Cucumber: lower leaves turning yellow

What it is likely to be on cucumber

1. Nitrogen deficiency

   Candidate 1common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Cucumbers — cultivation
   • Home and Garden Information Center — Clemson Cooperative Extension: Cucumber nutrition

   Checked 2026-08-13

   The classic pattern is uniform yellowing that starts on the OLDEST leaves and works upward, because nitrogen is mobile within the plant and is withdrawn from old growth to feed new. That directionality is what separates it from most other causes of yellowing, and it is observable without any equipment.

   Points towards it

   • · a cucumber under a heavy pick moves nitrogen out of its oldest leaves into the fruit faster than almost any other crop
   • · the leaf pales evenly, veins and all, working in from the margins
   • · growbags and pots hold only a few weeks of feed, and a greenhouse cucumber crops for months

   Argues against it

   • · green veins standing out against yellow tissue is magnesium, which more nitrogen will not fix — this rules it out entirely.
   • · a mosaic of light and dark green with distorted leaves is a virus rather than a shortage, and feeding will not touch it

   Check this next

   Work out how long the plant has been in that compost and count the picks. A greenhouse cucumber in a growbag exhausts the nitrogen in it within about six weeks of planting, so yellowing that starts at the base a month or two into cropping — evenly, on a plant still producing — is the bag running out rather than anything wrong with the plant.

   What to do now

   Start a regular liquid feed at every second watering, using a general or high-nitrogen feed rather than a tomato one while the plant is in growth. The new leaves will come through green within a fortnight; the yellowed ones will not recover and can come off once the plant is growing again.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Cucumbers — feeding

   Checked 2026-08-13

2. Magnesium deficiency

   Candidate 2common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Magnesium deficiency

   Checked 2026-08-13

   Yellowing between the veins of the OLDER, lower leaves while the veins themselves stay green. Magnesium is mobile within the plant, so it is withdrawn from old growth to supply new — which is exactly why the pattern starts at the bottom.

   Points towards it

   • · the veins hold their green while the tissue between them yellows, which is the magnesium signature
   • · the plant withdraws magnesium from its oldest leaves first to supply the fruit
   • · a high-potassium tomato feed given to a cucumber is the commonest way this is induced under glass

   Argues against it

   • · an evenly pale leaf including the veins is nitrogen rather than magnesium — this rules it out entirely.

   Check this next

   Look at where the yellow stops relative to the veins, and then ask what the plant has been fed. A cucumber given tomato feed all summer commonly shows marbled yellowing between green veins on its lower leaves — the compost has magnesium in it, and the potassium in the feed is what is stopping the roots taking it up, so the answer is to change the feed rather than add more of it.

   What to do now

   Switch off the high-potassium feed for two or three weeks and water in Epsom salts at about 20g per litre. Cucumbers want a balanced or nitrogen-leaning feed rather than a tomato one for most of their life, and the routine of feeding everything in the greenhouse from the same bottle is what causes this.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Magnesium deficiency — treatment

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on cucumber — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general lower leaves turning yellow page lists causes across all plants, and the Cucumber profile lists the problems it is prone to.