Nephrolepis exaltata
Quick answer
Yellow leaves on your Boston fern have a few possible causes, and on this plant the likeliest is thirst.
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How to tell
The soil is bone dry, and the yellowing arrives alongside crispy frond tips rather than soft, mushy growth.
The fix
If the soil is dry, water thoroughly (every 3 to 5 days in summer is the usual pace for a Boston fern), then settle into feeling the soil before every watering rather than watering on a fixed day.
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How to tell
The yellowing comes with pale, stretched new growth leaning toward the nearest window. That combination shows that the spot is too dim for a Boston fern.
The fix
Move it to a brighter spot, but keep it out of harsh midday sun, which scorches.
How to tell
The soil is still wet days after watering, the lower and oldest leaves yellow first, and the stem base may feel soft. 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 2 cm (0.8 in) of soil feel dry: ferns don’t like drying out. If the base is mushy or smells sour, unpot it and trim any brown, soft roots before repotting in fresh mix.
How to tell
Older leaves fade to an even pale yellow and growth has clearly slowed. On a plant that likes 60–80% humidity and regular water, this shows up after many months in the same tired mix once feeding has lapsed.
The fix
Feed with a balanced, diluted liquid fertilizer through the growing season, and refresh the top layer or repot if it’s been over a year.
How to tell
A single bottom leaf slowly yellows while the rest of the plant looks healthy and is pushing new growth.
The fix
Nothing is wrong. Snip the spent leaf off and carry on. Plants retire their oldest leaves.
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