Even if St. Patrick's day isn't coming up, you can still enjoy shamrocks by growing this cheerful plant, either as a houseplant or outdoors. Although several different plants might be called shamrocks because of their three-lobed leaves, the most commonly grown member of this group is also known as wood sorrel (Oxalis acetosella), which can grow outdoors in U.S. Department of Agriculture plant hardiness zones 3 through 7. It needs only basic care and a bit of extra attention now and then to thrive outdoors or as a houseplant.
Light and Soil
The best type of soil and proper lighting for a shamrock depends to some extent on whether you grow it outdoors or indoors as a houseplant.
The shamrock is a compact plant that's usually about 4 inches tall and spreads as a mound to cover about 1 foot. If you plant shamrocks outdoors, space them about 1 foot apart to allow them to spread fully. In the garden, the best location is one that's partially to totally shaded, such as near other larger plants that provide some cover. Some dappled sunlight is fine during the cooler morning hours, but shade is important during summer afternoons, when leaves might scorch from too much sun.
Indoors, a shamrock plant needs bright light that's indirect, such as near a lightly curtained south- or west-facing window. During winter, it can tolerate direct sun in an uncurtained window, but this is too strong in summer and might burn leaves. You can also use fluorescent lighting indoors to keep the plant in a dimly lit spot, but keep this on for 12 hours daily.
A garden-grown shamrock does best in organically rich soil, so supplement soil by mixing in 1 inch of compost before planting. You can also boost the soil's fertility by scratching an inch or two of compost into the soil under the plant in spring, when it starts growing.
If you grow a shamrock as a houseplant, pot it in any well-drained, commercial potting soil. For an extra boost, add about 1 inch of compost to the soil surface each spring, mixing it in with a small trowel or fork. Make sure the container has a drainage hole.
Moisture and Fertilizer
Whether grown outdoors or as a houseplant, a shamrock prefers even, steady moisture. Let the surface dry slightly between waterings, then water the plant deeply. If the plant is potted, let it drain fully after you water and never keep it in a water-filled saucer because this can promote fungal growth.
Although shamrocks aren't heavy feeders, they bloom best when fertilized during their period of active growth, generally in spring and early summer. Use a balanced, water-soluble fertilizer such as a 20-20-20 formula, diluted at half-strength, or 1/4 teaspoon per gallon of water, but check the product label for other instructions. Use the fertilizer solution in place of a regular watering, and feed the plant every two weeks.
It's natural for a shamrock plant to slow its growth and start to die back, entering a dormant period for a few months. Outdoors, this usually happens in late summer, when leaves start to dry up and drop, while a houseplant can become dormant at any time, although this usually begins in fall or winter.
When a shamrock slows its growth and becomes dormant, stop watering and fertilizing. Cut back dried stems for tidiness or let them drop naturally; if you cut them, wipe shears with rubbing alcohol between cuts to avoid spreading plant disease. If the plant is potted, keep it in a cool, dark place such as a basement until you see new growth, then move it back into the light and resume watering and feeding.