The most persistent weed in California gardens must be oxalis. People look at me in disbelief when I, at various times, tell them that I have eliminated most weeds from my garden, but I am quick to add that I have never vanquished oxalis.
If you are unfamiliar with this weed, you simply don't know it by its name. Many call it clover because of the similar leaf but this "clover" has little yellow flowers, not the bushy pink and lavender ones on true clover.
The observant gardener has probably noticed that there are at least three distinct kinds of oxalis. One is very pretty, sending up big, bright yellow, buttercup-like flowers. This is Oxalis pes-caprae and it grows from bulbs deep underground, so nothing you do to the top will affect it for more than a season. It will be back, unless you dig it out of the ground.
Much more common is Oxalis corniculata , which is the one that spreads on above-ground runners that root as they go. There is also a variety of this, Oxalis corniculata atropurpurea , which tends to grow as a little clump, not spreading far and wide, and the leaves are often purplish. Leaves on the variety tend to be flat, while leaves on the other tend to flair back. Both have small yellow flowers. These two are the ones you will never be rid of.
They grow in sun or shade; taller in shade, low and flat in the sun. The variety atropurpurea comes up every time you water from what must be millions of seed that are set when the plant is still a babe and that persist in the garden for what must be generations. They are best done in by a hoe and will not usually come back from the roots..
In my garden, plain old Oxalis corniculata is the worst because it spreads through all kinds of low growing plants. It has probably caused more work in the garden than everything else combined. Give it a week or two and it will so intertwine itself with small plants that you can only get rid of it by getting rid of the plant it has invaded--by starting over.
To weed it out you must be diligent. When you spot the leaves, follow them back to the runner and follow that back to the main clump of roots. This would be impossible were it not for the fact that the runners are often reddish in color so you can see them, if you look carefully, among the green of the plant it has invaded. But that's the only break this plant will give you.
It is constructed cleverly; the runners are very fragile near where they root so pull too hard and they break off leaving the roots and the rest of the plant in the ground and you holding but one of the many runners.
Exploding Seed Pods
You must also act before they set seed, because simply touching the plant will cause the seed pods to explode sending seed everywhere. At some times of the year you can actually hear them popping in the garden, scattering seed.
Oxalis, which are members of the Oxalidaceae family, are good at being weeds because they are native to just about everywhere in the world. O. pes-caprae is native to South Africa, which has a climate very similar to ours, though this oxalis has the common name of Bermuda buttercup because it is a weed of great stature there.
O. corniculata is native to Europe, though it has been a cosmopolitan weed for so long that no one knows quite where in Europe it originated. It goes under the common name of creeping oxalis.
The Irish shamrock is an oxalis, O. acetosella ,though it is native to Japan and much of Asia as well. Oxalis deppei is called the good-luck plant, or the lucky clover, because it has four leaflets. It is native to Mexico.
Even the remote Falkland Islands are not safe from oxalis. Scurvy grass is the common name of O. enneaphylla , which grows there and saved many a sailor on a trip around the Horn.
I don't know how anyone could call something with cloverlike leaves a grass, but children in Southern California also call Oxalis pes-caprae a grass. They like to chew on the sweet-sour leaves and call it "sour grass," a practice that will do them no harm. For generations, hikers have chewed on wood sorrel, Oxalis oregana , an oxalis native to California.
Convinced of the indomitability and universality of oxalis? Well, how about an oxalis that grows to six feet and is native to Chile, or a succulent oxalis, named O. herrerae , native to Peru? Oxalis grow on mountaintop and desert floor, forest or beach or garden.
I wish I could tell you some easy way to get rid of oxalis but I can't. O. pes-caprae has bulbs so deep you need a shovel, not a trowel, to get them out; the other two must be pulled or hoed. In lawns you can use herbicides made to take oxalis and other broad-leafed plants out of grasses (they are usually sold mixed with fertilizer granules), but elsewhere it's hands-and-knees stuff.