Many oxalis are valuable ornamental plants with their clover-like foliage and pink or yellow flowers, but a few can become serious weeds in the garden.
Means of spread
Spreading by bulbils
Some of the perennial, bulbous-forming species, such as the tropical American Oxalis debilis, are quite hardy. Seeds are rarely produced, but the tiny bulbils are easily spread and can remain dormant in the soil for several years.
Spreading by seeds
Oxalis corniculata is a yellow-flowered species which often grows as an annual, regenerating each year from seed. The stems root where they touch the ground but its main means of spread is by seeds, ejected forcibly from the ripe seed pods.
Cultural control
Oxalis debilis: Forking out is best done in the spring when the tiny developing bulbils are firmly attached to the parent plant. 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.
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 material. Where the weed 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 to prevent seed being formed.
Chemical control
Oxalis debilis: Some control can be gained by treating with the non-selective herbicide glyphosate. The most effective period for application is in spring when the oxalis is growing actively and vigorously. Be prepared for some regrowth later in the season or the following spring.
Oxalis corniculata: This weed is strongly resistant to the range of selective lawn weedkillers.
In rose beds and among certain ornamental trees and shrubs use the soil-acting granular weedkiller dichlobenil (Casoron G4 Weed Barrier), applied in February/early March. Check the label first for restrictions of use.
Helen Bostock