When houseplant (indoor plant) leaves turn yellow, it’s usually one of the first symptoms indicating that the plant is stressed. If the conditions causing the plant stress are not alleviated, the yellow leaves may eventually turn brown and drop.
Plant stress can be caused by a wide variety of issues, and can be categorised as either biotic and abiotic stresses.
- Biotic stress refers to damage caused by living things, such as pest insects and plant diseases.
- Abiotic stress is caused by non-living things, such as improper growing conditions.
The Main Causes of Yellowing and Leaf Drop in Houseplants
The leading causes of yellowing and leaf drop is houseplants is abiotic stress, caused by unsuitable environmental conditions related to a plant’s needs – light, water, soil and temperature.
Abiotic injury can be caused by a lack or excess of water, exposure to low light or extremely sunny conditions, or the use of poor-quality potting mixes (planting media).
Often, the problem may be caused by a combination of factors, rather than just a single one, so it’s important to investigate and identify all possible causes, and take corrective action to resolve them.
The main abiotic stress factors that cause of houseplant leaf yellowing are:
1. Overwatering and Underwatering
Overwatering is the leading reason why most indoor plants fail, and is the most common mistake beginners make.
Water a plant until it just begins to trickle from the bottom to ensure that the potting mix is evenly damp all the way through. Don’t let the plant sit too long in a saucer or drip tray full of water.
All plants need a wet-dry cycle as they experience in nature, so it’s important to let the first few centimetres of potting mix to dry before watering again.
A plant that is underwatered may wilt and droop, and the pot will feel really light when it is lifted. Peace Lilies (Spathiphyllum) droop quite dramatically when they’re short of water, making them great beginners’ houseplants that can teach good watering practices, and can grow in fairly low light too.
Typically, indoor plants may need watering once a week, or once every two weeks, depending on the indoor air temperature, humidity and the season of the year.
Some houseplants don’t require much water at all, need to be watered less often, and are very sensitive to overwatering. These include:
- Mother-in-Law’s Tongue or Snake Plant (Sansevieria trifasciata)
- Cast Iron Plant (Aspidistra elatior)
- Zanzibar Gem or ZZ plant (Zamioculcas zamiifolia)
- Fiddle Leaf Fig (Ficus lyrata)
2. Insufficient Light or Excessive Light
Indoor plants will yellow and drop their leaves when light levels that are too high or too low, or if a sudden change in light intensity occurs.
All indoor plants are really subtropical and tropical understory plants, and are adapted to grow in filtered light coming through the tree canopy of tropical rainforests. Some need more light than others, but they all have a limit as to how much or how little light they can tolerate.
Plants also have the capacity to adapt to their environment, and one way they can do this is by growing larger leaves when light is low to capture more light, and smaller leaves when getting light is bright and they’re getting too much.
Ficus plants, such as the Weeping Fig (Ficus benjamina) are infamous for dropping their leaves when moved to a new position with very different light.
Moving a plant suddenly to another location where the light is much brighter or much dimmer will stress the plant and cause it to drop its leaves and try to grow new leaves better suited to the new conditions. Therefore, it’s best when moving plants to new locations to always change light intensity levels gradually.
Houseplants That Grow Well in Low Light Conditions
In rooms where there is low light, select plants that can grow well under low light levels, such as:
- Chinese Evergreen (Aglaonema)
- Cast-Iron Plant (Aspidistra elatior)
- Parlor Palm (Chamaedorea elegans)
- Spider Plant (Chlorophytum comosum)
- Dumbcane (Dieffenbachia)
- Fishtail Palm (Caryota mitis)
- Grape Ivy (Cissus rhombifolia)
- False Aralia (Dizygotheca elegantissima)
3. Low Relative Humidity
Low humidity creates a dry air environment for plants leaves that increases moisture loss, which usually causes the browning of leaves, and eventual leaf drop.
Since houseplants are really subtropical and tropical understory plants that grow in humid rainforests, they can readily becomes stressed by dry air due to low humidity outside, or through the use of ducted/fan heating or air conditioning.
A simple way to increase humidity around indoor plants is by misting them with a fine spay of water using a spray bottle.
A more elaborate method is to construct a humidity tray, which is just a large plastic tray filled with scoria or large stones around 25mm in size, with water underneath, that indoor plant pots are placed on. The stones hold the bottoms of the pots well above the water beneath so they don’t get waterlogged, and the water evaporating from the tray increases the humidity around the plants.
4. Soil Drains Poorly or Remains Wet For Too Long
Sitting an indoor plant pot in a saucer of water for extended periods of time will cause the potting mix at the bottom of the pot to become waterlogged, which will suffocate and rot the roots.
Potting mix is porous to allow water to flow through it to reach the roots of plants, but the downward flow of water also pulls down behind air and carries it to the root zone too. All terrestrial (land-growing) plant roots require air! It’s no coincidence that healthy soils and good quality potting mixes have sufficient porosity to allow for 25% air in their structure.
The potting mix can stay too wet for prolonged periods of time if plants are overpotted (when small plants are planted in overly large pots), because the small root systems can’t take up enough of the water fast enough.
When indoor temperatures drop over winter, plants will transpire (take up water) less, and therefore need to be watered less often compared to the hot summer season.
5. Temperature Extremes
Houseplants are warm-climate plants and can be injured by low temperatures when exposed to a draft from an open door or window, or from an air conditioner.
When plants are exposed to blasts of hot air from a nearby heater or heating duct, or a situated too close to the glass of a window that receives direct midday or afternoon sun (which lacks air circulation and gets extremely hot), plant injury can also occur.
When exposed to such temperature extremes of hot or cold, plants can become stressed, and leaves are likely to yellow and drop. Low temperatures can also cause the browning of leaf tips.
6. Under-Fertilizing
In order to grow, all plants need nutrients, which serve as their food to fuel the growth.
It’s important to feed all houseplants either once (at the start of spring) or twice a year (start of spring and autumn) with a slow-release fertiliser, which are solid, either in the form of a powder, pellet or prill (tiny ball). Water-soluble liquid fertilisers are only meant as a quick, short-term top-up feed, and won’t sustain a plant through the whole year.
Many beginners forget to feed their plants, and when a plant runs out of food, it’s forced to draw nutrients from the older leaves (causing them to yellow), and redirect them into new growth, dropping the used-up old leaves in the process.
7. Leaf Senescence
All plant leaves have a natural developmental life cycle, where they emerge, mature, then decline. Leaf senescence is the last stage of leaf development. Senescence can be defined as the deterioration that ends the functional life of an organism or an organ, in this case, a leaf.
Evergreen plants, including houseplants, can and do drop very old leaves every now and then. Early in the process of leaf senescence, we see the loss of chlorophyll, the green pigment in leaves responsible for photosynthesis, resulting in the yellowing of leaves.
There is usually no need for alarm if an occasional leaf yellows and drops as a part of the natural yearly growth cycle of many plants.
On older plants, if many leaves continue to yellow, or the whole plant does all at once, then there is need for concern, as is the case when any leaves yellow on very young plants.
When All the Leaves of a Houseplant Are Yellowing
If the whole indoor plant has yellowing leaves, it’s most likely a symptom caused by overwatering.
Check that the drainage holes in the pot are not blocked, and if necessary, gently remove the plant from the pot to examine the roots. If most of the roots look dark, mushy, dead, or missing altogether (due to rotting away) then the issue is definitely overwatering.
If an indoor plant looks like it’s in a really bad state, it may be worth trying to rescue it. Repotting the plant using fresh potting mix may help. If there is extensive root loss, then it’s best to repot into a smaller pot.
Leaf yellowing of the whole plant may also be due to insufficient light, too little fertilizer, or the plant may have become weakened by pest insect or mites.
When Only the Lower Leaves of a Houseplant Are Yellowing
The lower leaves of a plant are usually its oldest leaves, and when a plant is suffering from a nutrient deficiency due to lack of fertilising, these leaves will yellow as the plant mobilises the nutrient to reallocate them to new growth.
These symptoms are usually caused by a nitrogen or iron deficiency, and nitrogen is a key major nutrient that plants require for producing new leafy green growth.
Plants are more susceptible to nutrient deficiencies when they become pot-bound (root-bound), this is where the roots fill all available space, so the root system no longer has adequate space to grow.
Apply fertilizer once or twice a year, depending on the fertiliser application rate recommended on the label, and where necessary, perhaps every 2-3 years, repot the plant into a slightly larger pot using fresh potting mix to avoid this problem.
The lower leaves of houseplants may yellow and senesce if the plants are allowed to become too dry for an extended time due to lack of watering. When plants are not watered, roots dry out and die off, and the result is that there are insufficient roots to provide water to all the leaves on the plant. To deal with the imbalance, plants have to drop some leaves, and they sacrifice the oldest leaves first.
When plants are subjected to prolonged period of stress, leaves yellow and fall, but more significant symptoms also appear, they become stunted and exhibit poor growth. By noticing the pattern and progression of symptoms, we can diagnose the cause of the problem and remedy the situation quickly to return our houseplants to a state of good health.
More articles on Garden Pests, Diseases and Problems
References
- University of Maryland Extension, Yellowing Leaves on Indoor Plants, Updated: March 26, 2021 – https://extension.umd.edu/resource/yellowing-leaves-indoor-plants
- Clemson University Cooperative Extension Service, Houseplant Diseases & Disorders Factsheet, HGIC 2251, Updated: Sep 10, 2021 – https://hgic.clemson.edu/factsheet/houseplant-diseases-disorders/
- Iowa State University Extension Service, Yard and Garden: Diagnosing Houseplant Problems, February 21, 2022, Aaron J. Steil – https://www.extension.iastate.edu/news/yard-and-garden-diagnosing-houseplant-problems
- Pennsylvania State University Extension, Preventing, Diagnosing, and Correcting Common Houseplant Problems, https://extension.psu.edu/preventing-diagnosing-and-correcting-common-houseplant-problems
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