If a leaf is turning yellow but the veins running through it are staying sharply green, you’re not looking at ordinary yellowing — you’re looking at iron chlorosis. It’s a specific, recognizable pattern with a specific cause, and treating it like generic yellow-leaf trouble (less water, more light, a random dose of fertilizer) usually doesn’t fix it. This post covers that one pattern in depth; for the broader range of things that turn leaves yellow, see why are my plants’ leaves turning yellow and signs of nutrient deficiency in houseplants.

What iron chlorosis actually looks like

The tell is interveinal chlorosis: the tissue between the veins fades to yellow or pale green while the veins themselves stay a noticeably darker green, so the leaf ends up looking like a green mesh laid over a yellow background. It shows up on new growth first and worst, because iron doesn’t move around within the plant the way nitrogen or potassium do — once it’s been used to build a leaf, the plant can’t relocate it to a newer one, so new leaves are the ones that go short first.

This is different from the overall, uniform yellowing you’d see with overwatering or plain old age, and different from the edge-and-tip yellowing that points to potassium deficiency. If you’re staring at a leaf trying to decide which yellowing you’ve got, veins staying green while the tissue around them fades is the specific thing to look for.

Why it happens — and why it’s usually not a lack of iron

This is the part that trips people up: iron chlorosis is rarely caused by there being too little iron in the soil or fertilizer. Most potting mixes and all-purpose fertilizers already contain enough iron for houseplants. The actual problem is almost always soil pH. Iron becomes chemically locked up and unavailable to roots when soil gets too alkaline — plants can be sitting in soil with plenty of iron in it and still be unable to absorb any of it.

Two things commonly push soil pH up into that range:

• Mineral buildup from hard water. Calcium carbonate in hard tap water is alkaline, and every watering leaves a little more of it behind as the water evaporates. Over months that mineral buildup nudges soil pH upward. See hard water vs. soft water for houseplants for how this accumulates and what else it affects.
• An overly alkaline potting mix. Some bagged mixes, especially ones amended with a lot of lime or certain composts, start out on the alkaline side. A plant potted in one of these can show chlorosis even with otherwise perfect watering and light.

This is also why dumping in more iron fertilizer often does nothing: if the pH is the actual blocker, the roots can’t take up the extra iron any better than they could take up what was already there. You end up with iron sitting inertly in the soil while the plant keeps producing pale new leaves.

How to actually fix it

Test the soil pH first. A cheap soil pH meter or test kit tells you whether you’re actually dealing with an alkalinity problem before you start changing anything. Most houseplants want slightly acidic to neutral soil, roughly pH 6 to 7; readings noticeably above that are consistent with iron lockout.

Flush accumulated mineral salts. If hard water has been building up mineral deposits for a while, run a generous volume of water through the pot — enough that it drains freely out the bottom several times over — to carry dissolved minerals out of the root zone rather than let them keep concentrating.

Switch to filtered or rainwater going forward. This addresses the cause instead of just the symptom. Filtered, distilled, or rainwater don’t carry the calcium and magnesium that push soil alkaline over time, which stops the problem from rebuilding after you’ve corrected it.

Use a chelated iron supplement as a targeted fix. Chelated iron is bound to a compound that keeps it available to roots even in soil that’s somewhat alkaline, so it can green up new growth while you’re working on the underlying pH. Treat it as a patch, though, not a permanent solution — if the pH and water source that caused the lockout aren’t addressed, the chlorosis will keep coming back in each new flush of leaves.

Consider repotting into fresh, appropriately acidic mix if the current soil tested high and has been in the pot a long time. Old, compacted, mineral-laden soil is harder to correct in place than it is to replace outright.

What to expect after treatment

Existing yellow leaves generally won’t turn back to solid green — whatever chlorophyll they lost, they lost — but new growth that emerges after you’ve corrected the pH and water source should come in properly green from the start. That’s the real marker that the fix worked: not the old leaves recovering, but the new ones never developing the pattern in the first place.