Why do leaves turn yellow and fall off of otherwise healthy-looking plants?

This is one of those questions that’s not easy to answer, since there are so many possible reasons for this symptom. When I get this question, I always feel like a doctor who can’t give you a definitive answer for why you have a persistent cough.  You don’t have a cold, you don’t have the flu, so it might be allergies, it might be your sinuses … it might be any number of things, so you’ll just have to wait it out until the symptoms pass.  I always feel frustrated by this response when I visit the doctor, until I remember that often, I have the same answer for people with plant questions.  Sometimes it’s just impossible to pin down a specific reason for particular symptoms.

In the case of yellowing leaves, it might be that the plant is getting too much water, or it might be just that the leaf is old.

It could also be that the plant is lacking in nutrients, so the plant decides to sacrifice the leaf, and no longer waste precious resources trying to keep it alive.  If a leaf is not green, it’s lacking in chlorophyll and unable to do its job: perform photosynthesis and supply the plant with carbohydrates for growth.  The remedy for this instance would be to fertilize the plant.

But if the plant is getting too much water, obviously, the solution is to cut back on watering.  You can tell if the plant needs water by pressing your finger down into the soil as far as you can.  That’s about 2 inches, and you don’t need to water if the soil is not dry at that depth.  If the top 2 inches never dry out, the soil below that definitely doesn’t.

To confuse you even more, another reason for healthy plants to develop yellowing leaves and drop them might be not ENOUGH water.  So the key to solving water issues is going to be watching the soil, and checking the moisture level quite often for a while.  It would be a good idea to purchase a moisture meter.  You can find very affordable ones at most nurseries, and there’s really no need to buy an expensive one.

The good news is that overall yellowing of older leaves is not a sign of disease, and is rarely a sign of insect issues.  So my best advice is to remove the yellow leaves and just pay close attention to your plant’s growing environment.  As with that persistent cough, this symptom will usually run its course, with very little effort on your part.