<em>Oxalis</em>: planting, growing and care

Description and botany

Oxalis are small herbaceous plants, often bulbous, with a spreading habit. They are decorative for their foliage, which resembles that of clovers, and for their star-shaped flowering. Oxalis comprises about 500 species, which can be annual or perennial. They have a very wide worldwide distribution, but greatest species diversity occurs in South America and South Africa. Eight species occur in the wild in France, including Oxalis acetosella and Oxalis corniculata, which are present across most of metropolitan territory. In the wild, oxalis grow in open habitats or underwood, which explains why some prefer sun while others favour shade.

Oxalis are not always very hardy. Some tolerate down to -10°C, but others are sensitive when temperatures approach 0°C and must be brought under cover for winter. Oxalis triangularis, for example, is commonly grown as a houseplant and can spend whole year in an apartment or house.

The name Oxalis derives from Greek oxys, meaning “acid, sour”, in reference to tart taste of its leaves. In French they are sometimes called oxalids, or “false shamrocks”. Oxalis acetosella is also called “wood-sorrel” or “small sorrel” in English.

Oxalis gave their name to family Oxalidaceae, of which they represent large majority of species (500 out of 600). Apart from resemblance of their foliage, oxalis have nothing to do with clovers (Trifolium sp.), which belong to family Fabaceae. Their flowers are very different.

Oxalis acetosella: botanical illustration

Oxalis are sometimes regarded as weeds, and some can be truly invasive… But this mainly concerns Oxalis acetosella and Oxalis corniculata, which grow wild in France and can be difficult to eradicate.

Oxalis contain oxalic acid, which gives leaves tart flavour similar to sorrel or rhubarb, but makes them toxic in large quantities.

Oxalis are low, creeping plants that can be used as groundcover. They measure between 5 and 30 cm in height and spread rapidly in width. Oxalis hedysaroides is an exception, as it can reach up to 1 m in height!

Oxalis triangularis has long, very thin petioles giving airy aspect. This characteristic, combined with triangular, purple leaflets, gives very graphic, modern appearance, making it a popular houseplant.

Depending on variety, flowering occurs between spring (May–June) and autumn… with exception of Oxalis versicolor, which has reversed cycle and flowers in winter. In general, Oxalis flowering lasts a long time.

Flowers are usually white, pink or yellow, sometimes red or mauve. Flowering is bright, often in light vivid tones. Centre of corolla is often shaded, either yellow or darker than rest of petals. Flowering of Oxalis versicolor is striking, since petals are bicoloured, white and red. One might think it an horticultural selection for its unusual flowers, but it is indeed a botanical species found in the wild. In Oxalis hedysaroides, yellow flowers stand out beautifully against dark purple foliage.

Oxalis flowers may be solitary or grouped in umbels. They are borne on long, fairly flexible peduncles. Flowers are star-shaped, very simple, composed of five petals, fused into a tube at base. They are surrounded by five free sepals. Flowers are regular, with radial symmetry, and can be cup-shaped or funnel-shaped. Tube of corolla is sometimes long and narrow, flaring into a funnel, as in Oxalis versicolor. Flowers also have ten stamens (five long and five short) bearing pollen.

Flowers are sensitive to light: they open fully in sun and close in evening or when sky is overcast. More generally, oxalis need a bright position to produce generous, abundant flowering.

Flowering of Oxalis tetraphylla (photo Wildfeuer), Oxalis versicolor (photo Nzfauna) and Oxalis triangularis ‘Atropurpurea’

Leaves of oxalis strongly resemble those of clovers. This is reason some species with four leaflets are popular—playing on idea that four-leaf clovers bring luck.

Leaves often have bright light green colour. They can also be purple, as in Oxalis triangularis… or bicoloured, as in Oxalis deppei, green on outer sides of leaflets and brown at centre, with sharp contrast. Oxalis adenophylla has grey-blue coloured leaves.

Generally, leaves consist of three cordate leaflets (heart-shaped) or triangular leaflets. They are often broad but can also be very narrow, almost linear (Oxalis versicolor). In Oxalis triangularis, leaflets are truly triangular with geometric shape, while margins are much more rounded in other oxalis.

When they have four leaflets, oxalis very much resemble four-leaf clover! Other species have leaves composed of many leaflets (up to about twenty), as in Oxalis adenophylla. Each leaflet is attached at centre at same point; when many, whole leaf takes rounded shape. Some oxalis have exceptional, highly divided foliage, such as Oxalis palmifrons, creating a palmate effect.

In most species, foliage folds at night or when sky is overcast, and unfurls by day (nastic movement). Plant thus reacts to light. One might almost say plant “sleeps” at night, entering rest by folding foliage and flowers!

Foliage of Oxalis tetraphylla (photo Wouter Hagens), Oxalis adenophylla and Oxalis triangularis ‘Atropurpurea’

Oxalis versicolor has particularity of doing opposite to many plants: it flowers in winter and goes dormant in summer.

Oxalis generally have bulbs, but can also form rhizomes or tubercles. Oca of Peru (Oxalis tuberosa), for example, produces edible tuber. Some oxalis can spread very quickly by producing new bulblets next to original bulbs.

After flowering, oxalis produce capsules, thin and elongated, containing seeds. These are expelled, ejected when capsule opens.

Main varieties of Oxalis

Most popular varieties :

Our favourite varieties :

Oxalis triangularis

Appreciated for its deep purple, very dark, compound leaves with three truly triangular leaflets. It also bears small white flowers, perfectly set off by the foliage. Often cultivated indoors as young plant!

• Flowering time August to October
• Height at maturity 20 cm

See the product sheet

Oxalis versicolor

This very unusual oxalis bears bicoloured flowers, white and red on the reverse, forming a spiralled pattern. It notably flowers in winter and enters dormancy during summer! Awarded Award of Garden Merit by Royal Horticultural Society (RHS). Not hardy.

• Height at maturity 10 cm

See the product sheet

Oxalis deppei

This oxalis has leaves with four leaflets, whose shape closely resembles that of the four-leaf clover. They are marked with dark brown at the centre and light green on the outside, producing a very clear, original contrast. Flowers are pink-red.

• Flowering time August, September
• Height at maturity 20 cm

See the product sheet

Oxalis triangularis subsp. papilionacea Atropurpurea

This oxalis has triangular purple leaves and also produces pale pink flowers. Awarded Award of Garden Merit by Royal Horticultural Society (RHS).

• Flowering time July to December
• Height at maturity 15 cm

See the product sheet

Oxalis adenophylla

This oxalis produces beautiful white and pink flowers, veined with dark pink. Foliage is highly decorative, bluish-grey, nicely dissected and taking on a plicate aspect. Awarded Award of Garden Merit by Royal Horticultural Society (RHS).

• Flowering time July, August
• Height at maturity 10 cm

See the product sheet

Planting oxalis

Where to plant?

Most oxalis need plenty of light: choose a sunny spot to allow their flowers to open. Some species, however, grow well in shade or partial shade, for example in woodland (Oxalis oregana, Oxalis acetosella…). For Oxalis triangularis, the brighter the position, the darker and more attractive the foliage will become, but avoid direct sunlight.

Oxalis prefer humus-rich, fertile soil; so do not hesitate to add some well-rotted compost to enrich soil. They also like light, well-draining growing media (excess moisture can rot bulbs), but soil should remain cool in summer. Oxalis also prefer slightly acidic soil.

The oxalis adapt well to being grown in pots or window boxes. This is a good option if you live in a region with a cold climate, as it allows you to move them under cover for winter.

Some oxalis can also be grown indoors or in a greenhouse year-round. This is notably the case for Oxalis triangularis. Place it in a pot or hanging basket in your home, in a bright spot but without direct sun.

When to plant?

We recommend planting oxalis in spring, as soon as risk of frost has passed (May–June).

How to plant?

Oxalis are sold as bulbs. We recommend planting them in small groups, placing at least 4 or 5 bulbs together each time. Once planted, they spread quite rapidly sideways. Space bulbs about 10–15 cm apart.

1. Work soil, incorporating well-rotted compost and, if needed, some coarse sand or gravel to improve drainage.
2. Dig small holes so bulbs sit about 8 cm deep.
3. Place oxalis bulbs in holes.
4. Cover with soil and firm lightly with the palm of your hand.
5. Water well.

Continue to water in following weeks.

You can also grow oxalis in pots.

To do this:

1. Choose a fairly wide container, for example a window box or hanging basket. Make sure there are drainage holes in base to allow water to escape.
2. Fill with a free-draining substrate (a mix of potting compost and sand works well). It is also preferable to add a drainage layer at pot base before adding compost.
3. Plant oxalis bulbs.
4. Cover with substrate and, if desired, plant other plants alongside them.
5. Water.
6. Place pot in a sunny spot; for indoor placement and for Oxalis triangularis, avoid direct sun.

Caring for Oxalis

Oxalis thrive in soils that remain relatively cool: remember to water them regularly during summer… even more so if you grow them in a pot! In that case, and during period of growth, you can water once a week. But in winter, when plant is in dormancy, reduce watering sharply to prevent bulbs, tubercles or rootstocks from rotting.

As Oxalis appreciate humus-bearing, fertile soil, we recommend enriching soil by adding well-rotted compost. If you grow Oxalis in a pot, apply liquid fertiliser about once a month while plant is in growth, especially in spring and summer.

After flowering, leave leaves in place, as they allow bulbs to store energy before entering dormancy. Leaves will naturally wither in autumn as temperatures fall; you can then cut them back.

If Oxalis you grow are frost-tender (such as Oxalis triangularis), remember to bring them under cover for winter. Stop watering and adding fertiliser then.

If you grow Oxalis in a pot or window box, repot from time to time.

Regarding diseases, Oxalis is susceptible to rust. Leaves then bear orange powdery spots. This disease is caused by a microscopic fungus and is favoured by a combination of warmth and humidity. It weakens plant and makes it less attractive, but is not seriously dangerous to its survival. We suggest removing affected leaves and treating with fungicidal products (horsetail decoction, sulphur…).

Oxalis leaves affected by rust

As for pests, Oxalis can be attacked by aphids. To get rid of them, spray a solution of black soap diluted in water. Slugs and snails may also nibble foliage. When grown indoors, Oxalis is sometimes attacked by red spider mites and by whiteflies.

Multiplication

Oxalis tend to spread and multiply by themselves. If you wish, you can also divide the clumps to replant elsewhere, pot up, or give to friends and family. Oxalis can also be propagated by sowing, but this method is slower and less commonly used.

Dividing clumps

Oxalis are very easy to divide. Do this in spring.

1. Choose a well-established clump and dig up part of it at the edge.
2. Replant in a new position after preparing the soil. Choose a bright spot with a fresh, humus-bearing and well-draining substrate. Backfill around and firm gently.
3. Water generously.

Keep watering over the following weeks.

Using and pairing Oxalis in the garden

You can plant oxalis in a bed, for example in front of a mixed border. Choose tangy blooms to accompany them: pink, yellow‑green, turquoise blue, orange‑red… You’ll create a bed with bright, luminous tones. Plant them with Phlox, Diascia, campanulas, pinks (notably Dianthus deltoides), hardy geraniums, Lychnis coronaria, alchemillas… Add a few small clumps of grasses to bring lightness and movement, choosing for example Stipa tenuifolia, Briza media or Carex. As oxalis are quite low, it is best to add some height and volume behind them: Echinops, Gaura, salvias, verbenas, Nepeta, Liatris spicata, grasses… Also consider gypsophilas, which will add volume and lightness.

You can incorporate oxalis into a bed with other colourful blooms and some grasses. Oxalis tetraphylla (photo Wildfeuer), Pennisetum setaceum ‘Rubrum’, Phlox paniculata ‘Peppermint Twist’, Lychnis coronaria and Dianthus deltoides ‘Albiflorus’

Create a planter arrangement or use a large pot, planting oxalis with annual euphorbia ‘Diamond Frost’ (Euphorbia hypericifolia), which offers a superb airy flowering display. Also consider hardy geraniums, nemesias, diascias, Laurentia, Lobelia erinus, Campanula muralis, hybrid verbenas, pansies… Prioritise colourful blooms, and optionally add a few plants with decorative foliage, such as ipomoeas, ivies and grasses.

You can create striking colour and contrast effects thanks to deep purple foliage of Oxalis triangularis. It will simply need bringing indoors for winter. You can also choose Oxalis deppei, which has attractive leaves, pale green at the edges and brown in the centre. Combine these oxalis with other decorative, colourful foliage, such as heucheras, Ophiopogon, Libertia, grasses, persicarias (Persicaria ‘Red Dragon’ or ‘Purple Fantasy’…), Sagina subulata, euphorbias… Also consider sedums, which offer great diversity in leaf shapes and colours.

Finally, indoors, Oxalis triangularis pairs very well with other exotic plants, possibly in hanging baskets. You can place it alongside Ceropegia, Pilea peperomioides, Ficus pumila, ferns, banana plants…

→ Discover more ideas for pairing oxalis in our advice sheet !

Did you know?

• Oxalis or clover?

Although oxalis can be mistaken for true clovers, these plants are easily distinguished by their flowering: oxalis produce star-shaped flowers, sometimes brightly coloured, whereas clover flowers are small, numerous, tubular, and grouped in globose spikes, often white or pink. Likewise, bulblets can generally be seen when digging up oxalis.

• An edible and useful plant

Many species of oxalis have edible leaves with a tangy flavour, and can be used as a substitute for sorrel. However, avoid eating large quantities because of oxalic acid, which is toxic in high doses.

Oxalis acetosella, for example, which grows wild in France, is edible and also has medicinal properties. It is nicknamed “Oxalis petite-oseille”, precisely in reference to its slightly acidic taste.

The Peruvian oca, meanwhile, is an oxalis whose swollen tubercle is eaten.