Written by Mary Free, Extension Master Gardener
The month of March is a good time to talk about the shamrock, a symbol often associated with Ireland and Saint Patrick’s Day. In Gaelic, shamrock translates to “seamrag,” which also means trefoil or clover. There is some debate over whether this celebrated plant is Trifolium dubium (small hop or yellow clover) or Trifolium repens (white clover), both native to Europe but now widespread in the United States.
Small hop clover has yellow flower heads and tear-shaped, trifoliate leaves. White, v-shaped “watermarks” sometimes distinguish the egg-shaped leaflets of white clover. Its tiny, white florets, which appear in ¾” heads, serve as the major nectar source for clover honey. When white clover seeds are mixed with grasses, it serves as excellent forage for livestock. Many of us, though, know these plants as lawn weeds, through which we searched for four-leafed clovers as children.
White clover occasionally produces four leaflets instead of three. [Even less frequently, mutations may occur with more than four leaflets.] However, there is a plant known as “lucky clover,” on which each and every leaf has four leaflets. In fact, it is not a clover at all; Oxalis tetraphylla (iron cross) is in the wood-sorrel family. Often purchased as a houseplant, this Mexican native has four, dark green leaflets with maroon centers and pinkish-red flowers. Planted outdoors as an ornamental, it prefers sun to part shade and is hardy to Zone 8. Although if mulched it might survive a mild winter in Zone 7 (Arlington and Alexandria are in Zone 7A), overwintering the bulbs in a cool place indoors and replanting them in spring is more reliable. It also makes a stunning companion to Oxalis triangularis.
Commonly named shamrock (false shamrock or purple shamrock), Oxalis triangularis is a South American native that produces pale pink flowers and beautiful 12” tall, purple to green, trifoliate foliage. Also grown as a houseplant, it may be hardy outdoors as far north as Zone 6. Oxalis triangularis performs best in containers or as an accent in borders in part to full shade and well-drained soil.
Many illustrations and advertisements depict the shamrock with three green, heart-shaped, instead of oval-shaped, leaflets. The heart-shaped leaves are representative of Oxalis, not clover, as in Oxalis stricta (yellow wood sorrel, pictured). Like the other Oxalis mentioned here, it exhibits nyctinasty—each leaflet folds in half at night and opens again in the morning (note the folded leaflets behind the firefly). Also, like the others, it is edible and used to flavor and decorate soups and salads and quench thirst. Oxalis contains oxalic acid, though, so its use in the diet should be spare. Also, it could potentially harm individuals with certain health issues (e.g., kidney problems, rheumatoid arthritis, etc).
Native to the lower 48 United States, this annual/perennial wildflower grows up to 6” tall and produces small, yellow flowers with five petals. Mostly, though, it produces angst amongst nurseries and landscapers who consider yellow wood sorrel an aggressive weed.
In container plants, hand weeding (before it produces seed capsules) and mulching prove the most effective controls for yellow wood sorrel. In lawns, good cultural practices offer the best defense: establishing a dense turf by choosing the right grass for your yard and maintaining it properly. When that does not work, herbicides may be effective in controlling weeds like Oxalis and white clover if the correct ones are applied at the correct time (i.e., primarily pre-emergence herbicides for Oxalis, only post-emergence herbicides for white clover). Chemicals, though, could harm other plants, animals or water quality so they must be chosen and used with great care.
Wood sorrel or clover, wildflower or weed, which shamrock(s) will you welcome into your yard?
For information on the use of herbicides or specific weed control products, call the Virginia Cooperative Extension office at (703) 228-6400 or visit the Virginia Tech web site and consult its Pest Management Guide: http://pubs.ext.vt.edu/456/456-018/456-018.html