Basil plant leaves turning yellow?

In a pot, basil leaves turning yellow usually mean water, drainage, or light stress; one or two oldest lower leaves can yellow as a tall plant shades itself, but fast or widespread yellowing is not usually just normal age.

Answer what you can see

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Push a finger two inches into the soil. What does it feel like?

Which leaves are affected?

Where does it live?

How fast did this happen?

Several causes can overlap: damp soil plus yellow lower leaves can be overwatering, low light, hungry soil, or downy mildew, so use the extra checks such as drainage, room brightness, feeding history, and leaf undersides to tell them apart.

The causes in full

Overwatering or poor drainageMOST COMMON

How to tell
In a pot, check for a wet saucer, no drainage hole, sour-smelling mix, or soil that stays wet; outside, this is more likely in a low spot or heavy soil after rain.

What to do
If the soil is soggy or wet 2 inches down, stop watering until it is dry and crumbly 2 inches down, empty the saucer, and move the plant to a pot with drainage if needed. Outside, open up crowded growth and improve drainage rather than adding more water.

Too dry or rootboundCOMMON

How to tell
The pot feels very light, the root ball may pull away from the pot, and outside plants fail fastest in hot wind or full afternoon heat.

What to do
If the soil is dry and crumbly 2 inches down, water slowly until the whole root ball is moist; if it is also pulling from the pot, set the pot in water briefly, then drain it well. After that, water only when the soil is dry and crumbly 2 inches down, and repot if roots are circling tightly.

Too little light or cold window stressCOMMON

How to tell
The plant is pale, stretched, or yellowing on the window side, and the soil stays damp because the plant is not using water quickly; outside, chilly nights can yellow basil even when watering is correct.

What to do
Move an indoor pot to the brightest warm spot you have, avoiding leaves pressed against cold glass or blasted by a hot pane. If the soil is damp or wet 2 inches down, do not water again until it is dry and crumbly 2 inches down.

Hungry plant or exhausted potting mixCOMMON

How to tell
Older leaves fade first, growth slows, and the plant has been in the same small pot or poor outdoor soil for weeks without fresh mix or light feeding.

What to do
Repot into fresh potting mix or feed lightly with a balanced plant fertiliser at label strength; outside, loosen crowded plants and improve the soil around them. Do not add water for this cause unless the soil is dry and crumbly 2 inches down.

Pests or downy mildew diseaseLESS COMMON

How to tell
Look under leaves: pests leave speckling, sticky residue, webbing, or visible insects, while downy mildew makes yellow patches with gray-purple fuzz underneath, especially outdoors after humid weather or on crowded plants.

What to do
For pests, isolate the pot if indoors, wipe off insects, and treat with a labeled insecticidal soap. For downy mildew, remove infected plants or heavily spotted stems and improve spacing and airflow.

Fertilizer, spray, or herbicide damageLESS COMMON

How to tell
Yellowing or bleached patches started soon after feeding, spraying, cleaning near the plant, or outdoor weedkiller drift, often on the newest leaves or the side that was hit.

What to do
Stop using the suspected product and remove badly damaged leaves after new growth starts. If excess fertilizer is likely and the soil is dry and crumbly 2 inches down, run plain water through the pot once; if it is damp or wet 2 inches down, wait until it is dry and crumbly 2 inches down before flushing.

Will it recover?

Yellow basil leaves usually do not turn green again; limp leaves from thirst may perk up, but judge recovery by healthy new green growth.

Stopping it happening again

Keep potted basil in a draining pot in the brightest warm spot available, space outdoor plants for airflow, inspect leaf undersides often, feed lightly during active growth, and water only when the soil is dry and crumbly 2 inches down.