Date: April 1989 (Revised April 1995)
Source: NDSU Extension Service Horticulturists
Leaves occasionally turn yellow and drop off indoor green plants. This may concern you, especially when several leaves start to turn light green or yellow.
There are several reasons why plants leaves fall off. Physical injury to a leaf may cause it to turn yellow and drop off. Something may have happened while the particular leaf was forming to weaken it. There are also many environmental causes. Dry soil, overly bright lights or cold temperatures can all make leaves turn yellow. Do not be concerned if a single leaf starts to decline, regardless of its location on the plant.
Light is important because it helps maintain healthy green leaves. This is especially true for those leaves near the base of the plant. There is a tendency for these leaves to be shaded and they may receive only modest light. When this happens the oldest leaves tend to turn yellow and drop from the plant.
Light influences the plant's ability to make food and keep its leaves in good condition. When new leaves appear, older leaves often turn yellow and fall off. This is not unusual because it is how the plant balances available light with the appearance of new leaves. The oldest leaves decline first when there is inadequate "food" to support the entire plant.
A declining root system can also turn leaves yellow. Root injury may occur due to excessive use of fertilizers. High populations of soil insects feeding on the root system can also trigger lower leaves to die.
Too much water in the soil and limited oxygen can also kill the root system. This may happen if there are no drainage holes in the pot. Water may accelerate damage and destroy some or all of the roots. Heavy soils often become waterlogged and roots may die from a lack of oxygen.
If you change the plant's location it can trigger older leaves to fall off. The plant must adjust to the new environment and in the process it may shed some leaves.
If many leaves turn yellow within a day or two, the plant may have been exposed to cold temperatures and possibly toxic gases. Plants near a window often lose leaves immediately after the first cold spell of the fall.
There are no simple answers to the question of why leaves often turn yellow. One must examine the history, environment, watering and soil conditions in hopes of identifying the primary source of the problem.
For future reference, you may want to obtain a copy of Extension bulletin PP-744, "House Plants Proper Care and Problem Solving," which is available at your county office of the NDSU Extension Service.
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