For cheerful little plants, few top those in the genus Oxalis, which contains about 800 species. Most are also called wood sorrels or shamrocks -- named for their leaves that are divided into three or four leaflets, making them resemble the Irish lucky charm. Usually 6 to 12 inches tall, they thrive outdoors under the right conditions. Oxalis also make good houseplants that do best in cool temperatures and need only bright light, regular water and a bit of fertilizer now and then.
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Growing Oxalis Outdoors
You can grow an oxalis plant outdoors, provided you choose a variety that's suited for your area and that can withstand local winter temperatures. For example, the common wood sorrel (Oxalis acetosella)and the broadleaf wood sorrel (Oxalis latifolia) can survive sub-freezing temperatures. The common type grows in U.S. Department of Agriculture plant hardiness zones 6 through 11, while broadleaf wood sorrel grows in zones 8 through 11. In colder parts of their range, these types die to the ground over winter, returning the following spring. Other varieties can't survive frost for long periods; these include volcanic sorrel (Oxalis vulcanicola "Zinfandel"), which grows in USDA zones 9 through 11. Grow this and other frost-sensitive oxalis plants as annuals.
Outdoors, most sorrels grow best in partial or full shade, which you can provide in the garden by locating taller, shade-producing plants nearby. They also need regular watering during the growing season to keep soil evenly moist but don't do well in constantly soggy conditions. Provide extra water during dry spells, but only water when the soil's surface feels dry to the touch.
Add 2 or 3 inches of organic mulch under the plant, to help retain moisture and suppress weeds.
Many oxalis varieties grow well indoors in pots, where they thrive in bright light such as in a west- or south-facing window. But protect the plant from direct summer sun, either by adding a sheer curtain to the window or moving the plant into less light during hot weather. You can also grow these plants indoors under fluorescent lights turned on for 12 hours each day.
Keep a houseplant at about 70 to 75 degrees Fahrenheit during the day; cooler temperatures between 50 and 65 degrees F are best at night. Keep the soil evenly moist, letting it dry slightly between waterings. The plant might become naturally dormant in winter, dying back and drying up -- stop watering and let its soil dry for about one month, then resume watering and new growth should appear.
Other Care and Possible Problems
During spring and summer, when an oxalis houseplant is actively growing and blooming, feed it monthly with a balanced, water-soluble fertilizer such as a 20-20-20 formula -- dilute it half-strength, or 1/4 teaspoon per gallon of water, but check the product label for additional directions. Reduce this to once per month after flowering, then stop when it's dormant. Fertilize an outdoor plant monthly with the same half-strength fertilizer until it becomes dormant.
Outdoor-grown oxalis plants are generally disease- and pest-free, although they might develop fungal problems when grown in overly wet conditions. Watering with a soaker hose to keep foliage dry helps prevent this, as does regularly clearing away dropped leaves and other debris Whether indoors or outdoors, the plant might attract spider mites, microscopic pests that produce visible webs on leaves. Control these by spraying with insecticidal soap every two weeks as needed. Dilute the soap at a rate of 5 tablespoons per gallon of water.
Potential for Invasiveness
Some types of oxalis plants can be invasive, spreading by putting down roots where stems touch the soil, so they're not good choices for planting near a naturalized area. Help control this tendency by trimming back plants regularly or by choosing a noninvasive variety such as "Iron Cross" oxalis (_Oxalis tetraphylla "Iron Cross"_), which doesn't spread aggressively; this variety and grows in USDA zones 7 through 10.