Yellow leaves are the most reported houseplant symptom and the least useful on their own. Almost everything that can go wrong produces them eventually.
Two questions narrow it down faster than anything else.
Start here
Which leaves are yellow?
The oldest leaves at the base of the plant, or new growth at the top? Plants shed old leaves as a matter of course, and a single yellow leaf near the soil on an otherwise healthy plant is usually nothing at all.
Is the soil wet or dry?
Push a finger 3-5cm (1-2 inches) in. This single check separates the two most common causes, which produce almost identical symptoms above the soil.
Overwatering
This is the most likely cause. Yellowing starts with the lower leaves, they feel soft and limp rather than crisp, and the soil is damp days after watering. The stem may be soft near the soil line, and the soil may smell sour.
Roots need air as much as water. Soil that never dries suffocates them, they begin to rot, and the plant sheds leaves because the damaged roots can no longer support them.
What to do
Stop watering. Check whether the pot actually drains and empty any saucer. If the stem base is soft, unpot the plant, cut away brown mushy roots, and repot into fresh, free-draining mix in a pot no larger than before.
Read the watering guide for how to avoid repeating it.
Underwatering
Less common, and it looks different. Leaves yellow and then go crisp rather than soft, edges brown first, and the soil is bone dry, sometimes shrunk away from the sides of the pot.
What to do
Water thoroughly until it runs from the drainage holes. If the mix has gone water-repellent, stand the pot in a tray of water for half an hour so it can absorb from below.
Not enough light
Yellowing spreads evenly rather than starting at the base, growth has slowed or stopped, and new leaves are smaller than old ones. The plant may be leaning hard toward the window.
Variegated plants show this first, dropping their pale sections and producing plain green leaves.
What to do
Move it closer to a window. The light guide has the distances that actually produce each light level.
Nutrient shortage
Uniform pale yellowing across older leaves, on a plant that has been in the same soil for a year or more and is otherwise well cared for.
What to do
Feed at half strength during spring and summer only. Do not feed a plant that is struggling for another reason, and do not feed in winter.
Normal ageing
Plants shed their oldest leaves throughout their lives. One or two yellow leaves at the base, on a plant that is otherwise producing new growth, is not a problem and needs no action.
Pull or cut them away once they have yellowed fully.
Pests
Yellowing accompanied by fine pale speckling, webbing, or sticky residue means spider mites or another pest rather than a care problem.
Check the leaf undersides and read the pests guide.
Telling them apart
| Overwatered | Underwatered | Low light | |
|---|---|---|---|
| Soil | Damp for days | Bone dry | Either |
| Leaf feel | Soft, limp | Crisp, brittle | Normal |
| Which leaves | Lowest first | Lowest first | Spread evenly |
| Stem base | May be soft | Firm | Firm |
| New growth | Stunted | Stunted | Small and pale |
Yellow leaves do not turn green again. Judge whether your fix worked by watching what the plant produces next, not by waiting for existing leaves to recover.
