Why Are My Oca Leaves Curling?

Aphids

Aphids (primarily peach-potato aphid) are the most common cause of leaf curling on oca during the summer months. They colonise the soft growing tips, causing the newest, smallest leaves to curl and pucker tightly around the colonies. Affected leaves may be coated in sticky honeydew and become blackened with sooty mould that grows on the honeydew. Large infestations can stunt the growing tips of the plants and slow vegetative growth during the critical period before day length triggers tuber formation.

What to do

• Apply insecticidal soap directly to aphid colonies on the growing tips. Natural predators (ladybirds, hoverfly larvae) usually establish within a few weeks and bring colonies under control without further treatment. Avoid nitrogen-heavy feeding, which produces soft, aphid-attractive growth.

Slugs

Slugs cause significant damage to oca in spring, particularly to the small, soft shoots as they emerge. The tender leaflets are attractive to slugs; damage produces ragged holes in the leaves which then curl and distort as the surrounding tissue expands around the damaged areas. A cold, wet spring can be particularly damaging, as slug populations are highest and oca growth is slowest under these conditions.

What to do

• Protect emerging shoots with nematode slug control (Phasmarhabditis hermaphrodita) applied to the soil around the plants. Check plants in the evening and remove slugs by hand. Growing oca in containers with copper tape around the rim significantly reduces slug damage. Once the plants are well established and growing vigorously, slug damage becomes less significant.

Drought

Oca leaflets have an Oxalis-family habit of folding upward and closing in bright sunshine and during drought, which can look alarming but is normal thermonastic and nyctinastic behaviour. Prolonged drought causes more persistent folding and, in severe cases, yellowing of the older leaves. Oca prefers consistently moist soil; in hot summers, the shallow root system dries out quickly in light soils.

What to do

• Water regularly during dry spells in summer. Apply a mulch around the plants to retain soil moisture. Temporary leaf closure in afternoon heat is normal and resolves overnight; if leaves remain closed for extended periods, check soil moisture. Container-grown oca needs more frequent watering than garden-grown plants.

Frost and cold

Oca is frost-tender: the leaves and stems are killed by even a light frost, which collapses the soft tissue and causes the leaves to turn dark, water-soaked, and limp. In the UK, this means tuber development stops in October as the risk of frost increases, and the aerial growth must be protected to maximise the short remaining growing season. Cold (but not freezing) temperatures below 5°C slow growth noticeably and cause older leaves to yellow and curl as the plant enters dormancy.

What to do

• Protect oca from frost from October onwards with cloches, horticultural fleece, or by potting up the crowns and bringing them under glass. Dig the tubers promptly after the first frost kills the tops, as the tubers are more frost-hardy than the aerial growth but should not be left in the ground through a hard freeze.

Frequently asked questions

Why are my oca leaves curling?

Oca (Oxalis tuberosa) leaves curl most commonly because of aphid infestation, slug damage on young plants, or drought. Oca is an Andean tuber crop related to wood sorrel, with clover-like, three-lobed leaves that have a pleasantly lemony flavour. Aphids (particularly peach-potato aphid) colonise the growing tips of oca in summer, causing the youngest leaves at the shoot tips to curl and pucker around the dense feeding colonies; the leaves may also show a sticky honeydew residue from the aphid colonies. Slug damage on the soft young growth in spring causes irregular holes in the expanding leaves, which then curl and distort around the damaged areas as they grow. Drought in a warm summer causes the small, clover-like leaflets to fold upward and close (a natural response common to Oxalis species) rather than the full leaf wilting as in many other plants.

How do I grow oca in the UK?

Oca (Oxalis tuberosa) is one of the most rewarding Andean tuber crops for UK gardens, producing small, colourful, waxy tubers (typically yellow, pink, or red depending on variety) with a lemony-earthy flavour. It is a short-day plant: it does not begin to bulk its tubers until the day length drops below about 12 hours, which in the UK occurs around late September. For this reason, tubers are not ready to harvest until October to December, and the crop must be protected from frost until as late in the season as possible to maximise yield. Plant tubers in pots under cover in March to April, and plant out after the last frost (May to June) into a sunny, sheltered position in free-draining but moisture-retentive soil. Water regularly in dry spells. In October, erect a frost-protection cloche or fleece over the plants, or pot up the crowns and bring them into an unheated greenhouse for the last 4 to 6 weeks of the season. After the first frost kills the tops, dig the tubers immediately. Tubers store well in a cool, dark, frost-free place.

Why does oca fold its leaves in the afternoon?

Oca (Oxalis tuberosa) is a member of the Oxalidaceae family, which includes wood sorrel, and shares the family trait of nyctinasty (leaf movement in response to light) and thermonasty (leaf movement in response to temperature). In bright sunshine and in drought conditions, oca leaflets fold upward and close, reducing the leaf surface area exposed to the sun and limiting water loss through transpiration. This is a normal, evolved behaviour and does not indicate a problem with the plant. The leaflets typically reopen in the evening or in cooler, cloudier conditions. If the leaves remain folded for extended periods even in low light, the plant may be experiencing drought stress; check the soil moisture and water if dry.

Is oca the same as New Zealand yam?

Yes, oca (Oxalis tuberosa) is commonly known as New Zealand yam in the Southern Hemisphere, particularly in Australia and New Zealand, where it was introduced as a minor vegetable crop. Despite the common name, oca is not related to true yams (Dioscorea species), which are a completely different genus of monocot plants native to tropical Africa, Asia, and the Americas. Oca is an Andean crop native to the high Andes of South America, where it has been cultivated alongside potato and quinoa for thousands of years. In the Andes, the tubers are often left in the sun after harvest (a process called 'soaking') which converts the oxalic acid to sugars and produces a sweeter flavour; UK-grown oca tubers are usually milder and can be eaten without this step.