Why Are My Leaves Turning Yellow?

Why Are My Leaves Turning Yellow?

I stared at my pothos last Tuesday morning, coffee in hand, and noticed three yellow leaves near the base. My first thought was panic. My second thought was to poke the soil with my finger like a weirdo and smell it. (Yes, I smell my plant soil. You probably should too.)

Yellow leaves freak us out because they feel like a cry for help, but here’s the thing: plants turn yellow for about a dozen different reasons, and figuring out which one is your problem takes a bit of detective work. I’ve killed enough plants to know that jumping to conclusions usually makes things worse. So let’s walk through the most common culprits, and I’ll share what I’ve learned from both my own mistakes and some actual research.

Overwatering: The usual suspect

When I first got into houseplants, I thought being a good plant parent meant watering on a schedule. Every Sunday, like clockwork. Turns out, that’s a great way to drown your plants.

Overwatering is the number one reason houseplants develop yellow leaves, and it’s sneaky because the yellowing doesn’t happen immediately. What happens is this: when soil stays wet for too long, the roots can’t breathe. They start to rot. Once the roots are damaged, they can’t take up water properly (ironic, right?), so the plant starts shutting down older leaves to conserve energy. Those leaves turn yellow, then brown, then fall off.

The tricky part is that overwatered plants and underwatered plants can look similar at first. Both get droopy. Both get yellow leaves. But here’s how you tell the difference: stick your finger about two inches into the soil. If it’s wet or even damp down there, and your plant has yellow leaves, you’ve probably been watering too much. Overwatered soil also tends to smell a bit funky, kind of like a basement. That’s the rot.

I learned this the hard way with a beautiful calathea I bought two years ago. I kept the soil moist because I’d read they like humidity. What I didn’t realize is that moist soil and wet soil are very different things. The lower leaves started turning yellow within a month. By the time I figured out what was happening, half the roots were brown mush.

If you catch it early, you can fix this. Let the soil dry out completely before watering again. If the roots are really rotted, you might need to take the plant out of its pot, trim away the dead roots with clean scissors, and repot it in fresh soil. I’ve saved a few plants this way, though I’ve also lost a few where I waited too long.

According to research from the University of Georgia Extension, root rot from overwatering is exacerbated in containers without drainage holes or in soil that’s too dense. They recommend using well-draining potting mix and always checking soil moisture before watering. I now use a moisture meter for my fussier plants because my finger test isn’t always accurate, especially in deeper pots.

Pests causing yellow spots

I didn’t think I had a pest problem until I looked closer. Like, really close. With my phone’s flashlight.

Spider mites, aphids, and thrips all suck the juices out of plant leaves, and this can cause yellowing. But it’s not the same kind of yellowing you get from overwatering. Pest damage usually shows up as speckled yellow spots or a general fading of color, especially on new growth. Sometimes you’ll see tiny webs (spider mites) or sticky residue (aphids). Sometimes you won’t see the bugs at all because they’re microscopic jerks.

I had spider mites on my monstera last summer. I noticed the leaves looking dull and pale, almost like they’d been dusted with powder. When I wiped a leaf with a white paper towel, I saw tiny red smears. That’s when I knew. Spider mites thrive in dry conditions, and my apartment gets pretty dry in the summer with the AC running.

The treatment depends on the pest. For spider mites, I’ve had success with wiping down every single leaf (top and bottom) with water, then spraying the whole plant with insecticidal soap. You have to repeat this every few days for a couple of weeks because the eggs keep hatching. It’s tedious. I almost gave up twice.

For aphids, I’ve just blasted them off with water in the shower. They’re soft-bodied and don’t usually come back if you’re thorough. Thrips are harder because they’re so small and they hide in the soil too. I’ve read that systemic insecticides work for thrips, but I haven’t had to deal with them yet (knock on wood).

The key with pests is catching them early. I try to inspect my plants every time I water now, which sounds obsessive but takes like two minutes. I look under the leaves, check the stems, and keep an eye out for anything that looks off. The sooner you spot a problem, the easier it is to fix.

Above: A close up look at the symptoms.

Nutrient deficiency signs

This one took me forever to figure out because I kept thinking houseplants didn’t need fertilizer. They’re indoors, growing slowly, how hungry could they be?

Turns out, pretty hungry. Potting soil only has so many nutrients in it, and once the plant uses them up, it starts pulling resources from older leaves to feed new growth. This causes the older leaves to turn yellow and eventually drop off.

Different nutrient deficiencies cause different patterns of yellowing, which honestly feels like overkill for someone just trying to keep a snake plant alive, but it’s useful to know the basics. Nitrogen deficiency causes older leaves to turn yellow all over, starting from the bottom of the plant and working up. Magnesium deficiency causes yellowing between the veins while the veins stay green. Iron deficiency causes the same interveinal yellowing, but it shows up on new leaves instead of old ones.

I noticed this pattern on my peace lily last winter. The older leaves were turning completely yellow, one by one, but the plant was still putting out new growth. The soil was moist (not wet), there were no pests, and the light hadn’t changed. I finally broke down and bought some balanced houseplant fertilizer, the liquid kind you dilute in water. Within a month, the yellowing stopped.

According to the Colorado State University Extension, most houseplants benefit from fertilizing during the growing season (spring and summer) but not during winter when growth slows down. I fertilize about once a month from April through September, then I stop. I also use a diluted solution, usually half of what the bottle recommends, because I’ve burned plants before with too much fertilizer. Yes, you can over-fertilize. The leaf tips turn brown and crispy. Ask me how I know.

One thing to watch for: if your plant is dropping yellow leaves in the winter and you haven’t fertilized in months, it’s probably not a nutrient problem. It’s more likely a light or watering issue.

Natural aging of leaves

Here’s the thing nobody tells you when you start keeping plants: sometimes leaves just die. It’s normal. It’s supposed to happen.

Every plant sheds old leaves eventually. They turn yellow, then brown, then fall off. This is especially common with plants like pothos, philodendrons, and monsteras that grow vines or large leaves. As the plant puts energy into new growth at the top, it drops the oldest leaves at the bottom. It’s not sick. It’s not dying. It’s just doing plant things.

I used to panic every time I saw a yellow leaf. Now I ask myself a few questions first: Is it just one or two leaves? Are they the oldest, lowest leaves on the plant? Is the rest of the plant healthy and putting out new growth? If the answer to all three is yes, I just cut off the yellow leaf and move on with my life.

The key is knowing what’s normal for your specific plant. My pothos drops a leaf every few months. My rubber tree has never dropped a leaf in two years (which actually worries me a bit, but that’s a different issue). My fiddle leaf fig is dramatic and drops leaves if I look at it wrong.

I found a good rule of thumb from a gardening blog I trust: if less than ten percent of the plant’s leaves are yellow and they’re all old growth, it’s probably natural aging. If more than ten percent are yellow, or if new leaves are yellowing, something else is going on.

Above: The tools you need to fix this.

Light stress yellowing

Light problems are weird because both too much light and too little light can cause yellowing, just in different ways.

Too little light usually causes a gradual, overall yellowing of the whole plant. The leaves don’t get the energy they need to stay green, so they fade. This happens slowly, over weeks or months. I see this a lot in winter when the days get shorter. My plants that are far from the windows start looking pale and washed out by February.

Too much light, on the other hand, causes bleaching or scorching. The leaves turn pale yellow or white, sometimes with brown crispy edges. This usually happens when you move a plant from a dark spot to a bright spot too quickly, or if it’s sitting in direct sun when it’s used to shade. I did this to a snake plant once. Moved it from my bedroom to a south-facing window in the living room. Within a week, the leaves looked sunburned.

The fix for too little light is obvious: move the plant closer to a window or add a grow light. I’ve started using a clip-on grow light for my desk plants in the winter, and it’s made a huge difference. The fix for too much light is also obvious: move the plant back or filter the light with a sheer curtain.

Here’s what I’ve learned about light through trial and error: most houseplants can adapt to different light levels, but you have to give them time. If I need to move a plant to a brighter spot, I do it gradually over a week or two, moving it a bit closer to the window each day. This gives the leaves time to adjust without getting shocked.

The University of Maryland Extension notes that light stress often compounds other problems. A plant that’s already dealing with overwatering or pests will show worse symptoms if it’s also in the wrong light. So sometimes fixing the light issue helps everything else fall into place.

The bottom line with yellow leaves is this: don’t panic, but do investigate. Look at the pattern of yellowing, check the soil, inspect for pests, think about when you last fertilized and whether the light has changed. Most of the time, the plant is telling you exactly what’s wrong if you know how to read the signs. And if you kill a few plants while you’re learning (like I have), that’s okay too. We’re all just figuring this out as we go.

References

University of Georgia Extension. “Houseplant Diseases and Disorders.” Accessed through UGA Extension publications on container plant care and root health.

Colorado State University Extension. “Houseplant Fertilization.” Master Gardener program publications on indoor plant nutrition and fertilizer application rates.

University of Maryland Extension. “Growing Houseplants.” Resource materials on light requirements and environmental stress factors for common indoor plants.