Shamrocks: These can become serious weeds
We found a 'weed', which measures 6.5 inches from fleshy root to top of the leaves which we keep finding in our veg patch. It looks like a member of the shamrock/clover family and has very strange roots. Please can you identify it and suggest how we can eradicate it? Oh, and thank you for your advice about runner beans in May – the replacement beans are cropping well! (K and A Hughes, Watton)
You definitely have a shamrock problem. Many shamrocks (oxalis) are valuable ornamental plants, with their clover-like foliage and pink or yellow flowers, but a few can become serious weeds. Several species such as Oxalis tetraphylla and O.adenophylla are grown as ornamental garden or glasshouse plants.
Although originating from warm regions such as South America, many oxalis species are hardy enough to survive outdoors in the UK and become naturalised, mostly in southern counties.
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The main problem species are Oxalis corniculata, O.debilis and O.latifolia.O corniculata often grows as an annual, regenerating each year from seed. Some of the perennial bulb-forming species, such as the tropical American O.debilis, rarely produce seeds, but the tiny bulbils are easily spread and can remain dormant for several years. O.latifolia reproduces both by seed and by bulbils formed at the end of the rhizomes.
To start the control process first look at nonchemical means such as digging out or suppressing with mulch.
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Otherwise, chemical controls may be necessary. Always read the label carefully before buying or using weedkiller; contact weedkillers or glyphosate-based products may suffice. Take particular care when using residual weedkillers, which can persist in the soil.
O.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 O.corniculata persists, it may be necessary to strip and destroy affected turf, then re-seed in spring or autumn. In herbaceous borders, repeatedly hoe to kill the weed and prevent seed being formed.
O.debilis and O.latifolia: Forking out is best done in the spring before O.latifolia produces seeds. Later in the season the bulbils detached easily, causing the weed to be spread. 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.
Some control can be gained by treating oxalis with glyphosate. Apply 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. Be wary of accidental spray drift, or apply glyphosate specifically to the leaves of the oxalis.
•This article was first published on September 3, 2011.