Yellow Leaves, Green Veins — Reading a Magnesium Deficiency

Magnesium deficiency: yellowing between the veins, starting low and old.

First time I saw it I thought the plant was dying artistically. The lower leaves had gone this stained-glass pattern — yellow panels, green lines — and I stood there admiring it for a good minute before it occurred to me that healthy plants don’t do stained glass.

The short version:

• Yellow creeping in BETWEEN the veins on the older, lower leaves — while the veins themselves stay green
• The green veins are the tell. Uniform yellow is nitrogen; this two-tone pattern is magnesium
• Check pH before you feed anything — lockout fakes this constantly
• Fix is Cal-Mag or Epsom salts, then judge recovery on the new growth, not the old leaves

Want the full breakdown? Keep scrolling.

Why are my leaves yellow but the veins still green?

The classic march: interveinal yellow, then rusty flecks, then crisp.

That pattern has a name — interveinal chlorosis — and on the lower leaves of a cannabis plant it points at magnesium. Magnesium sits at the centre of every chlorophyll molecule; no magnesium, no green. When the plant runs short, it strips magnesium out of its oldest leaves to feed the new growth up top, and the yellow spreads between the veins as the chlorophyll goes. The veins hang on longest, which gives you the stained-glass look.

It starts low and old and works upward. If the SAME two-tone pattern is on the newest top growth instead, that’s a different story — iron — and you want the iron deficiency guide.

Is it really a magnesium shortage, or is it pH?

Here’s the bit that saves you money: half the time there’s plenty of magnesium in the feed, and the plant just can’t get at it. Magnesium locks out when pH drifts out of range — under about 5.8 in coco and hydro, or out of the 6.2–6.5 window in soil.

So before you buy anything, test the pH of what’s going in. If it’s off, correcting pH IS the fix, and any supplement you pour on top of a lockout is wasted. Our pH lockout guide covers it properly — it hides behind more deficiencies than any other single cause.

How do I fix magnesium deficiency in cannabis?

Once pH is confirmed in range:

• Feed it. A Cal-Mag supplement covers magnesium and calcium together — on coco or RO water you should be running it anyway (see the Cal-Mag guide). Plain Epsom salts work too: pure magnesium sulphate, cheap as chips — half a teaspoon per 4 litres in the feed water dissolves clean.
• Be patient with it. Magnesium recovery is slow. The yellowed leaves may green up a little but mostly they’re spent — the plant already robbed them.
• Judge it on the new growth. Healthy new leaves coming in solid green means the problem’s solved, whatever the old ones look like. Give it a week to ten days before you change anything else.

And the standing rule applies: change one thing, then watch. Fix the pH AND double the feed AND add Epsom on the same day, and when she recovers you’ll never know which one did it.

FAQ

Will the yellow leaves turn green again? Mostly no. The plant pulled the magnesium out of them and it doesn’t go back. Watch the new growth — that’s your scoreboard.

Can I use Epsom salts instead of Cal-Mag? For magnesium alone, yes — it’s the same magnesium. Cal-Mag adds calcium as well, which matters on coco or RO water where both run short together.

What pH locks out magnesium? Below roughly 5.8 in coco and hydro it gets harder to take up. In soil, stay in the 6.2–6.5 window. Drift outside those and magnesium sits in the medium where the plant can’t use it.

Is it magnesium or nitrogen deficiency? Nitrogen yellows the whole lower leaf evenly, veins and all, and the leaf eventually drops. Magnesium yellows between the veins while the veins stay green. Whole-leaf yellow = nitrogen; stained glass = magnesium.

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Not sure this is your problem? Run her through the Diagnosis Buddy — five questions and you’ll know.