How Do I Know When My Sweet Potatoes Are Ready to Harvest?

Sweet potato does not give the same obvious harvest signals as some other vegetables. There are no fruits turning colour, no pods drying out, no external signs above ground that tell you definitively the tubers are at their maximum size below. Instead, harvest timing is guided by season length, vine behaviour and a test dig — combined with the hard deadline of the first killing frost, after which tubers deteriorate rapidly if left in the ground.

Days to maturity

Most sweet potato varieties take 90–120 days from transplanting slips to harvest. Count from the day you planted the slips in the ground — not from when you started rooting them. Keep track of this date and use it as your earliest harvest guideline. Do not dig before 90 days even if the vines look magnificent — the tubers will be undersized.

Reading the vines

As summer gives way to autumn and day length shortens, sweet potato vines naturally begin to slow their vegetative growth and the plant puts more energy into bulking the tubers. The vines may begin to look slightly less vigorous, some older leaves yellow and drop, and the growth tips slow their extension. This shift — usually in September in northern climates — signals that tuber development is in its final phase. This is not a reason to harvest immediately; it is a sign the plant is doing exactly what it should be doing.

The test dig

To check tuber development without committing to a full harvest, use your hand or a small trowel to carefully scrape away soil near the base of one plant to expose the largest tuber. If it is at least 5–7 cm in diameter, the crop is approaching harvestable size. If it is still small and smooth-skinned, give the plant another two to three weeks. Replace the soil carefully after checking to avoid disturbing the plant.

The frost deadline and harvest technique

Frost kills sweet potato foliage immediately and causes the tubers to deteriorate rapidly within days if left in the ground. Harvest all tubers before the first forecast frost — even if that means harvesting slightly before ideal maturity. Dig carefully with a fork at least 30 cm from the crown to avoid spearing tubers. Lift gently and handle like eggs — skin damage at harvest is the main entry point for black rot and other storage diseases.

Curing before storage

Freshly harvested sweet potatoes must be cured before long-term storage. Cure at 29–32°C and 90–95% relative humidity for four to seven days — a warm shed, airing cupboard or polytunnel on warm days works well. Curing hardens the skin and heals harvest wounds, extending storage life dramatically. After curing, store at 13–15°C in a dry, dark, well-ventilated space. Do not refrigerate — cold temperatures cause internal damage to sweet potato flesh.