When St. Patrick picked a leaf of clover, to illustrate the three-in-one nature of the Trinity, no one recorded exactly what plant it was -- just seamrog, Irish for little clover. Many people believe it was common white clover (Trifolium repens), which came from Europe to North America and spread all over. I like to see its white flowers in the lawn, but it when it creeps into flowerbeds it overwhelms other plants more than grass does, and is harder to get out.
Others say the shamrock was yellow-flowered hop clover. Or black medic, a clover look-alike also in the pea family two more successful immigrants here. But most varieties of all these have their three leaflets in an oval shape not the familiar heart-shape of the popular icon. I was able to find one photo of white clover with leaflets that shape, on a US Department of Agriculture database. However all the descriptions I could find, say the leaflets are oval.
Clovers are hard to grow indoors. Still sometimes you can buy little pots around this time of year. More commonly, florists sell an unrelated plant that looks like clover: wood sorrel (Oxalis acetosella). A native to moist woods both here and in Ireland, it does have those cute heart-shaped leaflets. But the flower looks nothing like a clover.
O. stricta, native yellow wood sorrel, resembles O. acetosella, except stricta has yellow flowers, and grows anywhere including in the pots of my houseplants.
My favorite oxalis is O. regnellii Atropurpurea. A South American native, its hardy in zones 7-10; around here it does fine as a houseplant: one of the few that likes the windows in the back of my house, which face Northwest, and get late afternoon sun.
From little bulblets (with not much of a root system), slender, almost translucent stems rise seven inches, to three-inch leaves, each composed of three triangular leaflets, looking like three butterflies. At night or when the plant gets too dry the leaves fold down toward their stems and inward along their central vein, making an odd, striking shape like a sort of pointy-top tricorn hat. Mostly a deep, almost-black purple, each leaf displays a dramatic, V-shaped blotch in a somewhat lighter purple.
Pendent little pale pink flowers have 5 petals in surprising contrast to the three or six youd expect on a plant with three-lobed leaves. Altogether, not much like a clover. Nevertheless, as an oxalis, its called a shamrock.