Why Are My Potatoes So Small and Thin?

You dig up your potato rows expecting a satisfying yield of chunky tubers and instead find a mass of small, marble-sized potatoes. It is one of the most frustrating harvests to face after a whole season of care. Small potatoes are very common and almost always have a clear, fixable cause. Unlike some potato problems where you have limited options once the season is under way, understanding what limits tuber size gives you specific steps to improve your next crop significantly.

Water shortage during tuber bulking

The single biggest cause of small potatoes is insufficient water during the tuber bulking phase — roughly from flowering until harvest. This is when tubers do the majority of their growing, and consistent soil moisture during this period is critical. If the soil dries out even for a week or two during active bulking, tuber development stalls and the final size is limited. Deep, regular watering every five to seven days during dry weather — around 20 litres per square metre — makes a very significant difference to final tuber weight. Sandy soils need more frequent watering than heavier soils.

Too many tubers competing

If there are many tubers on each plant — which happens naturally with vigorous plants in good conditions — each individual tuber receives less of the plant's resources and stays smaller. This is why first early varieties, which produce fewer tubers per plant, tend to produce larger individual potatoes than maincrops. You can encourage larger tubers in maincrop varieties by removing some of the smaller developing tubers (called "de-bulbing") once tubers are visible, though this is labour-intensive. More practically, choosing varieties known for large tuber size, ensuring each plant has adequate space, and maintaining water and feed levels achieves a good average size without intervention.

Planting too late

Potatoes planted late in the season have less growing time before the foliage dies back or frosts arrive. Late-planted maincrops may simply not have had enough weeks to build the tuber size they are capable of. The solution is to plant on time — first earlies in late March/early April, maincrops in April, and second earlies in between, adjusted for your local last frost date. Chitting seed potatoes before planting gives an additional head start that can be the difference between a modest and a full harvest in a season with an early autumn.

Poor soil fertility

Potatoes are hungry plants that benefit from well-prepared, fertile soil. Poor soil — particularly ground low in phosphorus (which drives root and tuber development) or potassium (which drives tuber bulking) — produces small crops even with good watering. Incorporating well-rotted compost or manure into the bed before planting, and applying a balanced fertiliser or a dedicated potato fertiliser (higher in potassium), produces noticeably larger tubers. Avoid excessive nitrogen, which drives leafy growth at the expense of the tubers below.