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Many different culprits, such as the environment, nutrition, or pests, can cause yellow leaves.  Whatever the reason, yellow leaves are often the most worrisome signs of plant stress that gardeners worry about.  In this article, we will outline all the possible causes and how you might diagnose and address them.

Why do leaves turn yellow?

Leaves are the powerhouse of the plant. Inside each beautiful leaf is a complex biological machine that turns sunlight and CO2  into water and sugar. Part of this machine is a green pigment called chlorophyll, which captures energy from the sun.  But sometimes leaves lose their characteristic green color and begin to turn yellow. This yellowing of the leaves is known as chlorosis. Leaves yellow when a lack of chlorophyll affects portions or the entire area of a leaf. 

Old Age

A vital feature of all living things is that nothing lives forever. Leaves can have natural lifespans anywhere from 2 weeks to 25 years! Most house plants have long-living leaves. Although there are many causes for yellowing, it is important to consider if the leaf in question is very old, as this simply may be a part of nature!

Inadequate Sunlight

Sunlight is an essential part of a plant's diet. After extended time with low light, leaves all over the plant will begin to fade to yellow. The leaves can also become limp. Move your plant into an area or window with more light if possible. You can also supplement your house plants with lights. These can be grow-lights, LED, or fluorescent bulbs. You can even place your plant directly under a floor lamp for a boost of light. 

During change of season, daylight becomes shorter and may affect plants that were otherwise happy.  Check out our article on what to do for plants in winter.

Overwatering

Many of the environmental factors that lead to yellow leaves involve water. All plant roots require water and air in the soil. When plants get overwatered, the roots can suffocate. Soggy soil also predisposes your house plant to pests and diseases that flourish in dark, wet areas like fungi. You may also notice fungus gnats, small flies that eat fungus from damp places. The lower leaves will be affected first; leaves can fade to yellow or dull light green. If overwatered consistently, the lowest leaves will begin to drop.  In severe cases, the base of the stem can become blackened with rot. 

To treat this, you need to re-pot your houseplant into new, clean soil. If your plant requires better draining soil, mix sand into your potting mix. Consider using a different pot with more drainage as well. Most importantly, you will need to build new watering habits. Test your soil by poking your finger up to the second knuckle. If the soil beneath the finger is still damp, wait a few days and check again before watering.

Underwatering

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Underwatered house plants have dry, crunchy leaves, sometimes curling inward. Leaves can be yellow or brown. The symptoms of an overwatered plant and an underwatered plant can look very similar. If you are unsure, check your soil. If your soil is wet or moist, overwatering might be the culprit. But if your soil is dry or you know that you have not watered your plant in a while, your house plant is thirsty! Treat underwatering by consistently checking on your houseplants. Ensure you thoroughly saturate the root system by soaking pots with drainage holes for 10-20 minutes in a bucket or tray of water.  

Water quality

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Another way water can impact your house plant is through minerals and ions in the local water supply.  Depending on your area, tap water can contain high fluoride, chlorine, or various salts. Low water quality leads to yellow and brown leaf tips and edges. You can validate this by checking for salt buildup on the surface of the soil. To treat this, let tap water sit out overnight to help evaporate some of the minerals. You can also purchase distilled water or use collected rainwater.

Pests  

Bacterial/fungal/virus

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Diseases caused by bacteria, fungi, and viruses can lead to yellow leaves. Some Infectious diseases can be spread via insects, but most are spread by human behavior.  These infectious organisms can be found in the air and soil outside, and under wet conditions, they can attack a plant. Avoiding over-soaked soil and keeping leaves dry helps to prevent infection. Unhygienic practices like using dirty pruning shears can spread diseases between plants.

A common disease on house plants is known as Brown Spot Disease. Either bacteria or fungi can cause brown spots. Brown spot disease causes spots of brown, necrotic areas surrounded by a halo of yellow. Remove and dispose of all damaged tissue and immediately. Bacteria and fungi spread quickly, so isolate the infected plant from the rest of your houseplants. Antifungal soaps, such as Copper soap, can be applied to prevent additional infections. Metal soaps coat plant leaves and, when wet, release ions that destroy fungal proteins. 

Viruses also cause yellow leaves. Viral diseases cause characteristic yellow splotches called a mosaic.  If you suspect that you have a viral disease, immediately isolate the plant. Viral diseases cannot be treated, so it's best to get rid of the plant if the disease progresses. 

 Nutrition

Aside from sunlight, plants require 13 nutrients from the soil. If you have ruled out environmental and biological causes, your plant could be experiencing a nutrient deficiency. Deficiencies can be due to a lack of minerals in the soil or the plant's inability to uptake a mineral, which can happen when your soil becomes too alkaline. 

The most common deficiency in house plants is iron.  Iron deficiency is easy to notice because it affects the youngest leaves first. Leaves become very pale with dark green veins. Most other deficiencies will affect the older leaves first.

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Nitrogen deficiency starts on older leaves, with yellow tips and veins, and eventually progresses to entirely yellow leaves.

Phosphorus deficiency will cause older leaves to be yellow and brown. If your houseplant is flowering, phosphorus deficiency can also prevent the formation of flower buds.

Photo Credit: JB Henry, DOI: 10.13140/RG.2.2.25313.40800

To treat nutrient deficiencies, use a balanced fertilizer (i.e., 10-10-10) once or twice in the spring or summer. You can also check the pH of your soil if you are worried about iron availability. 

What do you do with a yellow leaf?

Once a leaf starts to turn yellow, it cannot be reversed.  When you see yellow leaves on your plant due to light, watering, or nutritional deficiencies, you have a few options:

1. If only a small portion of a large leaf is yellow, cut out the damaged part and leave behind the healthy portion. If a leaf is yellow all over, remove the entire leaf. This allows the plant to focus its energy on growing new leaves instead of dealing with damaged ones. 

2. If you suspect you have an insect, bacterial, fungal, or viral infection, remove damaged leaves and dispose of them immediately. Just remember to never remove more than 30% of the foliage at one time.  Anytime you prune your plants, it's best practice to sanitize your scissors or shears with rubbing alcohol (Hand sanitizer also works). This is especially important if you suspect your plant has a disease, as unclean shears can quickly spread the infection to a healthy plant. 

Yellow leaves are a normal part of owning house plants.  It is most important to focus on your plant's overall health! Giving your houseplants and occasional inspection will help you notice any problems before they progress. By keeping an eye out for yellow leaves, you can help keep your plants happy and healthy.