How to grow Sweet Potato in New Zealand

Where to plant

Kūmara needs full sun, warmth and very well-drained soil. It does poorly in clay and heavy soil, so build raised mounds or ridges about 20 cm high from sandy, free-draining soil, mixing in river sand if needed. Leave plenty of room, as the vines spread across the ground.

A firm layer under the loose soil stops the tubers going too deep. Some home gardeners bury a sheet of iron under the bed for the same reason, so the tubers grow outwards and are easy to dig.

• Northland and Auckland: the best kūmara country. Most of New Zealand's commercial crop is grown in the Northern Wairoa district of Northland, around Dargaville.
• Waikato, Bay of Plenty, Wellington and Nelson: plant in November once the soil has warmed, in the warmest spot, and harvest before the autumn frosts.
• Central North Island: marginal. Warm lake-edge sites such as around Rotorua can work with black plastic mulch or a tunnel house.
• Canterbury, Otago and Southland: grow it in a large tub or tunnel house that can be covered or moved before frost, and expect a small crop.

Choosing a variety

Three main kūmara varieties are sold in New Zealand, and tipu of each are sold in garden centres in spring.

• Owairaka Red: red skin and creamy white flesh, the most common variety. It is a New Zealand cultivar from an early European introduction and was first grown commercially in 1954.
• Toka Toka Gold: golden skin and flesh and a sweeter taste than red. Another New Zealand cultivar, named after Tokatoka Peak in the Kaipara and first grown commercially in 1972.
• Beauregard: orange flesh, sweeter than red or gold. A United States variety introduced in 1993.

How to plant

1. Buy tipu from a garden centre in spring, or raise your own. In late winter, half bury a kūmara in moist sand or stand a halved kūmara cut side down in water in a warm place such as the hot water cupboard. Shoots appear in a week or two.
2. When the shoots are 10 to 20 cm long, ease them off the tuber and stand them in water to root, keeping them warm.
3. Build mounds or ridges of loose, free-draining soil about 20 cm high, enriched with compost or sheep pellets.
4. Once the frosts are over and the soil is warm, plant the rooted shoots about 30 to 50 cm apart, with the rooted base only 5 to 7 cm deep and the stem lying along the soil or set in a J shape so the roots grow sideways.
5. Water well after planting and every two to three days until they are growing strongly.

Watering and feeding

Keep the soil moist while the tipu establish, then water through dry spells or whenever the plants wilt. Do not let the beds dry out in summer, as this slows tuber growth. Pea straw mulch helps hold moisture.

Feed with a general vegetable fertiliser at planting and every four to six weeks, or a liquid seaweed and fish feed every couple of weeks. Avoid rich, high-nitrogen feeding, which gives lots of vine and few tubers.

Pruning and care through the year

• Lift the vines off the ground every week or two. Where a vine touches the soil it roots, and the plant then puts its energy into many small tubers along the vine instead of good-sized ones at the base.
• Keep the beds weeded until the vines cover the ground.
• Watch for the large green or brown caterpillars of the kūmara moth on the leaves, and pick them off.
• Kūmara is one of the plants the tomato/potato psyllid can feed on, so check under the leaves if psyllids are a problem in your garden.

Harvesting

Kūmara is ready about five months after planting, when the leaves start to yellow. In the north that is usually from March to May. Dig before the first frost.

Cut off the vines first, then dig carefully, as the skins bruise easily. Leave the tubers on top of the soil for a few days, covered with the cut vines, to cure in the sun. Curing toughens the skin and turns some of the starch into sugar, so kūmara grows sweeter in storage. Store in a dry, airy place, not in the fridge, and keep them safe from rats and mice.

Growing in pots

In cooler areas, grow kūmara in a large tub 50 to 60 cm across and at least 30 cm deep, filled with vegetable or potting mix, in full sun. Several tipu can share a big tub. Water every couple of days through summer and feed regularly, and move the tub under cover if frost threatens before harvest.

Kūmara in Māori history

The Polynesian ancestors of Māori brought kūmara to New Zealand when they arrived in the 13th century. The traditional kūmara were small, finger-sized tubers, and because the climate here was much cooler than in tropical Polynesia, they would not grow through winter. Tubers were harvested around March and kept over winter in underground storage pits called rua kūmara, some for eating and some to plant out the next spring.

Gardens, called mara kūmara, were laid out as mounds of loose soil on sunny, north-facing slopes, often with sand and gravel added to drain and warm the soil. Kūmara was linked with Rongomatane (Rongo), the atua of cultivated food, and planting and harvest were marked with karakia and tapu. The larger kūmara grown today descend from a North American variety brought in during the 1850s, which Māori and settlers both took up.