When to plant potatoes?

When, by region

The northern date is from Valle d'Aosta's Informatore Agricolo: plant by 20 April on the valley floor, after a month of chitting, so tubers form in May and June, before the heat. The central window is the traditional one in the regional proposal for Patata rossa di Colfiorito, an upland area: 1 March to 30 June, with harvest from 1 August through November. The south's three cycles are from Campania's integrated-production guide; October–November and August–September, and the 7 °C limit, are from Sicily's. Frost on young shoots, and the 15 cm trench, are from the RHS. The wider calendar is the planting calendar.

Seed tubers, not leftovers

Valle d'Aosta's Informatore Agricolo advises against replanting leftover tubers: they often carry viruses, and the crop can fall by 70–80%. The size it gives is 35–45 mm, about an egg, so you don't have to cut them. Cutting, the same article says, spreads viruses and rots the seed in a wet spring. Buy in March, a month before planting, while there is still a choice.

In Campania, cutting is suggested only for the early crop, on very light soil, at least a week ahead, leaving the two halves just attached so the cut can heal. On the main and second crops the tubers go in whole. Umbria's rules also allow only certified seed tubers.

Chitting and frost

Valle d'Aosta's chitting: about 20 days in the dark at 16 °C (61 °F), then 14 days in trays in the light at room temperature, until the sprouts are short and sturdy. The point is earlier emergence, and tubers forming in May and June.

The RHS chits early varieties in particular, for 4–6 weeks, from late January in mild areas to early March in cold ones. On maincrops the gain is small. Young shoots are scorched by April and May frosts: if one is forecast, cover the row or earth it up. In a pot on a balcony the compost gets as cold as the air: see the container garden.

Depth, spacing, earthing up

The RHS digs a trench 15 cm (6 in) deep, sprouts up. Earlies go 30 cm (1 ft) apart in rows 60 cm (2 ft) apart; maincrops 37 cm (15 in) apart in rows 75 cm (30 in) apart. In Campania the tuber sits 5–10 cm deep, deeper for the summer–autumn cycle, where the surface soil is hot and dry. At Colfiorito the rows are 70–90 cm apart. In Sicily, 75–90 cm between rows and 20–30 cm along the row.

Earth up once the plant has emerged, so the new tubers stay covered. Tubers left in the light turn green and are not for eating: the RHS says so. A thick mulch can stand in for part of that earthing up, as in the mulching guide.

Water, rotation, blight

Potatoes want steady water, especially while the tubers are swelling. In Valle d'Aosta, cracks and internal faults come with uneven water and with heat during tuber formation. In Campania, long spells above 30 °C mean the plant stores less starch. The summer–autumn cycle needs water as soon as the shoots emerge. How much is in the watering guide.

Don't put potatoes where potatoes, tomatoes, peppers or aubergines grew in the last 3–4 years: that is the same rule as for late blight, under pests and diseases. In Umbria the integrated-production minimum is 2 years; in a home garden the longer gap still applies. Tomato months and spacing are in when to plant them.