Why are my potatoes so small?

A potato plant builds its tubers in a fairly short window, and anything that interrupts the plant during that window costs you size rather than number. Working out which interruption you had is mostly a matter of remembering the weather and how the row was managed.

Step by step

1. Check you have not simply lifted too soon.First earlies are ready around flowering, but that is a signal not a deadline — a week or two more often doubles the size. Maincrop is not ready until the haulm has yellowed and died down. Scrape soil from one plant and look before committing the row.
2. Think back to watering at flowering time.The critical period is tuber initiation and swelling, which coincides roughly with flowering. A dry fortnight then is the single biggest cause of a small crop. Potatoes want a proper soak — enough to wet the root zone, not a sprinkle.
3. Check the spacing you used.Potatoes planted too close compete and produce many small tubers. Earlies want around 30cm in the row, maincrop nearer 40cm, with generous gaps between rows.
4. Consider how many sprouts you left on the seed.Every sprout becomes a stem, and every stem carries tubers. Lots of stems means lots of small potatoes. Rubbing back to three or four strong sprouts before planting gives fewer, bigger tubers.
5. Look at what the foliage did.Small yellow-green plants that never bulked up mean the plant never had the leaf area to fill tubers — usually poor soil, drought, or a container far too small. Big lush plants with small tubers usually means too much nitrogen.
6. Account for blight or an early frost.Anything that kills the foliage stops tuber growth dead. If the haulm went down in August with brown blotches, the tubers stopped at whatever size they had reached.
7. Check the compost volume if you grew in a container.A bag or pot under about 30–40 litres per plant will always give a token crop. Potatoes need depth and volume; three tubers in a 20-litre pot is a decoration, not a harvest.
8. Feed correctly next time.Potatoes are hungry. Well-rotted organic matter dug in before planting, plus a balanced or potash-leaning feed as they grow. High nitrogen alone gives magnificent tops and disappointing bottoms.

Tips

• Watering matters far more once flowering starts than it does early on. A thorough soak every week or ten days in a dry spell from flowering onwards is what makes the size difference.
• Hollow, brown-centred large tubers are the opposite problem — a growth check followed by a rush of growth. Steady watering prevents that too.
• In a container, water until it runs from the bottom, then leave it until the surface is drying. Potatoes hate sitting wet but hate drying out even more.
• Earthing up produces more tubers but does not by itself produce bigger ones — water and season length do that.

Common mistakes

• Judging readiness by the calendar — Weeks-from-planting is only a rough guide and moves a fortnight with the season. Looking under one plant costs nothing and settles it.
• Watering little and often — A light sprinkle wets the surface and never reaches the tuber zone. Potatoes need infrequent, generous watering that soaks down.
• Piling on nitrogen-rich manure just before planting — It drives leafy growth, delays tuber formation and can worsen scab and slug damage. Organic matter is best incorporated well before planting.

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