Picture credit: Lois Miklas
As St. Patrick’s Day approaches, nurseries and florists offer “shamrock plants” to celebrate the day. Even though the leaves look clover-like, the shamrock plant offered for sale as a houseplant is not the shamrock found in Ireland. The original shamrock may have been Trifolium dubium (lesser trefoil) or Trifolium repens (white clover). Instead, the shamrock houseplant is a member of the genus Oxalis from the wood sorrel family and is native to Africa and the Americas. Oxalis regnellii resembles a shamrock, because each leaf has three triangular shaped leaflets. (According to tradition, St. Patrick used the shamrock to explain the three persons of the Trinity when he brought Christianity to Ireland). 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. You can fertilize every two to three weeks during flowering using half of the recommended strength fertilizer. 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. If 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 of the tubers and replanting in new pots.
All members of the Oxalis genus 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 of these plants around pets, though they would have to ingest large amounts to be severely harmed.
An interesting characteristic of many Oxalis species is 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 of the leaf cells, and 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, but 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!