Most likely cause

Overwatering & root rot. Stop watering. Make sure the pot can actually drain, and wait until the top 3 cm of soil feel dry to the touch. Because this plant’s roots are delicate, unpot it at the first sign of a soft or sour-smelling crown rather than waiting.

Describe what is visible before choosing a cause

01Yellowing leaves
02Mushy leaf bases
03Root rot
04Soft leaf stalks
05Crispy brown edges
06Curling leaves

What’s causing it

How to tell

Overwatering & root rot. The soil is still wet days after watering, the lower and oldest leaves yellow first, and the crown may feel soft. This plant’s dense, fine roots rot faster than a tougher foliage plant’s, and yellowing leaves is usually the very first sign.

Tap-water minerals (a real trigger on this plant). Unlike most houseplants, Round-leaf calathea is genuinely sensitive to the salts and fluoride in ordinary tap water: a general, all-over yellowing with no soggy soil and no light problem often traces back to this rather than watering technique.

Too little light. The yellowing comes with dull, fading leaf patterning and pale new growth leaning toward the nearest window. Round-leaf calathea wants medium, indirect light. Together, those changes show that the spot is too dim.

Thirst (underwatering). The soil is bone dry, and the yellowing arrives alongside crispy brown edges rather than soft, mushy growth.

Hungry (nutrient shortfall, but easy to overcorrect). Older leaves fade evenly and growth has slowed after many months unfed, but this genus is also easily salt-burned by fertilizer, so don’t overcompensate.

Simple old age. One of the older, largest leaves slowly yellows from the outside in over a couple of weeks, while the new furled leaves emerging from the centre look completely healthy. This is normal retirement on a Round-leaf calathea, not a symptom.

The fix

Overwatering & root rot. Stop watering. Make sure the pot can actually drain, and wait until the top 3 cm of soil feel dry to the touch. Because this plant’s roots are delicate, unpot it at the first sign of a soft or sour-smelling crown rather than waiting.

Tap-water minerals (a real trigger on this plant). Switch to filtered, distilled, or rested rainwater, and flush the soil every few months to clear built-up minerals. Sensitive plants like this one often green back up within a few weeks of the switch.

Too little light. Move it to a brighter spot. The patterning sharpens back up within a few new leaves, but keep it out of harsh midday sun, which scorches.

Thirst (underwatering). Water thoroughly (every 5 to 7 days in summer is the usual pace for a Round-leaf calathea), then settle into feeling the soil before every watering rather than watering on a fixed day.

Hungry (nutrient shortfall, but easy to overcorrect). Feed at a quarter to half the label strength through the growing season, flush the mix with plain water every couple of months, and refresh the top layer or repot if it’s been over a year.

Simple old age. Nothing is wrong: snip the spent leaf off at its base and carry on. Plants retire their oldest leaves.

Prevention

Switch to filtered or rainwater since this plant is genuinely salt-sensitive, check the mix regularly rather than on a fixed schedule, and keep humidity up.