Plants are supposed to be green so when plant leaves turn yellow, something’s going wrong in the growing conditions.
It’s a frustration even the most experienced gardeners and botanical experts can be left exasperated trying to understand the cause of so they can rectify things and restore a plant to full greenery and blossoming.
When plants turn yellow, it’s the plant telling you to change something in the growing conditions.
Understanding the Reason for Yellowing Plant Leaves
For a plant to grow healthy, it needs optimum growing conditions, including the right amount of water, air, nutrients, light and heat for photosynthesis to happen.
If any of those are on either an oversupply or an undersupply, that’s when plant leaves turn yellow.
Restoring the green foliage is done by inspecting the growing conditions, identifying the cause(s) and altering the conditions, so don’t reach for any fertilizer just yet until you know that the basics are covered.
Causes of Chlorosis
Chlorosis is the term used to describe a plant with yellowing leaves. It’s a condition caused by a lack of chlorophyll, which is needed for photosynthesis.
Chlorophyll is the pigment in plants that give them the green color by absorbing light wavelengths of the blue and red spectrum mostly, although it still does need yellow and green light – just in lower amounts.
The chloroplasts on plant leaves trap the energy from light sources, which is then used for photosynthesis. The intensity of light reaching the plant also affects its shade, which is why there are different shades of green on the leaves of plants.
A lack of sunlight can cause chlorosis, but so too can many other factors.
The Most Common Reasons for Plant Leaves Turning Yellow
Stress from Moisture
Plant leaves can turn yellow because of either too much or too little water. In either case, it’s going to cause moisture stress on the plant. When the plant’s stressed, the yellowing on the leaves is often the first tell-tale sign.
While it’s difficult to pinpoint watering accuracy as it’s mostly a case of trial and error when you introduce a new plant until you get familiar with it’s watering frequency, the soil test is a good start to see if your plant is under or overwatered.
By dipping your finger into the soil, you’ll get a feel for how much moisture is there. If it’s dry, it’s underwatered, in which case, quench its thirst.
If, on the other hand, the soil is wet and clumping together, it’s going to cause a lack of oxygen, starving the plant roots, which will eventually lead to root rot and your plant dying.
Too much water in the soil isn’t always because you’ve added too much. It could be the soil quality that has insufficient drainage.
If your soil is thick, adding course grit can increase drainage.
Tropical indoor plants don’t do well with drafts and generally start showing yellow leaves when temperatures drop.
Check around your plant for any drafts that could be affecting it and if needed, move it to somewhere with a steady temperature.
Avoid placing them near air conditioning units, fans, or anything else that causes temperature fluctuations.
Lack of Light
A Lack of sunlight is a common cause of yellowing leaves on houseplants. In particular, if you have them placed away from a sunny window, such as in the hallway or the top of a staircase with not much natural light.
Unfortunately, houseplants can’t be placed anywhere. They need sufficient sunlight every day to maintain growth and color.
Artificial grow lights are an option for certain types of indoor plants, but for ornamental plants that you don’t want to have specialist lamps ruining their aesthetics, the only options are to increase the plant’s exposure to natural sunlight or to give it increased bright and indirect light.
When plants are lacking nutrients, it can cause the leaves to yellow. The main nutrient deficiencies in plants with leaves turning yellow include:
When any, all, or a combination of the above nutrients are lacking, plant leaves will turn yellow. The solution to any of the above deficiencies is to fertilize your plant with a 10,10,10 balanced fertilizer to feed the plant the main three (macronutrients), which are the first three listed above – Nitrogen, Phosphorous and Potassium.
The other secondary and micronutrients such as iron, and magnesium are only needed in trace amounts and can be added using organic mulch in your soil.
Usually, when plant leaves turn yellow because of a nutrient deficiency, it’ll be lacking the macronutrients, which is what a balanced fertilizer will fix.
Plants can suffer from any number of pest problems. However, if the only change you’re seeing is the leaves turning yellow, check it for aphids.
There will be more than one and the plant will need to be treated as these reproduce at rapid speed and will transfer onto other plants around them.
Aphids suck the sap from the leaves of plants, causing the yellowing of leaves, but even treating the plant by spraying it with neem oil or another alternative method to get rid of aphids, there’s still the aftermath to contend with too.
Sooty mold is a black residue that’s left behind after treating an aphid problem. The thick coating over the leaves of the plant blocks sunlight so if you do need to treat a plant for aphids, neem oil is a good first treatment method to get rid of the garden pests, then wash the leaves again to remove any residue that’s left behind.
Other pests that can wreak havoc on your plants, causing leaf discoloration are red spider mites and whiteflies. If you need to treat your plant for a pest problem, see this article about how to get rid of bugs on plants.
Chlorosis Viruses Caused by Whitefly
A chlorosis virus can affect various plants but are more prone to cause problems on tomato plants, watermelon, and cucumber plants. The virus is transmitted by whiteflies, which is the most atrocious and destructive insect to various plants as its responsible for transmitting as much as 114 viruses.
Whiteflies are similar to aphids in that they suck the sap from the leaves of plants. In the early stages of a whitefly presence, you’ll see spotty flecks of yellowing, but you need to look closer at the leaf to spot whiteflies as they are tiny.
In addition to being so sneakily small, making them hard to detect, they’ll lay hundreds of eggs on the underside of leaves. Oftentimes, by the time you’ve noticed the pest problem, the numbers have increased significantly.
Here’s a closeup video on how whiteflies multiply and destroy plants.
As you’ll see in the video, it’s not only mature the whitefly that’ll latch onto plant leaves to suck the sap from them, but as each of the hundreds of eggs they lay hatch, the nymph insects lay immobile on the leaves, feeding from the sap too.
Left untreated, whitefly will cause direct and indirect damage. The direct damage happens from sucking the sap from the leaves.
Indirect damage happens later as a result of adult whiteflies excreting honeydew, which covers the leaves of plants and eventually turns into sooty mold.
Although the mold is easily washed off with a hose or spray, the main problem is that whiteflies will transfer diseases by traveling between plants.
These insects fly in swarms until they find a host plant to feed on and lay eggs. They tend to be attracted to warmer growing conditions such as greenhouse growing, which a lot of gardeners will use for growing tomatoes and other vegetative crops.
It’s not necessarily that tomato plants are more prone to whitefly infestations, but more about the environmental conditions being more attractive to insects. They’ll latch onto any plant they can feed from.
Not all yellowing is a sign of a catastrophe. It could just be natural that some of your plant’s leaves turn yellow. Aged leaves will yellow as more energy is diverted from the plant to new healthy leaf growth.
The older plant leaves that turn yellow just wither away, eventually dropping off, or being pruned to help channel the energy into new growth.
Take a look at all the leaves on your plant. If the majority, including new foliage are showing signs of yellowing, it’s likely one or more of the above problems are present.
If the yellowing is isolated to older and larger leaves without the new growth being affected, chances are it just needs a little tidying by pruning back older leaves, or just left until the discolored leaves fall off naturally.
When plant leaves turn yellow, don’t assume it is age. It could be a plant-saving sign to inspect your plant, the growing conditions, test the soil for water drainage, and possibly increase fertilizing the plant to help it get the nutrients it needs for photosynthesis.
Run through the five basic requirements of plants (water, air, nutrients, light and temperature) and make sure it has enough of each.
Increasing the green color on the plant’s foliage could be as simple as increasing sunlight exposure, altering the watering frequency, aerating the soil or adding a fertilizer. Or, it could be something more sinister like aphids or whitefly that are causing a nutrient deficiency in the plant.
When plant leaves turn yellow, it’s a plant’s signal that something isn’t right with the growing conditions. Healthy plants grow green. Yellowing plants are showing symptoms of a problem that should be addressed sooner rather than later.