How to Grow Sweet Potatoes in Australia

Sweet potato (Ipomoea batatas) is a trailing vine in the morning glory family, Convolvulaceae, grown for its sweet storage roots and edible shoot tips. It is not a true potato, and it is a different plant again from yacon and Jerusalem artichoke. With four to five frost-free months, a few slips can fill a bed; the usual mistake is overfeeding, which grows vines instead of tubers.

Three mistakes that matter

1. Planting slips into cold soil. Below about 15°C at night the plants stop growing, and a cold, wet bed rots young slips. Wait for the soil temperature in When to plant, usually well after the last frost.
2. Feeding for leaves. Nitrogen-rich manure or fertiliser before the tubers form makes lush vines and few, small tubers. Dig in compost at bed preparation, give only the light feeds in Care by growth stage, and never side-dress with nitrogen.
3. Storing tubers in the fridge. Sweet potatoes suffer chilling injury below about 13°C, and the flesh turns pale, spongy and pitted. Cure them first, then store them as Harvesting Tubers describes.

When to plant

Plant rooted slips once the soil holds at least 17°C and nights stay warm, as Three mistakes that matter explains. In Melbourne and cooler districts, Melbourne Cup weekend is a sound marker. One planting a season is usual, from a tuber sprouted weeks ahead, as Planting Slips describes.

Late in a window, choose 'Northern Star', the earliest of the three varieties, so the crop can be lifted before autumn frost. In the tropics, plant on mounds if the crop will sit through wet-season rain.

Site, soil and setup

Plant each year in a bed that did not grow sweet potato the year before, because weevils, nematodes and soil diseases build up.

Planting Slips

Gardeners grow sweet potato from slips (rooted shoots) sprouted from a seed tuber; true seed is kept for breeding. Due to quarantine restrictions and our shipping policy, we cannot send sweet potato tubers to Western Australia or Tasmania.

Choosing a variety

Growing slips

Start the tuber four to six weeks before planting, and until then hold it somewhere airy and above 13°C, never in the fridge. Half-bury it on its side in a tray of moist potting mix, in a warm, bright spot such as a sunny windowsill. Shoots appear in two to four weeks. Planting the whole tuber instead gives one plant, not eight or ten.

Snap each shoot off at 15 to 20 cm, strip the lowest leaves and stand it in water; roots usually appear within a week. How to sprout sweet potato slips covers the jar method and what to do if nothing happens.

Planting out

• Harden off rooted slips (accustom them to outdoor sun and wind) over a few days, starting in a sheltered, shady spot and adding morning sun.
• Plant in the late afternoon, laying each slip at a slant with two thirds of the stem buried, 7 to 10 cm deep.
• Bury at least three nodes (the joints where leaves grew), because the tubers form from those nodes.
• Space slips 30 to 40 cm apart, in rows 75 cm to 1 m apart.
• Expect slips to wilt for a day or two after planting; they recover once rooted.
• Mulch with straw once the slips stand up again.

Care by growth stage

Once rooted, sweet potato needs warmth, even moisture and restraint with feed. Storage roots start forming five to eight weeks after planting.

Every few weeks, lift the runners and lay them back down, so they do not root along their length and set many small tubers. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched into the soil can help.

Companion planting

The vines smother anything low beside them, so companions work best under taller plants or before the vines run.

Grows well with

• Fruit trees in frost-free gardens, where the vines make a living groundcover beneath the canopy.
• Lettuce, sown between young slips and cut before the vines close over.

Keep apart

• Morning glory and other wild Ipomoea vines, relatives that host sweetpotato weevil.
• Small, low crops such as herbs and bush beans, which the running vines shade out.

Harvesting Tubers

Lift sweet potatoes 16 to 22 weeks after planting, when the leaves start to yellow, and before the first frost. To check, cut a tuber: sap that dries white means it is ready, while sap that dries dark or green means it needs longer.

• Cut the vines off five to ten days before digging, so the skins toughen.
• Fork gently from the edge of the bed, well back from the stems, because the skins bruise easily.
• Lift the tubers by hand and brush off the soil; do not wash them.
• Set damaged tubers aside to eat first, since wounds let rot in.
• Keep a few sound, medium tubers from healthy plants to sprout next spring.

Curing

Curing heals skin wounds, cuts storage losses and turns some starch to sugar, so cured tubers taste sweeter. Start straight after digging, with the tubers unwashed.

Store cured tubers dark and airy at 13 to 16°C, wrapped in paper or in a single layer. They usually keep for four to five months. Roast, mash or bake them, and steam or roast 'Eclipse' rather than boiling, which leaches out the purple.

Common problems

Tuber rots instead of sprouting. A tuber kept cold and wet goes soft and smells; start a fresh one somewhere warmer, since slow sprouting usually means too cold.

Many small tubers along the runners. Runners that root at their nodes set small tubers of their own and split the crop; lift them as Care by growth stage describes.

All vine and few tubers. Nitrogen fed early, as Three mistakes that matter warns, or a bed kept wet in hot weather grows leaves at the expense of storage roots. Once the vines run, keep to the weekly deep soak in Care by growth stage.

Cracked tubers. A dry spell followed by heavy rain or a deep soak splits swelling tubers along their length; keep moisture even from week eight.

Ribbed or knobbly tubers. Cold soil while tubers first form, uneven watering or nematodes misshape them, and 'Northern Star' is prone to it; plant into warm soil.

Tubers rot in storage. Digging wounds and warm, humid storage above 20°C let rot in; cure first, and see Three mistakes that matter for chilling.

Holes chewed into tubers. Curl grubs (Heteronyx and Heteronychus spp.), the white, C-shaped larvae of scarab beetles, chew large holes that let rot in. True wireworms (Elateridae) and false wireworms (Gonocephalum and Pterohelaeus spp.) leave smaller holes. Fork over the top 15 cm before planting and pick out any larvae you turn up. Read more

Tubers dug up or gnawed. Rats, bandicoots, brush turkeys and wallabies all eat sweet potatoes, and native animals are protected. Fence the bed or cover it with wildlife-safe netting (white, taut, with mesh too small for a finger). Read more

Pests and diseases

Most leaf-chewers do little lasting harm, but the weevil can spoil a crop, particularly in tropical and subtropical gardens.

Sweetpotato weevil (Cylas formicarius). This slender, ant-like weevil, up to about 7 mm long, has a blue-black head and abdomen with an orange thorax and legs. Its grubs tunnel through tubers and stems, and even light feeding leaves the flesh bitter. Dig promptly once the crop is ready, because damage grows the longer infested tubers sit.

Root-knot nematode (Meloidogyne spp.). These microscopic soil worms blister, crack and roughen the roots, and the plants yellow and wilt easily. Damage is worst in sandy soil and warm weather. Rotate beds as Site, soil and setup describes, and note from Choosing a variety that 'Beauregard' is susceptible. Read more

Scab (Sphaceloma batatas). Brown, scabby lesions form on stems and leaf veins in wet weather, and the leaves cup and deform. Spores spread in rain splash and on planting material, so never take slips or tip cuttings from an affected plant.

Sweet potato feathery mottle virus (a potyvirus). Aphids and infected planting material spread this common virus. Leaves show mottled spots or feathery yellowing along the veins, and tubers grow thin and long, sometimes covered in fine cracks. Never sprout slips from a plant that showed these symptoms.

Convolvulus hawk moth (Agrius convolvuli). Its caterpillars, green at first and later olive or brown, grow up to 120 mm long and chew the leaves. They are most common in the northern wet season and in summer in NSW and Victoria. Pick them off by hand.

Silverleaf whitefly (Bemisia tabaci). Tiny white adults and flat nymphs suck sap under the leaves, causing yellow or purplish stippling, though the effect on the crop is usually minor. Avoid routine spraying, because this whitefly resists many common insecticides. Read more

Prevention first

• Sprout slips from clean seed tubers, not supermarket stock, and renew your own saved line every few years.
• Keep the soil moist and hilled over the tubers, so weevils cannot reach them through cracks.
• Clear away vines and missed tubers after harvest, since they carry weevils into the next season.
• Hose aphids off young shoots, because they spread feathery mottle virus.

Tips

1. Pick young shoot tips as a green vegetable, cooked like spinach in stir-fries, soups and curries. A light picking costs the tubers little.
2. For a few early tubers, find a thickened stem at ground level, feel beneath it and lift one, leaving the plant to grow on.
3. Fill gaps or start a second bed with 20 to 30 cm tip cuttings from the vines, which root and grow like slips.
4. Tubers dug in autumn are often reluctant to sprout, so give saved seed tubers extra time and warmth when you start them in spring.