Every March, grocery stores and florists sell plants in green wraps as shamrocks while the real shamrocks quietly begin their annual growth in local lawns. Oxalis species, the florists' shamrocks, grow from perennial bulbs and produce tiny, lily-like flowers. Unlike Trifolium dubium -- which is the Irish shamrock and an aggressive species of clover -- it is a welcome spring visitor.
An Irish legend attributes to the shamrock the role as St. Patrick's visual aid in teaching the Irish about the three-part nature of the Christian deity. The plant, a species of clover common in Ireland, became a national symbol of the saint, spring and rebirth, according to Dr. Leonard Perry. Trifolium species, Medicago lupulina, or black medic, and several species of oxalis are all referred to as shamrocks, but Trifolium dubium, a yellow-flowered clover, is the official Irish shamrock. Clover and black medic are considered weeds and both make poor houseplants. The large-leaf houseplant sold as a shamrock in March is an oxalis plant.
Only a few of the approximately 500 oxalis species appear as St. Patrick's Day shamrocks. Dozens -- types of wood sorrel or sorrel -- are either native or foreign perennial herbs listed as invasive or, at the least, rank as weeds. One, the tender Oxalis pes-caprae, or Bermuda buttercup, is common in Europe but grows only in Florida, California and Arizona in the United States. Wild sorrels generally do not adapt well to indoor growing.
Several oxalis species, however, do grow successfully indoors. Oxalis "shamrocks" grow from bulbs or tubers and their compact root systems allow crowding in containers. Leaves open fully in bright indoor light and fold closed at night. Oxalis acetosella, a common St. Patrick's Day shamrock, grows about 6 inches tall with three dark-green, triangular leaves atop each fragile stem with light rose flowers. Oxalis purpurea, the cape oxalis, grows only 4 inches tall and has cream, white or pink flowers. Oxalis regnellii's bright white flowers contrast strongly against the purple reverse of its leaves. Oxalis deppei, also called Oxalis tetraphylla or the good-luck plant, sprouts maroon flowers under stems with four leaves.
Oxalis bulbs and tubers require dormant periods of as long as three months. When plants enter dormant periods, watering and fertilisation should stop and dead foliage removed. Plants kept in a cool, dark place will sprout new shoots as dormancy ends; watering and fertilisation with houseplant fertiliser every 2 to 3 weeks can resume. Plants grow best in bright light and cool 55- to 65-degree F nighttime temperatures. Warm temperatures hasten dormancy cycles and encourage leggy growth. The delicate plants respond best to moist air and well-drained soil that is kept moist and allowed to dry a bit between watering.