The Botany Behind the Shamrock

Every March, as St. Patrick's Day approaches, the images of green shamrocks abound. No one plant is considered a shamrock, but several living plants are associated with it, including a popular houseplant. Look at shamrock lore and how you can care for your "shamrock" plant.

What Was the Original Shamrock?

The original shamrock may have been Trifolium dubium (yellow clover or lesser trefoil) or Trifolium repens (white clover). Legend holds that St. Patrick used the three clover leaflets to explain the Christian concept of the Holy Trinity—Father, Son, and Holy Spirit—to the Irish people in the fourth century. The word "shamrock" derives from the Irish words seamair óg or young clover. According to Smithsonian Magazine, yellow clover (Trifolium dubium) is the species most often sold as a "shamrock" in Ireland. T. dubium, T. repens, and red clover, T. pratense, have all been worn by the Irish as a symbol of St. Patrick's day—a tradition dating back to the late seventeenth century.

Are Shamrocks and Four-Leaf Clovers the Same?

Shamrock and four-leaf clover lore are often confused since both are associated with good luck. As noted above, the Irish shamrock has three leaflets. Occasionally, due to a fluke of genetics and environmental factors, the white clover plant (Trifolium repens) will produce four leaflets on one stem. Scientists estimate 1 to 2 four-leaf clovers for every 10,000 conventional three-leaf clovers. The association of four-leaf clovers with good luck seems to go back to ancient Celtic druids. The first documented reference to the lucky four-leaf clover is by Sir John Melton, 1620, who wrote, "If a man walking in the fields find any four-leaved grass, he shall in a small while after, find some good thing."

What Is the Shamrock Sold as a Houseplant?

The shamrock houseplant is a member of the genus Oxalis from the wood sorrel family and is native to Africa and the Americas. Oxalis triangularis (formerly called O. regnellii) resembles a shamrock because each leaf has three triangular-shaped leaflets. Oxalis also produces many five-petalled, small white flowers.

Oxalis grows from tubers. All prefer cool conditions in bright light and well-drained soil. Their roots are shallow, and they seem to prefer being pot-bound. Using half of the recommended strength fertilizer, you can fertilize every two to three weeks during flowering. Overwatering can lead to root rot. In late fall, the foliage will yellow and die, signaling the start of dormancy, which can last for one to several months. When this happens, stop watering and fertilizing and store pots in a cool, dark location until new growth begins to show. You can divide them by separating some tubers and replanting them in new pots.

All members of the genus Oxalis contain oxalic acid, which gives them a sour taste that helps to protect them from grazers. Ingesting large quantities of oxalic acid can be toxic, so be careful with these plants around pets. However, they would have to consume large amounts to be severely harmed.

An interesting characteristic of many oxalis species is the movement of their leaves in response to light levels. They open in bright light and close at night or when the day is overcast. These nyctinastic or sleep movements are due to rhythmic changes in turgor pressure within some leaf cells. These changes are tied to a built-in circadian clock in the plant. It is obvious why it is adaptive for the leaves to open horizontally during the day to capture more sunlight. However, the value of folding at night is still unknown. Hypotheses have suggested greater protection against nocturnal herbivorous insects or even decreasing the effect of moonlight on the setting of the plant's biological clock. Regardless of the reason, it's fun to watch the leaves very slowly unfurl each morning!