Oxalis: Strong as an Ox, Sour as a Lemon

Bermuda buttercup (Oxalis pes-caprae)

There is a certain plant that has remarkable strength.  I always think of an ox when reading about it or hearing its name and for an obvious reason: it’s called oxalis.  Yet the etymology of oxalis has nothing to do with an ox.  The word is derived from oxys, a Greek word for sharp, and sorrel, its common name, is cognate with sour.  Indeed, oxalis or sorrel is of a sharp sour lemon flavor — which you can quickly confirm by plucking its foliage and briefly chewing on it.  Some people toss oxalis greens into their salad, which is fine in moderation but be careful here since oxalis contains calcium oxalate which may lead to kidney problems if excessively consumed.

I’ve been thinking about oxalis, of which there are 900 (!) species, since receiving an email from Judy Ennis who is seeking an organic solution to her oxalis problem.  “We have their yellow flowers blooming all over our hills here in La Habra Heights,” she wrote.  Based on those unmistakable yellow flowers, she must be talking about buttercup oxalis (Oxalis pes-caprae).  She added that she needs to remove it by May 1, presumably to comply with fire department regulations concerning weed abatement. Native to South Africa, buttercup oxalis was imported to the US as an ornamental bedding plant but now is considered an invasive species.

Buttercup oxalis, also called Bermuda buttercup, is fascinating because, despite its ability to rapidly take over large expanses of open ground, it is sterile, incapable of producing seeds.  Its proliferation is achieved solely through vegetative propagation of its scaly bulbs.   If oxalis is running wild over your hills, it got there initially by means of urban wildlife — squirrels or birds or who dug up the bulbs from someone’s garden or open lot and deposited them in protected niches where they could remain viable until the rains came.

A quick organic fix to control buttercup oxalis and many other weeds is to fill a spray bottle with distilled white vinegar to which a few squirts of dish soap are added.  Spraying the base of the clumping oxalis foliage should bring the plant down within 24 hours.  A more permanent long term solution to this and other large scale weed problems is soil solarization, but you will have to wait until the hot weather comes in June or July to begin.  After thoroughly soaking the weedy area, cover it with clear plastic that contains an ultraviolet light inhibitor (to prevent degradation of the plastic).  Make sure the plastic is held in place by large rocks or cinder blocks.  After 6 weeks, the soil will have been steam heated to temperatures that should have kill oxalis bulbs and any other plant propagules, including seeds, that inhabit the top several inches of soil.

Another organic technique for killing weeds and lawn grass, too, is to layer wet cardboard or newspapers over the target vegetation and then pile on a thick layer of mulch  — straw, hay, or leaves.  After the cardboard or newspapers and mulch have decomposed, incorporate them into the soil with a spading fork and then plant your vegetables or ornamentals in the mixture of soil and rotted mulch.

pink wood sorrel (Oxalis rubra)

Dalmatian bellflower (Campanula poscharskyana) with purple shamrock (Oxalis regnellii triangularis)

By the way, those shamrocks that you see associated with St. Patrick’s Day and many company logos are oxalis leaves.  They are plainly visible on pink wood sorrel (Oxalis rubra) that grows nonchalantly in sun or shade and without any attention whatsoever although it will need some water in hotter garden spots.  Its deep pink blooms grow in bunches and may suddenly appear at any time.

Purple shamrock (Oxalis regnellii triangularis) is a drought-tolerant oxalis that is a fine addition to a shade garden.  Although it is blooming now in pale mauve, it shows off large burgundy-purple, triangular leaflets throughout the year.

Tip of the Week:  Four-leaf clover is a myth, since this classic good luck symbol actually has four-leaflets on a single leaf.  Oxalis tetraphylla (tetra = four, phylla = leaf or leaflet) does exist and is called iron cross on account of the markings at the base of its leaflets.  But it is still an Oxalis species, not in any way related to true clover.   A four-leaflet clover (speaking of true clover) is a mutation of common three-leaflet white clover (Trifolium repens) that grows in lawns and is notable for its spherical white flowers.  About one in every 5,000 of these little ground cover plants has four instead of three leaflets.  White clover has been used as a lawn substitute.  It is rather drought tolerant and, since it’s a legume, never needs fertilization and is always bright green.  Dogs love to roll around in it, too, yet dog traffic does not damage it.

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