Growing Oxalis Triangularis Outdoors in the UK

Updated/Fact-Chacked on June 21, 2023 by Gareth James

Oxalis triangularis has become my latest plant to experiment with in outdoor containers.

The plant is usually sold as an indoor plant in the UK, but I saw it being used in a container in June’s Gardeners World Magazine and fell in love with the dark foliage.

Types of oxalis

Oxalis, also known as wood sorrels, is a broad genus within the Oxalidaceae family, comprising over 800 species, each known for their diverse and vibrant characteristics.

The most widely recognized types include oxalis triangularis, noted for its distinctive purple, triangle-shaped leaves, and Oxalis pes-caprae, also known as Bermuda buttercup or African wood-sorrel, renowned for its luminous yellow blooms. Another species, Oxalis corniculata or creeping woodsorrel, stands out with its diminutive yet appealing flowers and heart-shaped leaves.

Oxalis acetosella, commonly known as ‘wood sorrel, is native to the UK and can be found in various parts of the country. This delicate, shade-loving perennial thrives in damp, nutrient-rich soils and is often spotted in the undergrowth of deciduous woodlands, alongside streams, and in cool, shady hedgerows.

Oxalis acetosella in UK woodland

What oxalis varieties are invasive?

Despite their beauty, certain oxalis species have a reputation for invasiveness. Notably, oxalis pes-caprae and oxalis corniculata are reputed to spread rapidly through both runners and self-seeding, which could potentially overrun your garden if not appropriately managed.

Invasive oxalis (African wood-sorrel, Bermuda buttercup, Bermuda sorrel, buttercup oxalis, Cape sorrel, English weed, goat's-foot, sourgrass, soursob)
Image source: Wikipedia. The invasive Oxalis pes-caprae

These species are listed on the UK Invasive Species List, underscoring the need for responsible cultivation.

Oxalis are nyctinastic. What does this mean?

Nyctinasty is a fascinating plant behaviour involving movements in response to circadian rhythms or changes in environmental conditions such as light or temperature.

Oxalis species, including oxalis triangularis, exhibit this phenomenon by closing their leaves when darkness falls or during periods of low light, such as on cloudy or rainy days. This mechanism is thought to help conserve energy and protect sensitive plant parts from potential harm.

Oxalis triangularis common names

Oxalis Triangularis is colloquially known as the false shamrock, purple shamrock, or love plant. It’s often misidentified as a shamrock due to its similar trifoliate leaf structure, despite not being a member of the Trifolium genus.

Leaves show why it is called 'false shamrock'

Difference between oxalis triangularis and shamrock leaves

While the leaves of Oxalis Triangularis and traditional shamrocks (from the Trifolium genus) both share a trifoliate structure, there are notable differences between the two. Oxalis triangularis, also known as purple shamrock, has distinctive triangular-shaped leaves that are a striking shade of deep purple, almost burgundy, with a fascinating three-dimensional, almost-folded shape that adds depth and intrigue. This is why I’m adding this plant to a container (see planting companions below).

In contrast, the leaves of traditional shamrocks, such as Trifolium repens (white clover) or Trifolium dubium (lesser trefoil), are typically green, smaller, and have a flat, round, or heart-shaped appearance. Also, the veins in shamrock leaves are often lighter than the rest of the leaves, creating a beautiful contrast.

Origin of oxalis triangularis

Native to tropical regions in South America, particularly Brazil, oxalis Triangularis has gained international popularity due to its aesthetic appeal and adaptability, allowing it to thrive in various climates and environments.

How to plant your oxalis triangularis

Planting Oxalis Triangularis in the UK should ideally be done in spring; saying that I’m planting mine out at the end of May, but temperatures have been very mild for this time of year.

Choose a spot with partial shade to protect the plant from intense sunlight. Ideally, east or west-facing borders so the plant can get a few hours of sunlight.

Use a well-drained balanced garden soil rich in organic matter would be ideal as it ensures appropriate nutrition and moisture retention. The soil pH should ideally be slightly acidic to neutral, within a range of 6.0 to 7.0, although oxalis triangularis is generally tolerant of a wide range of soil pH levels.  I used a basic Levington compost that I found on offer in Tesco.

Plant the bulbs 1-2 inches deep and space them 3-4 inches apart to allow sufficient room for growth.  If planting plug plants, position them in your container and plant around it.

Oxalis triangularis pests and diseases

Like most plants, oxalis triangularis can fall victim to pests such as aphids, spider mites, and mealybugs. Regular inspection and prompt response to infestations can help maintain the plant’s health.

Diseases like root rot could afflict your Oxalis Triangularis if overwatered or planted in poor-draining soil.

Oxalis triangularis maintenance in UK Climates

Despite its tropical origins, Oxalis Triangularis adapts well to UK climates. It prefers cool, shaded conditions, hence a location away from the harsh midday sun is ideal.

During winter, when it may enter a dormant phase, reduce watering and allow the plant to rest. It’s best to bring the plant indoors in winter, a west-facing window sill would be perfect. The RHS gives the plants a hardiness rating of H3 (-5 to 1 degrees); anything below these temperatures will likely kill the plant.  They specify good for coastal regions, but here on the  North-East coast, temperatures can easily fall below that.

Oxalis Triangularis flowers 2

Oxalis triangularis planting companions

June’s Gardeners World magazine inspired this whole article, after I saw the ‘container of the month’ – it’s not online, so I cannot show the container. The container comprised Oxalis triangularis with Petunia, light-coloured Glechoma hederacea and Saliva ‘Amistad’ standing tall at the back of the pot.  The dark foliage of the Oxalis triangularis really made the whole container pop.

Planting companions can enhance the aesthetic appeal of your Oxalis triangularis. Shade-loving plants such as ferns, hostas, and bleeding hearts complement the vibrant purple foliage of the Oxalis Triangularis well, creating a lush and diverse garden landscape.

As this oxalis does not like full sun, you have to juggle the requirement of the other plants.

Oxalis triangularis propagation

Propagation is achieved via bulb division, preferably done in late winter or early spring just before new growth commences. During this period, the bulbs can be gently separated and replanted in well-draining soil, ensuring that each new plant has an ample root system to support its growth.

Oxalis triangularis toxicity

It’s important to note that while Oxalis Triangularis is a sight to behold, it is toxic if ingested, especially for pets such as cats and dogs. The plant contains oxalic acid, a compound that can cause symptoms ranging from vomiting and diarrhoea to weakness and trembling in animals.

Where to buy oxalis triangularis in the UK

You can find Oxalis Triangularis bulbs at various gardening centres across the UK. Also, numerous online retailers such as Gardening Express, Crocus, and Thompson & Morgan offer these bulbs, often providing nationwide delivery.

Ensure that you buy from reputable sources to guarantee the health and quality of your bulbs.

Growing oxalis triangularis indoors

Oxalis triangularis also makes an excellent indoor plant, thanks to its distinctive foliage and easy maintenance. It thrives in bright, indirect light indoors and prefers a well-draining potting mix to prevent waterlogging.

You can bring your outdoor plant indoors to overwinter and plant it back outside in spring.

Oxalis Triangularis indoors 1

Water the plant when the top inch of soil feels dry to touch and reduce watering during winter when the plant enters dormancy. Regularly rotating your plant will also promote even growth, as it will naturally lean towards the light source.

With careful attention and appropriate care, you can cultivate a thriving Oxalis Triangularis, making it a vibrant addition to your UK garden or indoor plant collection.