The shamrock (Oxalis acetosella) isn't just an Irish symbol for St. Patrick's Day -- it's also an attractive small plant that grows both outdoors in the garden in U.S. Department of Agriculture plant hardiness zones 3 through 7, and as a cheerful houseplant. This is an easy-to-grow plant that has divided dark-green leaves, usually with three leaflets.
The Best Outdoor Conditions
The shamrock plant belongs to a family of plants called wood sorrels, which are native to moist forests or shaded rocky areas. If you grow the plant outdoors, give it partial shade for best results. Because the shamrock is only about 6 inches tall, shade from taller nearby plants is helpful, or filtered shade from overhead trees. Because it's a forest native, the shamrock grows best in organically rich soil that contains lots of humus. Boost your soil's organic content by adding 2 inches of compost under the plant every spring, mixing it into the top of the soil with care to avoid disturbing roots.
An Indoor Environment
To grow shamrock as a houseplant, keep its pot in a spot with bright, indirect light, such as in a lightly curtained south- or west-facing window. The plant can stand a bit of direct sun, but too much can cause scorching of the leaves, so minimize this.
Shamrock grows best in a cool environment; a nighttime temperature between 55 and 65 degrees Fahrenheit is ideal, while during the day it prefers a temperature that doesn't exceed 75 degrees F. You can grow several shamrocks in a single pot because they don't have extensive root systems, but if the plants wilt often, you might need to change to a larger pot with space for more soil to provide better moisture retention.
Moisture and Fertilizer
Whether a shamrock grows outdoors or as a houseplant, it prefers consistent moisture, so water whenever the top of the soil feels dry to the touch. Adding 2 or 3 inches of organic mulch under a plant in the ground helps retain soil moisture between waterings, but keep the mulch back a few inches from the plant's base to discourage fungal problems.
Feed an outdoor plant monthly from spring through summer with a balanced, all-purpose, water-soluble fertilizer such as a 20-20-20 formula. Dilute 1/2 teaspoon in 1 gallon of water and use this instead of a regular watering, but check your product label for further directions. For a houseplant, use the same fertilizer at the same dilution as for an outdoor plant, feeding it monthly while the plant's growing actively.
A shamrock grown outdoors goes into dormancy naturally when cold weather arrives, so stop fertilizing in early fall to prevent tender new growth that might be damaged by low temperatures. Indoors, a plant may wilt, drop leaves and become dormant two or three times yearly, making it unsuitable for mixed containers with other types of plants. Stop watering and feeding a houseplant during its dormant period and clear away dead leaves, then resume watering and feeding when you see new growth.
The shamrock is usually trouble-free, but it might develop fungal problems such as powdery mildew or leaf spots. To avoid these, keep plants well-separated for good air circulation and water at the base to avoid wetting foliage. The plant could attract spider mites, microscopic pests that produce web-like coverings on leaves. Control these by spraying with insecticidal soap diluted at a rate of 5 tablespoons per gallon of water. Wear gloves for protection, spray until the plant is dripping and repeat every two weeks as needed.