<em>Oxalis</em>: to plant, grow and care for

Description and botany

Oxalis are small herbaceous plants, often bulbous, with a spreading habit. They are decorative for their foliage, which resembles that of shamrocks, and for their star-shaped flowering. Oxalis comprise around 500 species, which can be annual or perennial. They have a very wide global distribution, but greatest diversity of species is found in South America and South Africa. Eight species occur wild in France, including Oxalis acetosella and Oxalis corniculata, which are present across much of metropolitan territory. In wild conditions, oxalis grow in open habitats or woodland understorey, which explains why some tolerate sun while others prefer shade.

Oxalis are not always very hardy. Some tolerate down to −10 °C, but others are sensitive when temperatures approach 0 °C and need to be brought under cover for winter. Oxalis triangularis, for example, is commonly cultivated as an indoor plant and can spend whole year in a flat or house.

Name of Oxalis comes from Greek oxys, meaning “acid, sharp”, referring to sour taste of its leaves. In French they are sometimes called oxalides, or “false shamrocks”. Oxalis acetosella is also known as “wood sorrel” or “small sorrel”.

Oxalis give their name to family Oxalidaceae, which they dominate in species number (500 of 600). Apart from resemblance of their foliage, oxalis have no relation to shamrocks (Trifolium sp.), which belong to family Fabaceae. Their flowers are, moreover, very different.

Oxalis acetosella: botanical illustration

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

Oxalis contain oxalic acid, which gives leaves a tart taste, like sorrel or rhubarb, but makes them toxic in large quantities.

Oxalis are low, creeping plants suitable for use 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 fine petioles, giving it an airy aspect. This characteristic, combined with its triangular, purple leaflets, gives a strikingly graphic, modern appearance, making it a popular indoor plant.

Depending on variety, flowering takes place between spring (May–June) and autumn… with exception of Oxalis versicolor, which follows an inverted cycle and flowers in winter. In general, oxalis flower for a long period.

Flowers are usually white, pink or yellow, sometimes red or mauve. Flowering is bright, often in clear, vivid shades. Centre of corolla is often shaded, either yellow or darker than remaining petals. flowering of Oxalis versicolor is rather surprising, since petals are bicoloured, white and red. One might think it is a horticultural variety selected for its unusual flowers, but it is indeed a botanical species found in wild. In Oxalis hedysaroides, yellow flowers set off dark purple foliage beautifully.

Oxalis flowers can be solitary or grouped in umbels. They are borne on long, upright stalks that are quite flexible. Flowering is star-shaped, very simple, made of five petals, joined into tube at base. They are surrounded by five free sepals. Flowers are regular, with radial symmetry, and may be cup-shaped or funnel-shaped. Corolla tube can be long and narrow, flaring into funnel, as in Oxalis versicolor. Flowers also have ten stamens (five long and five short) carrying pollen.

Flowers are sensitive to light: they open fully in sun and close in evening or in dull weather. More generally, oxalis need a bright position to deliver generous, abundant flowering.

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

Leaves of oxalis closely resemble those of shamrocks. This is reason some species with four leaflets have enjoyed popularity – playing on idea that four-leaf shamrocks bring luck.

Leaves often have a lovely light-green hue. They can also be purple, as in Oxalis triangularis… or bicoloured, as in Oxalis deppei, green on outer edges of leaflets and brown at centre, with strong contrast. Oxalis adenophylla has grey-blue leaves.

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

When they have four leaflets, oxalis strongly resemble four-leaf shamrock! Other species have leaves made up of many leaflets (up to around twenty), as in Oxalis adenophylla. Each leaflet is attached at centre, at same point; when numerous, this gives whole leaf a rounded form. Some oxalis have exceptional, highly divided foliage, like Oxalis palmifrons, creating a palmate effect.

In most species, foliage folds up at night or in dull weather and unfolds by day (nyctinasty, a nastic movement). Plant thus reacts to light. One might almost say plant “sleeps” at night, resting by folding leaves and flowers!

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

Oxalis versicolor has peculiarity of doing opposite to many plants: it flowers in winter and enters dormancy in summer.

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

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

Main oxalis varieties

Most popular varieties:

Our favourite varieties:

Oxalis triangularis

Appreciated for its very dark purple leaves, each composed of three distinctly triangular leaflets. It also bears small white flowers, perfectly set off by the foliage. Often grown as an indoor plant!

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

See the product sheet

Oxalis versicolor

This very unusual Oxalis produces bicoloured flowers with white and red reverse, forming a spiralled pattern. It flowers in winter and enters dormancy during summer. Recipient of the Award of Garden Merit from the Royal Horticultural Society (RHS). Not hardy.

• Height at maturity 10 cm

See the product sheet

Oxalis deppei

This Oxalis bears leaves with four leaflets, their shape very reminiscent of a four-leaf clover. Leaflets are marked dark brown at centre and light green at margins, creating a striking 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 bears pale pink flowers. Recipient of the Award of Garden Merit from the Royal Horticultural Society (RHS).

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

See the product sheet

Oxalis adenophylla

This Oxalis produces attractive white and pink flowers veined with dark pink. Foliage is very decorative, bluish-grey, finely cut, taking on a plicate appearance. Recipient of the Award of Garden Merit from the 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 good light: choose a sunny spot to allow their flowers to open. Some species, however, do well in shade or partial shade, for example in woodland understorey (Oxalis oregana, Oxalis acetosella…). For Oxalis triangularis, the brighter the position, the deeper the foliage colour will be, though avoid direct sunlight.

Oxalis prefer humus-rich, fertile soil; therefore, don’t hesitate to add some well-rotted compost to enrich the soil. They also like light, free-draining growing medium (excess moisture can rot the bulbs), but it is preferable that the soil remains cool in summer. Oxalis also prefer slightly acidic soil.

Les oxalis adapt well to growing in pots or planters. This is an option to favour if you live in a region with a cold climate, as it allows you to bring them under cover for the winter.

Some oxalis can also be grown indoors or in a greenhouse all year round. This is notably the case for Oxalis triangularis. You can place it in a pot or hang it in your home, in a bright position but out of direct sunlight.

When to plant?

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

How to plant?

Oxalis are supplied as bulbs. We recommend planting them in small groups, placing at least 4 or 5 bulbs together each time. Once planted, they spread fairly quickly sideways. Leave about 10–15 cm between bulbs.

1. Work the soil, incorporating some well-rotted compost and possibly a little coarse sand or grit so the growing medium is more free-draining.
2. Dig small holes so the bulbs are 8 cm deep.
3. Place the oxalis bulbs.
4. Cover them with soil and press down lightly with the palm of your hand.
5. Water well.

Continue to water over the following weeks.

You can also grow oxalis in pots.

To do this:

1. Choose a fairly large container, such as a planter or a hanging basket. Make sure there are holes in the bottom to allow drainage.
2. Use a free-draining compost (you can use a mix of potting compost and sand). It is also best to put a drainage layer at the bottom of the pot before adding compost.
3. Plant your oxalis bulbs.
4. Cover them with compost, and optionally plant other plants alongside them.
5. Water.
6. Place the pot in a sunny spot; for Oxalis triangularis, keep indoors and out of direct sunlight.

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 pots! In that case, and during growth, you can water once a week. But in winter, when plant is in dormancy, greatly reduce watering to prevent bulbs, tubercles or rootstocks from rotting.

As Oxalis appreciate fertile, humus-bearing soils, we recommend enriching soil by adding well-rotted compost. If you grow Oxalis in pots, apply a 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. They will wither naturally in autumn as temperatures fall; you can then cut them back.

If Oxalis you grow are frost-tender (such as Oxalis triangularis), bring them indoors for winter. Then stop watering and feeding.

If you grow Oxalis in pots or planters, remember to repot from time to time.

Regarding diseases, Oxalis is susceptible to rust. Leaves then show orange powdery spots. This disease is caused by a tiny fungus and is favoured by combination of heat and humidity. It weakens plant and makes it less attractive, but is not serious, survival not threatened. We suggest removing affected leaves and treating using a fungicidal product (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, also called “mouches blanches”.

Multiplication

Oxalis tend to spread and multiply on their own. If you wish, you can also divide clumps to replant elsewhere, pot up, or give to friends and family. It is also possible to multiply Oxalis by sowing, but this technique is slower and less commonly used.

Dividing clumps

Oxalis are very easy to divide. This should be done in spring.

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

Continue watering in the weeks that follow.

Using and pairing oxalis in the garden

You can plant oxalis in a border, for example in the foreground of a mixed border. Choose tangy-coloured blooms to accompany them: pink, yellow-green, turquoise blue, orange-red… You will obtain 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 ornamental grasses to bring lightness and movement, choosing for example Stipa tenuifolia, Briza media or Carex. As oxalis are quite low, it is preferable to add some height and volume behind them: Echinops, Gaura, sages, verbenas, Nepeta, Liatris spicata, grasses… Also consider gypsophilas for volume and airiness.

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

Create a container arrangement or plant in a large pot, planting oxalis with annual euphorbia ‘Diamond Frost’ (Euphorbia hypericifolia), which offers a superb vapoury flowering. Also consider hardy geraniums, nemesias, diascias, Laurentia, Lobelia erinus, Campanula muralis, hybrid verbenas, pansies… Prefer colourful blooms, and optionally add a few decorative-leaved plants such as Ipomoea, ivies and grasses.

Create striking colour and contrast effects with the superb purple foliage of Oxalis triangularis. Just remember to bring it indoors for winter. You can also choose Oxalis deppei, which offers attractive leaves, pale green on the outside and brown in the centre. Pair these oxalis with other decorative, colourful foliage, such as heucheras, Ophiopogon, Libertia, grasses, persicarias (Persicaria ‘Red Dragon’ or ‘Purple Fantasy’…), Sagina subulata, euphorbias… Also think of sedums, which offer great diversity in foliage shapes and colours.

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

→ Discover other 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 gathered in globose spikes, often white or pink. Similarly, bulblets can generally be seen when digging up oxalis.

• Edible and useful plant

Many species of oxalis have edible leaves, with a tangy flavour, and can replace sorrel. However, avoid consuming large quantities because oxalic acid 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 », a reference to its slightly acidic taste.

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