Purple Shamrock Plants, AKA "Black Oxalis"

How to Grow a Saint Patrick's Day Favorite

Black Oxalis
David Beaulieu

You may know this striking specimen as a purple shamrock plant, but in the language of plant taxonomy, it is Oxalis regnellii. An alternate botanical name is O. triangularis. One type you can grow is "Velvet," which is part of the "Charmed Series" promoted by Proven Winners. This series features plants distinguished from each other by leaf color. Velvet is the one closest to being black in color. Meanwhile, "Wine" is more purple. "Jade" is the outlier in the series, bearing merely green leaves.

In common lingo, you will also hear "black shamrocks" or "black oxalis" used as names for dark-leaved members of this genus.

This tropical specimen is treated as if it were an annual plant in the North (or brought indoors as a houseplant during the winter), although, where the weather is warm enough, it is a perennial. It grows from a rhizomatous bulb that is sometimes called a "pip." The plant belongs to the wood sorrel family (Oxalidaceae).

Features of the Plant

Velvet Oxalis regnellii is one of the prized black plants. Usually one prefixes a mention of that group with "so-called," because most of its members, while admittedly dark, are a long way from being a true black, being, instead, more of a dark purple. But Velvet can just as legitimately be called a "black shamrock" as a "purple shamrock." It is that dark. Yes, there is some purple in it, and how closely it approaches true black depends on lighting and other growing conditions. But along with black mondo grass, this is one of the few plants that one can be comfortable designating as "black," without much qualification.

Of course, it is not all about the pretty leaf color in the case of this unusual plant. The shape of the individual leaflets on the trifoliate leaves (that is, they come in threes) is also a major selling point, that shape being 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.

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 a very light pink in color.

The plant attains a mature height and spread of 1 foot (or a bit more).

Where the Plant Comes From and Where It's Hardy

Oxalis is indigenous to South Africa and tropical America. It is a perennial in growing zones 8 to 11.

Another kind of "purple shamrock," namely, Trifolium repens atropurpureum, is hardy to USDA planting zone 4.

Sun and Soil Requirements, Plus a Warning

This colorful plant is best grown in partial shade and in a 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.

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

Pest Control, Care for Purple Shamrock Plants

Relatively little care is required during the growing season for these plants. Water adequately to get them established, after which point their water needs are minimal. As for fertilizing, a little compost should be all the feeding they require (if that).

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 watering 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, but not to worry: Just give it a haircut in spring, and new growth will emerge.

The dormant phase that this plant goes through becomes a real blessing when a pest-control problem arises. And one very likely will arise over time if you use it every winter as a houseplant. Houseplants, in general, are notorious for becoming infested with some combination of aphids, spider mites, and whitefly. If at some point during the growing season, you find that any of these 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 (see above). 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.

Uses in Landscaping

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, etc. 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.

Noteworthy Features of the Plant

These are drought resistant plants with unique leaves, noteworthy for their color and shape alike. Give them away as St. Patrick's Day gifts, or celebrate March 17, yourself with a display of your own purple shamrocks in your favorite room.

An oddity worth noting is that their leaflets fold down toward the stem at night, then rise again the next day. You have heard of flowers "puckering up" at the end of the day, right? Well, the leaflets of purple shamrocks "pucker down" as night falls.

For Those Who Want Four-Leaf Clovers

O. regnellii has three leaflets. This fact may be disappointing for you gardeners seeking the famed four-leaf clover. Luckily for you, there is another plant in this genus that does bear four leaflets. O. tetraphylla 'Iron Cross' tells you in its very species name that it is four-leaved since that is what the name tetraphylla means. As a bonus, the leaves are bicolored, being 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. 

Are They the True Shamrocks?

Many gardeners searching for Irish shamrocks with which to celebrate authentically for St. Patrick's Day wonder if there is a "real McCoy" when it comes to growing so-called "shamrocks." The fact is, there isn't. But considering the beauty of the black and purple shamrock plants, they are as fine a choice as any to grow with an eye toward decorating for St. Patrick's Day.

By the way, not all Oxalis species are desirable. Yellow wood sorrel or "sourgrass" (Oxalis stricta) is a common lawn weed. If you have ever tried getting rid of it, you know that it is as mischievous as any leprechaun.