Oxalis is a large and diverse group of plants that includes some attractive ornamental varieties. Unfortunately it’s often more familiar to gardeners as a weed, because some species are extremely invasive. Problem oxalis species are either deep rooting with fibrous roots that regrow from any pieces left in the soil, or with roots that form many tiny bulbils that are easy to leave in the ground when weeding, and which will also regrow. Paving cracks, path crevices and greenhouse soil are favourite spots for oxalis to take hold. These problem species often arrive in the garden on other plants, hitching a ride by growing on the surface of the compost in a pot.
Not all oxalis are villains – some are non-invasive ornamental varieties, with colourful flowers that make excellent plants for rockeries, raised beds and pots, as well as our native wood sorrel, Oxalis acetosella, which bears dainty white flowers in spring and is lovely to naturalise in a woodland garden under trees.
Oxalis is poisonous to pets, but its bitter taste should deter most animals from eating it.
How to identify oxalis
Oxalis is also known as shamrock, due to its clover-like leaves. The most common weed species is yellow oxalis, Oxalis corniculata, which has small reddish-purple leaves and tiny yellow flowers. Its mat-forming growth rapidly spreads by means of runners (shoots that root as they spread in contact with the ground) and, if allowed to flower and set seed, also by ‘exploding’ seed pods. The dark foliage and creeping growth can make it hard to spot, especially as it often grows under the canopy of other plants.
How to control oxalis without chemicals
Prevention is the most effective method of control, by being vigilant and buying plants that are free of weed growth or turning down a well-intentioned gift if it looks weedy. If oxalis is already in your garden, prevent the flowers maturing and producing seed – remember one plant can become the parent of many! Hand-weed or hoe regularly to prevent seeding and, when time permits, use a weeding knife to dig up the roots. Don’t put weeded-out oxalis plants on the compost heap where seeds may persist, put them in the dustbin or garden waste bin instead. If oxalis is growing in spots where it’s hard to dig them out, a weed burner (garden flame gun) is an effective means of control.
Chemical control of oxalis
Oxalis can be killed with the systemic weedkiller glyphosate, which is applied to the leaves and is then taken down into the roots as the plant grows. Spring is the most effective time to apply glyphosate, and a second application later in the year may be required to completely kill the weed. Glyphosate comes in several formulations including gel, ready to use spray or concentrate to dilute and apply in your own sprayer.
Bear in mind that glyphosate has been linked to insect declines and cancer in humans, and has been deemed ‘probably carcinogenic’ by the World Health organisation (WHO). Always follow all safely instructions. Take great care to avoid getting it on garden plants or lawns as it kills everything it touches.
How to grow ornamental oxalis
Ornamental varieties of oxalis make handsome summer plants for pots, troughs, and rockeries outdoors or as pot plants to grow indoors. Some species are frost tender. The clover-like leaves can be bright green, with dark markings, or reddish-purple in colour. Tubular pink or yellow flowers are borne from early to late summer, usually opening during the day and closing up at night, or on overcast days. Buy and plant bulbous species in autumn and buy perennial species as ready-grown plants. Grow in full sun and in fertile soil or compost with good drainage. Water and feed regularly whilst in flower.