Improper watering can cause leaves to turn yellow. While too little water causes wilting, too much water is indicated by yellowing. Oxalis prefers soil that is barely moist and can handle soils that dry out slightly between waterings. To determine if overwatering is the cause of the yellowing, take the oxalis out of its container and inspect the roots. If the roots are brown and mushy, overwatering is responsible; cut back the dead roots and do not water the plant until new growth emerges.
All bulbed oxalis, no matter how healthy and well-cared for, enter a period of dormancy about two or three times each year. When an oxalis begins to die back or simply has not bloomed for an extended period, it is likely ready for a period of dormancy. This often coincides with warming temperatures. A period of dormancy is indicated by a yellowing or browning of leaves not caused by overwatering.
Care During Dormancy
When this plant is entering dormancy, stop watering it. Allow the leaves to die back and remove the dead, brown leaves. Keep the plant in a dark, cool place for about three months. When the bulb begins sending out new, green shoots it is ready for light and water again. If the plant must be divided, the division should occur at the end of this dormant period.
A few different pests can attack oxalis and cause yellowing of leaves. Two possible pests include spider mites and aphids. If a plant is kept outdoors, carefully inspect it for pests before bringing it inside, as it may spread pests to other houseplants. Any infestations on the oxalis are typically of no concern and can be left untreated, as they will not survive the next period of dormancy.
Additional Care Considerations
Oxalis generally require direct sunlight to reach full bloom potential and prefer cool temperatures, generally between 50 and 65 degrees Fahrenheit at night and no higher than 75 degrees during the day. If the plant is growing thin and lanky, it may require more light. Oxalis should only be fertilized when the plant is actively growing. Leaflets may naturally fold up at night or during overcast weather.