Why your hydroponic lettuce is turning yellow (and the fix)

Yellow lettuce is the most common message we get, and it is nearly always one of three causes. The good news is that you can tell them apart just by looking at which leaves went yellow.

Cause 1: pH drift, and the food is locked out

What you see: yellowing between the veins of the newest leaves, while the veins themselves stay green. The plant looks pale from the top down.

What is happening: your reservoir is full of nutrients the plant cannot reach. Outside roughly 5.5 to 6.5, iron and several other elements stop being chemically available. Nothing is missing. It is simply locked up.

The fix: test pH. If it is above 6.5, add white vinegar a few drops at a time, stirring and retesting between additions. If it is below 5.5, add a small pinch of baking soda the same way. New growth should green up within about a week. Leaves already yellowed will not recover, and that is fine.

Cause 2: not enough nitrogen

What you see: the oldest, lowest leaves yellow first, evenly across the whole leaf rather than between the veins. The plant looks pale from the bottom up.

What is happening: nitrogen is mobile within the plant, so a hungry plant strips it from old leaves to feed new growth. That is why the damage starts at the bottom.

The fix: usually the solution is too dilute, or it has been in use too long and the balance has drifted. Replace the reservoir with a fresh mix at the full label rate. If you had been running half strength for seedlings, this is the moment to move to full.

Cause 3: the roots are failing

What you see: general yellowing with wilting, even though the reservoir is full. Lift the net pot and the roots are brown and slimy rather than white and fuzzy, and there is a swampy smell.

What is happening: root rot. The plant cannot take up water or nutrients regardless of what is in the reservoir. Usually caused by solution above 75F (24C), a stalled air pump, or light reaching the reservoir.

The fix: rinse the roots under cool water, trim the slimy brown material with clean scissors, scrub the reservoir, and refill with fresh solution plus a teaspoon of 3% hydrogen peroxide per gallon. Then fix the cause, or it returns within a fortnight.

The one case that is not a problem

And one thing worth ruling out

If your lettuce is pale, stretched, and tall as well as yellowish, that is a light problem rather than a nutrient one. See the lighting guide.

For every other symptom: the full troubleshooting guide