False shamrock houseplants (Oxalis triangularis) are part of a large genus of plants with pleasing and familiar, clover-like leaves that are also known as wood sorrels. They are found in plant shops and nurseries under a number of synonyms including Oxalis regnellii and Oxalis triangularis ssp. papilionacea (the later is still recognized as a name for a dark-leafed form by some). The common name, false shamrock, differentiates Oxalis from plants in the Trifolium (clover) genus, that are often associated with shamrocks. Native to South America, false shamrocks have a growth cycle that differs from many pot plants.
False shamrock plants may die-back and enter a period of dormancy following flowering. A rest period protects and restores the plant during stressful conditions and this less attractive part of the life cycle needs to be supported by a home gardener. Your plant may do this in some years and not in others, which can be confusing. This behavior will vary with growing conditions and the species, cultivar or subspecies that you are caring for.
Oxalis are otherwise easy and lovely houseplants. They like a home kept on the cool side. Both flowers and leaves fold at night, a phenomenon called nyctinasty, a movement related to the onset of darkness. Leaves of cultivars vary in color and markings and flowers range from white to pink and pale violet. Individual plants can live for decades.
Shamrock plants prefer some bright, direct, early light but need protection from harsher afternoon sunlight. Close to an east-facing window or a more filtered spot in a south-facing room suits these plants. A west-facing window is suitable as long as the plant is not exposed to strong, direct, early-afternoon light. Moving to a slightly more protected light situation after flowering may reduce the chance of dormancy for some plants.
Wet soil is a quick way to kill a false shamrock plant. It must have fast-draining soil, be in a pot with a drain hole and be watered with attention. Water thoroughly, making the potting mixture moist and checking back after 15 minutes to remove any water sitting in the plant's run-off dish. When the plant is in active growth, keep the soil barely moist and allow the soil to just dry between waterings. After flowering, if the plant begins to die-back, taper watering and then keep the soil dry during the period of dormancy.
Humidity above 50% when in active growth will support a healthy plant with attractive leaves. Keep your plant on a tray of wet pebbles or moss. Employing a simple hygrometer near the plant will help you monitor whether the air moisture is staying in the targeted range.
Keep your Oxalis at cool household temperatures of 60 - 70º F. and 15 degrees cooler at night. Higher temperatures (above 75ºF.) can trigger dormancy, which will cause it to drop its leaves. Keeping the plant on the cool end of its comfortable range after flowering can reduce the chance of dormancy for some plants. Avoid locations near a heat or air conditioning vent. The rapid changes in micro-climate are too stressful for the plant.
Re-pot annually, in the fall as the plant resumes growth, with a soil-based potting mixture that is amended with perlite and peat. Keep the plant in a container that drains from the bottom and move up to a pot one size larger if your false shamrock appears crowded in the pot. Without an annual change of soil, even if a new container size isn't needed, the potting mixture can become too compacted and a build up of fertilizer salts can damage tender roots. Do not bury the plant any deeper than it was in the previous pot.
Nutrition needs are light and a 1/4 strength feeding with a balanced, organic fertilizer every month during active growth is all that is necessary.
Many false shamrock plants go through a period of dormancy, most often in summer after the flowering period, though it is possible to skip a year or more. Taper watering when it is clear that your plant is becoming dormant. You can remove the dead leaves when they have dried out completely. Keep your plant in a dark, cool spot without watering and watch for growth to emerge again after 1 to 3 months. Check in occasionally to see if your plant is reawakening. Then you can return the plant to its sunny position and resume normal care.
Plants will sometimes react to environmental conditions by becoming dormant at other points in the year. If hot temperatures, shock or extended dryness induce dormancy, follow the same dormancy care routine but begin to look for renewed growth after a few weeks. If the precipitating stress was brief, your false shamrock may not become fully dormant and can return to full vigor once conditions are corrected.
Plants that are kept cool (under 70ºF.), adequately watered and in somewhat reduced light after flowering are less likely to experience dormancy, though the pattern is also variable amongst plants and plant types.
Oldest leaves fading on the plant, may simply be leaves aging and falling off, which is natural and nothing to worry about.
Leaves gradually fading to a bleached color and stems collapsing, particularly in the summer, may indicate that your plant is becoming dormant. This unusual timing for dormancy can be a shock to plant owners. Some Oxalis houseplants enter dormancy and rest for a period of time. The leaves will get pale, then brown and fall off. This is also natural and not a worry.
The same fading color and drooping can occur, however, as a sign of distress for other reasons. Is your plant getting the recommended amount of light? Has the soil become too dry or the temperature too high? Those conditions can be easy to correct and a plant may recover quickly.
When a plant is over-watered or the soil is compacted or is not draining well, the drooping leaves may be harder to recover from. Check whether the soil is staying wet (Oxalis likes soil to dry between waterings), the drain hole is blocked and re-pot the plant if the soil is no longer loose and free-draining. Check that the roots are firm, not mushy, when you un-pot the plant.
Leaves and flowers folding at night or on a day of dim light, is a charming natural behavior of this plant called nyctinasty, a movement related to the onset of darkness.
Leaves can burn if the sun is too strong. If your false shamrock leaves look dry around the edges with some brown patches, they may need to move to a position where afternoon light is less intense. Avoid splashing water on leaves which can magnify the sun's rays.
The Houseplant Survival Guide by 
