How to Grow Yam Bean in Australia

Yam bean (Pachyrhizus erosus), also sold as jicama, is a climbing vine in the legume family, Fabaceae, grown for its crisp, sweet tuberous root. It is not a true yam, only the root is eaten, and green beans are a separate crop, covered in growing beans. A long, warm season suits it, from Cape York to the Sunshine Coast, and the error most gardeners make is letting the vine flower.

Three mistakes that matter

1. Sowing late or into cold soil. Tubers swell as the days shorten in autumn, so a late vine runs out of warm weather while its roots are still small. Sow at the soil temperature in When to plant, with at least five frost-free months ahead.
2. Leaving the flowers on. Flowers and pods draw strength from the root, and heavy flowering shrinks the tubers. Pinch off each flower spike as it appears, as Care by growth stage describes, and the toxic pods never form.
3. Feeding it like a leafy crop. Too much nitrogen builds a huge vine over a small tuber, and as a legume the plant makes much of its own. Use the potassium-rich feeds in Care by growth stage instead.

When to plant

Sow once the soil holds at least 21°C at sowing depth and all danger of frost has passed, because cold, wet soil rots the seed. The vine grows through the long days of summer, and the tubers swell as days shorten in autumn. Each crop takes six to eight months, so one sowing a year is the rule.

At the cool edge of a window, raise seedlings in pots on a seedling heat mat 8 to 10 weeks before the last frost. Where fewer than five frost-free months remain, expect smaller tubers, or grow the vine in a greenhouse.

Site, soil and setup

Yam bean fits a rotation with corn, sweet potato and pumpkin. Avoid beds where root-knot nematode has galled earlier crops, since it damages the tubers.

Sowing Seeds

Sow seed direct at the foot of the trellis, or start it in pots where the warm season is short.

Sowing direct

• Sow one or two seeds per spot, 20 to 30 mm deep and 15 to 25 cm apart.
• Keep the soil moist but never soggy, since saturated soil rots the seed.
• Seedlings usually appear in 7 to 21 days.
• Where both seeds come up, snip the weaker seedling at soil level.
• Keep young plants weed free, and water only the planting sites.

Use fresh seed, because it loses viability quickly when stored somewhere hot and humid. Vines raised from seed vary a little from plant to plant, which is normal.

Starting in pots

• Sow one seed per pot, 20 to 30 mm deep, and keep the mix warm on the heat mat.
• Plant out once frost has passed and the soil holds the warmth When to plant gives.
• Harden off (accustom seedlings to outdoor sun and wind) for about a week first.
• Plant the root ball whole, then water it in with seaweed solution at half strength.

Care by growth stage

Steady moisture, potassium and flower removal decide the size of the tubers. Weeks count from sowing and shift with the sowing date and day length.

Guide the first runners onto the trellis. Trim runners that pass the top, keeping the vine to about 1 to 1.5 m, so its strength goes to the root.

Flowering often starts about eight to ten weeks after sowing and can continue for three months. Pinch off every spike of blue or white flowers as it appears, and check twice a week.

Companion planting

Yam bean does well beside tall summer crops and badly beside other climbers on its trellis.

Grows well with

• Corn, its partner in traditional Mexican plantings and a sound crop to rotate with it.
• Pumpkin, a good crop to follow yam bean in the same bed.

Keep apart

• Climbing beans and peas, which compete for the trellis and draw green vegetable bug.
• Tomato and carrot in the bed before it, where root-knot nematode is present, since both host the nematode.

Harvesting Tubers

Tubers are usually ready six to eight months after sowing, once they reach about 10 cm across; scrape soil from the crown to check.

• Lift before the first frost, even if the tubers are still small.
• Leave them to grow to about twice that width at most, since very large tubers turn fibrous and bland.
• Dig with a fork well out from the crown, and avoid skinning the tubers, because damaged skin rots in storage.
• Trim off the vine and the thin roots, then wash the tubers.
• Dry them in the shade before storing.

Store whole tubers somewhere cool, dark and dry, ideally at 12.5 to 15°C, where they keep for one to four months. Stems start to sprout after about two months. Keep them out of the fridge, because 10°C or below damages the flesh within one to three weeks.

Peel off the brown skin, then eat the crisp flesh raw in salads, or use it like water chestnut in stir-fries, soups and stews.

Common problems

Huge vine, small tubers. Too much nitrogen, flowers left on or a late sowing sends the plant's strength into leaf; see Three mistakes that matter.

Split tubers. Irregular watering makes the swelling root crack, so keep moisture even, as the last rows of Care by growth stage describe.

Misshapen or forked tubers. Heavy clay or waterlogged soil distorts the root and makes it hard to dig; grow on mounds, as Site, soil and setup describes.

Seed never comes up. Cold or saturated soil rots the seed, and old seed germinates poorly; see damping off in Pests and diseases.

Fibrous, tasteless tubers. Tubers left in the ground too long lose their crisp, sweet flesh; lift at the size Harvesting Tubers gives.

Tubers soften or rot in storage. Chilling in the fridge or skin damaged at digging lets rot in; see Harvesting Tubers.

Pests and diseases

Yam bean is generally free of serious pests and diseases, and its hairy leaves discourage many insects.

Root-knot nematode (Meloidogyne spp.). These microscopic soil worms are very common in Queensland soils and can damage yam bean tubers. Plants look stunted and unthrifty, and dug roots carry lumpy galls. Rotate beds, and dig in compost, which feeds the nematodes' natural enemies. Read more

Green vegetable bug (Nezara viridula). These bright green, shield-shaped bugs, 13 to 15 mm long, are found throughout Australia and swarm in coastal Queensland. They move onto legumes at flowering and suck the pods, and they can also damage yam bean leaves. Check in the early morning, when bugs bask at the top of the vine, and pick them off. Read more

Damping off (Pythium spp. and Rhizoctonia solani). These soil moulds rot seed and collapse seedlings, and Pythium has also rotted yam bean roots in overseas trials. They thrive in wet soil, especially when temperatures do not suit the crop. Sow only into warm soil, and avoid overwatering. Read more

Prevention first

• Rotate beds as Site, soil and setup describes, and add compost before each crop.
• Earth up soil over any tuber shoulders that show, so rats and mice and insects cannot reach them.
• Keep up the flower removal in Care by growth stage, which leaves pod-feeding bugs little to eat.
• Water the soil rather than the leaves, and never let the bed stay waterlogged.

Tips

1. To save seed, let one healthy vine flower and set pods. Pick the pods as they turn brown, often in late winter, before they split. Store the seed dry, cool and labelled, out of reach of children.
2. Wear gloves and long sleeves when working among the vines, because the fine hairs on the stems and leaves can irritate skin.
3. After lifting, compost the vine tops but not the pods or seed. A legume returns much of its nitrogen to the soil in its leaves and stems.