Yellowing leaves on your houseplants can be caused by a number of conditions. Sometimes the cause is obvious, which means that you can diagnose and fix it immediately. There are other times when the problem is more of a mystery. In these cases, you'll need to try changing one thing at a time until you see improvement in your plant.
Even after you correct the problem, it's still likely that the yellow leaves will fall off with time. Don't worry, if the plant regains its health, it's possible that new leaves will fill in during the next growing season. Growing plants is always a matter of patience. Do your best to eliminate these common reasons for yellowing leaves, then wait to see what happens.
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Overwatering or underwatering are the most common culprits when a plant's leaves turn yellow. With potted plants, it is crucial that you only water as much as the plant needs.
If you have a plant with yellow leaves, check the soil in the pot. Is it dry? Is it soaked?
If plants don't receive enough water, they drop leaves to prevent transpiration (essentially, a plant's way of sweating) to conserve water. Before they drop, though, the leaves will typically turn yellow. If the soil is dry and this is happening, make it a point to get the plant on a regular watering schedule.
Too much water can be just as damaging to leaves. When the soil doesn't drain well, an overdose of water leaves the soil waterlogged and root systems can literally drown. Without oxygen, roots start to die.
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Cold drafts on tropical plants will often cause the leaves to turn yellow and drop. 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 in summer or a drafty window in winter, move it to a less turbulent place. Keep an eye on it to see if the yellow leaves spread any further. It's also a good idea to mist tropicals that you're overwintering to increase the humidity.
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Lack of Light
Plants that receive too little light will often start to yellow on the lower leaves before those leaves drop. If this is your issue, there is a clue that you can look for.
A plant that is yellowing from a lack of light will typically yellow on the side that is away from the light source. The leaves near the window, for instance, are getting all the light and blocking the opposite side.
If this is the case, move the plant to a sunnier location and see how it does. If window light is tough to come by in your home—especially in winter—you might need to rig up an artificial plant light or two.Continue to 5 of 6 below.
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Plant leaves may also turn yellow if a plant is not receiving all of the nutrients it requires. This can be caused by too much calcium in the water if you're using hard water or by a nitrogen deficiency.
If this is the problem, the plant's top leaves may be the first to go yellow. In other cases, you might notice an unusual pattern to the yellowing. For instance, the veins may remain dark while the tissue between them turns yellow.
The nutrients a plant requires varies based on the species and some are pickier than others. It's important to try and diagnose the problem properly or you might kill a plant that can otherwise be brought back to health.
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If your plant has a viral infection, it might show up as blotchy, spreading yellow patches on leaves throughout the plant. This may be accompanied by deformed leaves and stems, as well as discolored flowers.
Viral infections in plants cannot be cured and can infect all susceptible plants nearby. While it may be painful if it's a favorite, discard any plants that you suspect are infected. Wash and sterilize any pruning tools or pots before using on other plants.
Overwatering. Missouri Botanical Garden, 2020
Perry, L. Why Houseplants Drop Leaves. University of Vermont Department of Plant and Soil Science
Diagnosing Nutrient Deficiencies. University of Missouri Extension
Hosack, P and Miller, L. Preventing and Managing Plant Diseases. University of Missouri Extension