What is oxalis?

Although attractive looking, with several species such as Oxalis tetraphylla and O. adenophylla being grown as ornamental garden or glasshouse plants, some oxalis species can become a real nuisance in the garden.

Despite originating from warm regions such as South America and southern Africa, many oxalis species are hardy enough to survive outdoors in the UK. Some species have escaped from gardens and become naturalised, mostly in southern counties.

The main problem species are; Oxalis corniculata, O. debilis and O. latifolia.

Appearance

All oxalis have three-part, clover-like leaves.

• Oxalis corniculata has a creeping habit and small yellow flowers followed by upright seed capsules. A purple-leaved colour variant is quite common
• O. debilis has deep pink flowers from July to September
• O. latifolia also has a deep pink flower, borne May to September. As the name suggests, this species has particularly broad leaves, with distinct triangular leaflets, making it easily distinguishable from the above

How does oxalis spread?

Oxalis spreads via seed or bulbils:

• Oxalis corniculata often grows as an annual, regenerating each year from seed. The stems do root where they touch the ground but the main means of spread is by seed, ejected forcibly from the ripe seed pods
• Some of the perennial bulb-forming species, such as the tropical American Oxalis debilis, rarely produce seeds, but the tiny bulbils, which are attached to the parent plant, are easily spread when digging and can remain dormant in the soil for several years
• Oxalis latifolia reproduces both by seed and by bulbils: seeds are orange-yellow and have the explosive character of O. corniculata; bulbils are formed on the end of 10cm (4in) long stolons from the base of the bulb, distinguishing this species from the above. 

Control

First, consider whether control can be achieved using non-chemical means such as digging out or suppressing with mulch.

Where these methods are not feasible, chemical controls may be necessary. Choose a weedkiller that is most appropriate for that purpose by reading the label carefully before buying or using. Contact weedkillers or glyphosate-based products, which have low persistence in the soil, may suffice. Take particular care when using residual weedkillers, which persist in the soil for several weeks or months, as they can move deeper or sideways in the soil, leading to possible damage of underlying roots.

Non-weedkiller control

Oxalis corniculata

• In an established lawn, try feeding and top-dressing to improve turf vigour
• Vigorous wire raking in mid-September will remove much of the weed
• Where Oxalis corniculata persists, it may be necessary to strip the affected turf from the site and destroy it, then re-seed the area in spring or autumn
• In herbaceous borders, repeatedly hoe to kill the weed and prevent seed being formed

Oxalis debilis and O. latifolia

• Forking out is best done in the spring when the tiny developing bulbils are firmly attached to the parent plant and before Oxalis latifolia has opportunity to produce seeds. Later in the season they are easily detached, causing the weed to be spread further around the garden
• On infested shrub borders apply a heavy mulch of leaf litter, and replenish as necessary to keep the oxalis well buried. The mulch may need to be maintained for several years
• In a single small bed it may be possible to remove all the soil to a depth of several inches, bringing in fresh replacement soil

Weedkiller control

Lawn weedkillers

Unfortunately, Oxalis corniculata (which is problematic in lawns) shows strong resistance to the full range of selective lawn weedkillers.

Glyphosate

Some control can be gained by treating oxalis with the non-selective weedkiller glyphosate (e.g. Roundup Fast Action, SBM Job done General Purpose Weedkiller or Doff Glyphosate Weedkiller). The most effective period for application is in spring when the oxalis is growing actively and vigorously. Be prepared for some re-growth later in the season or the following spring, which will need a repeat application of weedkiller. Protect grass and garden plants from accidental spray drift, or apply glyphosate specifically to the leaves of the oxalis.

Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

Weedkillers for home gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 4)

Links

Chemicals: using safely and effectively
Chemicals: using a sprayer
Chemicals: using spot and broad-scale weedkillers
Weeds: non-chemical control