Purple Shamrock Plant Profile

Black Oxalis
David Beaulieu

Oxalis regnellii is one of the prized black plants. Most of its members of this plant family, while admittedly dark, are a long way from being a true black and are often more of a dark purple. This plant can legitimately be called a "black shamrock" (or a "purple shamrock"). It really is that dark. While there is some purple in it, how closely it approaches true black depends on lighting and other growing conditions. Along with black mondo grass, this is one of the few plants that one can be comfortable designating as "black," without much qualification.

  • Botanical Name: Oxalis regnellii
  • Common Name: Black Shamrock, Black Oxalis, Wine Shamrocks
  • Plant Type: Annual plant in the North. Where the weather is warm enough, it is a perennial.
  • Mature Size: Height and spread of about 1 foot
  • Sun Exposure: Partial Shade
  • Soil Type: Well-draining soil
  • Soil pH: 6.1 to 6.5
  • Bloom Time: Blooms repeatedly
  • Flower Color: Rose, Mauve, Lavender
  • Hardiness Zones: It is a perennial in growing zones 8 to 11
  • Native Area: Indigenous to South Africa and tropical America

How to Grow Purple Shamrock Plants

This plant grows from a rhizomatous bulb that is sometimes called a "pip." Relatively little care is required during the growing season for these plants. Since it is not hardy North of zone 8, Northerners grow this striking tropical plant as a summertime specimen that serves as an accent for the yard. Often they grow it in containers to adorn shaded patios during the warm-weather months. That way, not only can the plant be moved around to satisfy one's plant-combination needs, but the bulbs are also all ready to be brought inside at a moment's notice once cold weather approaches. If, by contrast, you live in zone 8 or warmer, use Oxalis regnellii in shade gardens.

The shape of the individual leaflets on the trifoliate leaves (they come in threes)are triangular. Each of the three triangles directs one of its points to the center of the leaf, where they all meet at one small hub; likewise, each thrusts out a broadside to form the interrupted leaf perimeter.

Among the bigger leaves on a purple shamrock plant, each of these broadsides can measure 3.5 inches in length. The visual effect is one of floppiness. It is as if one were gazing at a butterfly that had a third wing. An oddity worth noting is that their leaflets fold down toward the stem at night, then rise again the next day. The leaflets of purple shamrocks "pucker down" as night falls.

Flowers do emerge out of the mounding clumps, although, competing against the striking leaves, the blooms are bound to take a back seat. The trumpet-shaped blossoms are very light pink in color.

Light

This colorful plant is best grown in partial shade. Bright sun will be too much for this plant.

Soil

Grow in well-drained soil. If you are growing it in the ground, make compost (if you do not already have some), so that you can work it into the ground to loosen the soil and provide nutrients.

Water

Water adequately to get them established, after which point their water needs are minimal.

Temperature and Humidity

This plant thrives in moderate temperatures. It will grow best when daytime temperatures do not exceed 75 degrees Fahrenheit and nighttime temperatures are between 50 and 65 degrees Fahrenheit.

Fertilizer

A little compost should be all the feeding they require. Even that is not necessary for these easy to care for plants.

Varieties of Purple Shamrock Plants

Plants in the same family include:

  • O. tetraphylla or Iron Cross is a four-leaved shamrock with bicolored leaves that are green around the outside and dark red in the middle. Flower color is mainly pink. The plant reaches a height of 6 to 12 inches with a spread of 8 inches. It can be grown outdoors year-round in zones 8 to 10. 
  • Yellow wood sorrel or "sourgrass" (Oxalis stricta) is a common lawn weed.

Toxicity of Purple Shamrock Plants

Take care in growing purple shamrock plants around pets. They are known to be poisonous plants for cats and dogs.

Pruning and Wintering

When the growing season in the North is over and your purple shamrock plant begins to drop its leaves, simply cease watering and allow it to enter its dormant phase. Trim the remaining vegetation down to ground level. Pot up the plant (if you have not been growing it in a container all along) and bring it indoors for use as a houseplant. Cut back quite a bit on water for several weeks, then begin a period of light watering. When leaves reappear, water normally again. Grow it in bright light indoors, but remember, this is a shade plant. Do not place it in direct sun, where it will fry. The plant is likely to become leggy over the winter indoors. Just give it a haircut in spring and new growth will emerge.

If at some point during the growing season, you find that any pests have made a home on your purple shamrock plant, you can easily take care of the problem at the end of the growing season. You will be trimming the plant anyway and removing the leaves removes the bugs at the same time. Just be sure to dispose of the cut foliage properly, so that the pests do not have a chance to attack any other plants.