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. Small dark flies around moist potting medium may be fungus gnats. They suggest conditions suitable for breeding, without proving that the surface never dries.
The fix
Stop watering. Make sure the pot can actually drain, and wait until the top 3 cm (1.2 in) 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.
Other possible causes
Not a match? Compare the signs below to find the one you have.
Tap-water minerals (a real trigger on this plant)
How to tell
Round-leaf calathea is more sensitive than most houseplants to the fluoride and salts in tap water. The sign is yellow or brown edges and tips that spread slowly inward, not an even yellowing of whole leaves. All-over yellowing points to watering, light or feeding instead, so check those first.
The fix
Water with rainwater or distilled water, and flush the soil every few months to clear built-up minerals. Damaged edges won’t turn green again, but new leaves should come in clean.
Too little light
How to tell
The yellowing comes with dull, fading leaf patterning and pale new growth leaning toward the nearest window. Round-leaf calathea wants bright, indirect light. Together, those changes show that the spot is too dim.
The fix
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)
How to tell
The soil is bone dry, and the yellowing arrives alongside crispy brown edges rather than soft, mushy growth.
The fix
If the soil is dry, 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)
How to tell
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.
The fix
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
How to tell
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 center look completely healthy. This is normal retirement on a round-leaf calathea, not a symptom.
The fix
Nothing is wrong: snip the spent leaf off at its base and carry on. Plants retire their oldest leaves.
Thrips
How to tell
Silvery patches with tiny black dots point to thrips.
The fix
Rinse the plant, cut off the worst leaves and treat in rounds, as the thrips guide explains.
