Why are my plant's leaves turning yellow, and what should I do about it?

Article for :Beginner and plant killerExperienced plant lover

Some people agonize over their first wrinkle, others dread their first gray hair. Plant addicts, on the other hand, live in fear of the yellowing leaf. Has your favorite Begonia rex caught the jaundice? We'll help you identify the causes and explain what to do about it.

Yellowing leaves are a normal phenomenon

Even in a healthy plant, leaves can turn yellow. Photo by Quinn Dombrowski /Flickr

Before breaking out the heavy artillery, make sure that leaf yellowing is not the result of the plant's natural aging.

In fact, even in evergreen species, it's normal for the leaf blades to turn yellow, dry out and fall off. This is part of plant development. Like flowers, leaves wither. Older leaves die and give way to new ones.

If your plant has a yellowing blade and then nothing for months, there's no need to be alarmed. But if your house is littered with dead leaves, you've got a problem.

Should I cut off yellowing leaves?

To prevent the spread of disease, clean your pruning shears before pruning.

When a yellow leaf appears, the first instinct is to remove it to keep the plant green and healthy. Once dry, the leaf blades are unlikely to regain their natural color. They are no longer of much use. You can cut them off with clean pruning shears or remove them by hand.

But if you remove them as you go along, you'll never know how many leaves are affected and how often. If you leave the discolored leaf blades on the plant for at least a few weeks, you'll gather valuable clues as to why the foliage is yellowing and how to remedy it.

You've over-watered

Over-watering is a question of frequency, not quantity. Photo by Teona Swift/ Pexels

Over-watering is the primary cause of yellowing and wilting leaves. Behind every discolored Monstera deliciosa often lies an overzealous owner. To be sure, stick a finger in the soil. If it's damp or worse, muddy, the culprit is obvious. And action is urgently needed to prevent the plant from dying.

Excessive watering lowers soil nitrogen and oxygen levels. Without them, roots can no longer breathe. Eventually, they rot. This manifests itself on the surface with yellowing, softening leaves.

If your plant is turning yellow because it's been watered too much and too often, simply leave it alone until the root ball dries out. Just drain off any stagnant water in the saucer or planter. But if the soil is very soaked, it's best to repot. Choose a perforated pot and place a drainage layer (clay balls, small pebbles) at the bottom. Remove the soggy substrate from the roots and repot in fresh potting soil. Water for the first time to help the roots take root.

Want to avoid a repeat? Check the moisture content of the substrate before watering. And unless otherwise indicated, always use a pot with a hole in it. The hole is essential to allow excess water to drain away.

Your plant lacks water

If your plant looks like this, you need to take action.

Leaves that turn from green to yellow can also be the result ofinsufficient watering. There are three clues to confirm the diagnosis:

you can't remember the last time you took out your watering can;
the soil is completely dry;
the pot is very light.

When a plant is thirsty, it can no longer feed itself properly. It wilts, turns yellow, dries out and eventually dies. Here again, don't wait to take action.

In most cases, watering from above is enough to bring the unfortunate plant back to life. Water with non-calcareous water at room temperature until the water runs off through the drainage holes. If the water starts dripping immediately, the substrate has become too dry to absorb the liquid. Water from below instead. Can't get your plant to drink, even by capillary action? Do the drastic thing and plunge the pot into a container filled with temperate water. When the air bubbles disappear, take the plant out and let it drain.

Abundant watering after a period of drought causes yellowing of the lower leaf blades in certain species, such as goeppertia rufibarba. Don't worry, this is a temporary phenomenon. Your plant will stop losing its foliage and generate new, green leaves when you resume regular watering.

Substrate too poor

Repotting is best done during the growing season

Watering is not the only culprit. Foliage sometimes turns yellow because the substrate lacks nutrients. In this case, the leaves turn pale and the plant slows down. It doesn't produce new shoots or flowers.

In the open ground, plants have access to rich, fertile soil that is constantly renewed. In pots, they exhaust the resources of their potting soil after a few months. When there isn 't enough iron or magnesium, the edges of the leaves lose their color. The yellow gradually spreads to the entire leaf blade, with only the veins remaining green.

There are two solutions to this problem. During the growth period, fertilize your plant with a fertilizer adapted to its needs (we explain here the type of fertilizer to choose). If your plant has been living in its pot for two or three years, give it brand-new soil by repotting or resurfacing.

But beware of overdosing! It causes browning and discoloration of the leaves. Here are four tips to avoid over-fertilization:

don't fertilize a plant in poor health;
respect the dose of fertilizer indicated by the manufacturer;
don't fertilize within a month of repotting;
stop fertilizing at the end of the growing season.

To ensure that your plant doesn't suffer from deficiency again, or from a disease such as chlorosis, fertilize it during its vegetative phase, generally between March and October. Repot your plant when it's too small. If it's too big to be potted, just top-dress it once a year.

Unsuitable exposure

Too much sun can harm your plant

If you're a pro at watering and haven't missed a single fertilization, the problem may lie in exposure. A plant that lacks light has leaves that turn yellow and discolor. It produces long, bare, etiolated stems. Wilted stems and yellowing leaves are symptoms of excessive light.

All plants need sunlight, but not all require the same amount. Check the amount of light required by consulting the grower's sheet on the Monstera application. Even if we recommend a sunny exposure, don't stick your pot behind the bay window in midsummer. Prefer a window screened by a curtain.

Was your Maranta leuconeura right under your velux window? Move it a few meters to half-shade. Is your Beaucarnea recurvata wasting away in a corner of your bookcase? Move it closer to your French window to stop its leaves turning yellow. You can even send it to spend the summer in your garden, where it can benefit from natural light.

Some species, like ficus benjamina, can't stand change. A move, even of a few meters, often causes a few leaves to fall off. But don't panic! Once your shrub is used to its new location, it will resume its usual growth.

Cold weather

In winter, the temperature next to a poorly insulated window will be lower than in the rest of the room. Photo by Woodley Wonderworks / Flickr

A cold snap can cause yellowing and leaf blade drop. Every plant has a hardiness temperature, a minimum temperature it can tolerate without flinching. Below this, it loses its leaves. Most of our houseplants are tropical varieties. They appreciate the temperature of our homes and hate to see the mercury drop below 15°.

To find out if your plant is hypothermic, check that the room temperature matches its requirements. If your Bird's nest fern is in a draught, move it. Find a location away from the cold and heat. Radiators tend to dry out the air and turn the leaves brown.

One last tip: when airing out in winter, don't leave your plant in the room. Thermal shock and draughts could be fatal.

Yellowing leaves due to a parasite

Leaf discoloration is also a sign of pest infestation. Photo by Scot Nelson / Flickr

Yellowing leaves aren't always your fault. Parasites and fungi can also wear their hats. Discoloration is then accompanied by other symptoms, such as deformed leaf blades or brown spots. To prove your innocence, scan your plant for suspicious spots, insects, larvae and webs.

Once you've caught the culprit, examine it carefully to decide what treatment to apply. When it comes to spider mites or whiteflies, you can get away with a little soapy water. But when your plant is infected by a fungus, the problem requires a more radical solution. Remove diseased leaves and air the room more regularly, to improve air circulation.

By Servane Nemetz
on 21-08-2023 at 07h10