yellow and you get confused. What gives? You may think it’s the natural annual death of the plant since basil is not perennial, but indoor basil plants can produce multiple crops each year — so seasonality is rarely the cause. These are 6 of the most common reasons your basil plant may be discoloring.

1. Too much or too little water

Watering issues is one of the most common of basil leaves turning brown or yellow. Unfortunately, both overwatering and underwatering can cause this issue.

How to tell if you’re watering too much or too little

If your basil is suffering from overwatering, in addition to the leaves turning yellow or brown, your plant may wilt and feel limp or soft to the touch. Try watering less often, and allow the top of the soil to dry out before watering again.

If you are underwatering, or watering too little, the leaves may turn brown or yellow, but they will likely feel dry to the touch rather than limp. If this is the case, try increasing the frequency of watering. 

The drainage of your soil can also contribute to over or under watering. If your soil is too sandy, the water will pass through and quickly become dry. If your soil is to clay-y, it may fail to drain and your plant may experience symptoms of overwatering even if you are not watering frequently.

2. Not enough sunlight

Basil leaves can turn yellow and the stems can turn brown if they’re not getting enough sunlight. If this happens to your basil plant, there are a few things you can do to bring it back to health. 

Moving your basil to a south-facing window will help it get the longest hours of sunlight. If that’s still not enough, for example during a dreary winter with very short daylight hours, you can consider adding grow lights to help the plain get enough light. 

3. Soil is too acidic

Basil thrives in soil that ranges from 6.0-7.5 pH. When the soil is too acidic, the leaves and stems of your basil plant might turn brown or yellow.

If you recently re-potted your basil, you may have used soil that isn’t the correct pH to support the health of the plant. But if your plant has been living in the same soil for a long time, it’s unlikely that the acidity of the soil is to blame unless you recently added a lot of organic material to it (including compost).

4. Fertilizer burn

Every avid gardener runs into the issue of fertilizer burn at some point or another. We call it “loving a plant to death”. The intention is to provide the beautiful plant with some yummy nutrients so it can grow big and strong, but too much of a good thing can harm, or even kill, your plant.

Fertilizer should be applied to basil only once or twice per growing season. If your plant is turning brown and you know that you recently fertilized, you can salvage it by trying to remove the fertilizer from the soil and by watering a lot to flush the fertilizer out of the soil. Be sure to discard the water rather than allow the plant to soak in it.

5. Insects and pests

Insects and pests including aphids, scales, and spider mites can suck the vital juices from your basil plants, causing the leaves to turn brown. This is not a particularly common cause of browning leaves, especially in indoor basil plants.

If you suspect pests are to blame for the browning of your basil, you can spray the plant lightly with insecticidal soap.

6. Fungal diseases

Fungal problems like root rot, fusarium wilt, and damping-off are the final potential cause of leaf browning on basil plants. Overwatering is the most common cause of fungal diseases in indoor herb gardens. 

Fungal problems like damping-off can be treated with fungicides, but fusarium wilt cannot be treated and the plants should be immediately removed from your herb garden and destroyed so that the disease does not spread to your other herbs. 

Luckily, the browning of leaves on basil plants is usually caused by simple factors like water and light, which are easily manipulated.