It’s always disappointing when a cherished houseplant suddenly looks sickly. It always seems to be the more expensive specimens that succumb to plant collapse, too. You won’t find much pleasure in growing indoor plants unless you learn how to avoid plant issues such as plant leaves curling, wilting, or dropping off.
You might be surprised to learn that specific pests and diseases aren’t usually to blame. In most cases, plant collapse is caused by too much or too little of one or more of the essential growth factors.
Diagnosing Plant Collapse
When a beloved houseplant goes from health to the brink of death in a breathtakingly short time, act fast. However, you should also make careful observations before taking action. You don’t want to make it worse by misdiagnosing the symptoms.
If you’re wondering, “Why is my prayer plant curling?” your first step is matching the conditions of a Brazilian tropical rainforest. Knowing the origins and natural habitat of your plant helps identify why its leaves are curling, browning, or dropping off:
The second step in rectifying plant collapse is to look at the symptoms. Has there been a change in the leaf color? Are the leaves of your houseplant curling? Do they fall off after curling?
Why Do Plants Collapse?
Common symptoms for improper care include curling leaves, crispy brown leaf edges, yellowing leaves, wilting, or leaf drop.
The reason varies according to the type of plant, its native habitat, and its growing conditions. Let’s look at a few of the most common causes of houseplant demise.
Overwatering
The single most common cause of houseplant death is overwatering. Along with adequate moisture and nutrition, plant roots need oxygen.
Excess moisture damages plant roots. The fine root hairs rot, which means the plant is drowning in rather than drinking in water.
Excess moisture also results in fungal infections, which is often the last straw that kills your favorite houseplant.
It’s easy to overwater houseplants. When the plant can’t take up water without fine roots, the leaves droop. So, you water it even more, which makes the situation worse. 
Symptoms of overwatering
Many overwatered plants have yellow leaves that fall off. Other plants might form brown patches on the leaves. Look for mushy stems or a foul smell to the soil.
Always check the soil before watering. A soil moisture meter (or hygrometer) is easy to use and inexpensive.
Dry Soil
Many plants cope with infrequent watering in winter and, in fact, when they’re dormant, you should reduce watering.
However, if they don’t get enough water during the growing season (spring and summer), the plant will look very sorry for itself.
If your houseplant has curly leaves or they’re dropping off, it may need more water. Eventually, your plant will become too stressed to survive.
Signs of underwatering
Symptoms of underwatering in houseplants include leaves that curl upward. Other symptoms to look for are yellowing leaves that turn brown and fall off.
While overwatering turns leaves yellow, when the problem is a lack of water, the leaves feel dry and brittle.
Dry soil also feels compacted, which makes it hard for water and oxygen to reach the roots. Many potting mediums become hydrophobic when they become too dry. Ironically, the more dried out they become, the less able they are to absorb moisture.
If you have plants with high moisture requirements, such as tropical plants, consider repotting them into planters with self-watering reservoirs.
Low Humidity
Plants require air humidity similar to that in their native habitats, and hot dry air stresses many houseplants. During winter, central heating reduces humidity in the air and stresses delicate houseplants.
Like humans, plants “sweat.” Except in the case of plants, it’s called transpiration. They produce water on the underside of their leaves as a normal part of their metabolism.
While they replace lost moisture with adequate watering, adequate humidity helps retain the correct balance of moisture in their leaves.
Symptoms of low humidity
There really isn’t any universal setting for perfect plant humidity. However, symptoms include leaf curl. The leaves may also turn brown and brittle around the edges. Some plants will drop new leaves as well as old ones.
One sure sign of inadequate humidity is spider mites. For some reason, they’re attracted to dry, suffering specimens with plant leaves curling.
To improve humidity, group several plants together to create a microclimate with higher humidity. Also, try adding a humidifier to the room.
Cold Stress
A less common cause of houseplant collapse is cold stress. If you keep plants too close to doors and windows, they can become stressed during an overnight freeze.
This is especially true if the plants are normally kept under warm conditions during the day; it’s the sudden fluctuation in temperature rather than cold air which usually causes the damage.
Frost causes leaf damage at a minimum. So, plants kept on windowsills are the ones most likely to suffer. Never leave pots between the windows and drawn curtains on a cold night. If you expect frost and the room is unheated, then move the plants away from the window.
Most houseplants are tropical in origin, specifically from jungle understories. Many of these will look stressed at temperatures as high as 50 degrees Fahrenheit.
Signs of cold shock
When temperatures drop, the water inside the plant freezes. This is normally what kills off most plants in the winter. The frozen water inside plant cells freezes, which damages the leaves. This leads to droop and wilting.
Afterward, damaged leaves will turn color. Black is pretty common, but some turn red, yellow, or white. Eventually, the damaged leaves will drop off.
Remove the plant from the cold immediately and provide water if necessary. Remove damaged leaves with sanitized snips. Hopefully, there will be enough left for photosynthesis.
With warmth, light, luck, and a little love, your plant may grow back.
Make sure to move all plants away from doors and uncovered windows when the weather forecast predicts a sudden freeze.
If you have too many plants to move, or they’re simply too large to maneuver indoors, cover them. Frost blankets offer the best protection, but even an old sheet or blanket is better than nothing. 
Drafts
When a door and window are opened in a room, a cross-current of air occurs. Drafts can be a significant factor in plant problems, such as leaf curl or leaf drop.
Another spot subject to drafts is the windowsills where there are cracks in the window frame. If you decided to grow delicate plants on a windowsill, seal up the cracks. A silicone sealant is usually quick and effective.
Symptoms of draft damage
It’s difficult to tell exactly which symptoms are caused by drafts. That’s because the moving air could be too cold, too hot, too dry, or too moist.
For this reason, avoid placing plants in a direct line between doors and windows.
Harsh Sunlight
Some houseplants quickly succumb in direct sunlight, even if the air temperature is not unusually high. Many flowering plants, however, such as Pelargonium, thrive in a sunny window. But harsh sunlight burns even sun-loving varieties.
Strong sunlight cause leaf scalding and even leaf curl. Most houseplants are found naturally on the shaded floors of forests, where they thrive in low light. These plants are very susceptible to sunburn.
Signs of sun scald
Symptoms to look for are blanched leaves. They present first as white or yellow spots. Eventually, they become brown and brittle. Leaf drop is sure to occur next.
Along with sunburn, strong sunlight also bakes the potting soil. This damages roots and beneficial soil microbes.
The best course of action is to move the plant immediately. Trim off damaged leaves if you prefer. They will not turn fresh and green again after being burned.
Lack of Light
Poor light conditions in an average room usually don’t kill off plants. More often, the result is generally pale, weak growth, and no flowers.
There is a level, however, at which the amount of light is not sufficient to support houseplant life, and this occurs in dark hallways or the corners of large rooms
Symptoms of inadequate light
Inadequate light for houseplants is one of the easiest plant problems to identify. Plants need sunlight to produce chlorophyll, which is the substance that turns your plant green. Without enough light, your plant will be pale and even white looking.
Plants also turn and stretch toward a source of light. If nearby light is inadequate, they will stretch out with thin, weak stems and very few leaves. Low light results in leggy plants.
You’ll find the leaves are spaced out far along the stems. The plant is concentrating its energy on finding an adequate source of light before it bothers to produce leaves.
Variegated houseplants often lose their interesting coloring without enough light.
Luckily, this is also one of the common houseplant problems that are easy to remedy. The quickest solution is to move the plant closer to a window.
If that’s not an option, then consider adding grow lights. You don’t need expensive, industrial light fixtures. You’ll find grow bulbs that fit into standard lamps and provide the right kind of light for verdant plant growth.
Conclusion
There are many reasons for plant leaves curling, turning colors, wilting, or dropping off. Unfortunately, there is no one single reason. These symptoms are a response to some kind of stress. It’s up to you to investigate and use deductive reasoning to identify the cause of that stress. The good news is that you can catch it before it leads to complete plant collapse.
Editor’s Note: This post was originally published in November 2012 and has been completely updated.