Yellowing leaves can be caused by a number of conditions. If the cause is obvious, you can fix it immediately. If the cause isn't so obvious, try changing one thing at a time until you fix it. Even after you correct the problem, however, it's likely that yellow leaves will fall off with time. Don't worry: if the plant regains its health, it's very possible that new leaves will fill in during the next growing season.
Here are the most common reasons for yellowing leaves:
- Normal aging. As many plants age, the lower leaves will turn yellow and drop off as a normal part of growth. This is especially true of foliage plants like dieffenbachia and dracaena. In this case, don't worry. If the plant becomes too leggy, consider trimming back the main stem to promote new growth and bushiness.
- Overwatering. This is probably the number one cultural fault that causes leaf yellowing. Only water as much as the plant needs! Very few plants appreciate water left to sit in their trays (called sublingual watering), and even tropical plants don't want to grow in mud unless they are semi-aquatic plants, like some of the alocasia species.
- Cold drafts. Cold drafts on tropical plants will often cause leaf yellowing and dropping. This is different from short periods of exposure to intense cold, which will cause outright browning on the foliage or pale, transparent spots to appear between veins. If your plant is near an air-conditioner vent, move it to a less turbulent place and see if it stops spreading.
- Lack of light. Plants that are receiving too little light will often start to yellow on the lower leaves before dropping leaves. One clue: if the yellowing begins on the side away from your light source, it might be caused by too little light reaching these back leaves.
- Nutrient deficiency. If the top leaves are yellowing, or if there is an unusual pattern to the yellowing (i.e., the veins remains dark while the tissue between them turns yellow), it is most likely a nutritional problem. This can be caused by too much calcium in the water, if you're using hard water, or by a nitrogen deficiency.
- Viral infection. A viral infection might show up as blotchy, spreading yellow patches on leaves throughout the plant. This might be accompanied by deformed leaves and stems and discolored flowers. Viral infections in plants cannot be cured and can infect all nearby susceptible plants. Discard plants you suspect of being infected.