Yellow Seedling Leaves – Why Are My Seedlings Turning Yellow
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By: Liz Baessler
Have you started seedlings indoors that began healthy and green, but all of a sudden your seedling leaves turned yellow when you weren’t looking? It’s a common occurrence, and it may or may not be a problem. Keep reading to learn more about yellowing seedling plants and how to treat them.
Yellow Seedling Leaves
The first thing to establish is which of your seedling leaves turned yellow. When seedlings emerge from the soil, they put forth two starter leaves called cotyledons. After the plant becomes more established, it will begin producing differently shaped leaves that are characteristic of its species.
The cotyledons are designed to get the plant started in the very beginning of its life, and once it’s producing more leaves, these aren’t really needed anymore and will often yellow and eventually fall off. If these are your only yellow seedling leaves, your plants are perfectly healthy.
Why are My Seedlings Turning Yellow?
If it’s the larger, more mature leaves that are turning yellow, you do have a problem, and it could be caused by any number of things.
Are you giving your seedlings the right amount and intensity of light? You don’t need to buy a fancy grow light for healthy seedlings, but the bulb you do use should be trained about as close as possible directly over your plants and attached to a timer that keeps it on for at least 12 hours per day. Make sure you do give your plants a period of darkness too, of at least eight hours.
Just as too much or not enough light can cause yellowing seedling plants, too much or too little water or fertilizer could also be the problem. If the soil around your plants has been completely dried out between waterings, your seedlings are probably just thirsty. Overwatering, however, is a very common cause of sickly plants. Let the soil begin to dry a bit between waterings. If you’re watering every day, you may very well be doing too much.
If water and light don’t seem to be the problem, you should think about fertilizer. Seedlings don’t necessarily need fertilizer so early in their lives, so if you’ve been applying it regularly, that may be the problem. Minerals from fertilizer can build up very quickly in seedlings’ small containers, effectively strangling the plants. If you’ve applied a lot of fertilizer and can see white deposits around the drainage holes, flush the plant gradually with water and don’t apply any more fertilizer. If you haven’t applied any and your plant is yellowing, try a single application to see if it perks up.
If all else fails, plant your seedlings in your garden. New soil and steady sunlight might be just what they need.
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Overwatering or underwatering are the most common culprits when a plant's leaves turn yellow. With potted plants, it is crucial that you only water as much as the plant needs.
If you have a plant with yellow leaves, check the soil in the pot. Is it dry? Is it soaked?
If plants don't receive enough water, they drop leaves to prevent transpiration (essentially, a plant's way of sweating) to conserve water. Before they drop, though, the leaves will typically turn yellow. If the soil is dry and this is happening, make it a point to get the plant on a regular watering schedule.
Too much water can be just as damaging to leaves. When the soil doesn't drain well, an overdose of water leaves the soil waterlogged and root systems can literally drown. Without oxygen, roots start to die.
Improper Watering Habits
When you over-water your cannabis plants you prevent the roots from getting oxygen. This will prevent your plants from metabolizing nutrients and cause yellow leaves.
The leaves of an over watered plant will look droopy because they are full of water.
Under watering your cannabis plants will cause the leaves to wilt and turn yellow over time.
The solution is simple: adjust your watering habits. If you see over-watering symptoms, water your plants less frequently and vice-versa.
The rule of thumb is to water your cannabis plants once the top inch of soil is dry. Cannabis plants like soil that dries out a little between watering. Finding the right balance will depend on the size of the plant, the size of the planter, and temperature.
“Water your cannabis plants once the top inch of soil is dry.”
Both over and under-watering cannabis plants can lead to chlorosis. Unfortunately, water is a tricky thing to get right. You need to water your crop the perfect amount – and at the right times.
Too much water can reduce the oxygen content of the soil, which starves the root system and leads to death. You will know if you have over-watered your plants because their leaves will be swollen, droopy, and yes – maybe even yellow.
Under-watering is less common, since most cultivators are hyper-aware that their crop will die without proper hydration. However, under-watering does happen, and under-watered plants may appear weak, thin, brittle, and yellow.
The solution here is simple: stop watering plants that are over-watered, and water under-watered crops more! Get to know your plants well, and get to know the environment they thrive in. Get a feel for the weight of the growing medium when it’s dry, as well as when it’s saturated. You can even invest in a humidity meter to view the water content of your soil more accurately.
(By the way, poor soil quality/poor grow medium can also lead to watering issues. Make sure you invest in soil and containers that provide adequate drainage).
A common cause of yellowing on any houseplants is caused by either over-watering or under-watering. The same is true of the Calathea, however, more important is the type of water you use.
Rainwater is purer than tap water so if you can harvest some rainwater, do that. If not, distilled, or at least leaving your water out for 24 hours for some of the chemicals to evaporate will do the world of good.
The problem with tap water is that it’s been treated to make it fit for human consumption. Salt and other minerals such as fluoride are added at water treatment plants.
Excess chemicals from treated water accumulate in the soil making it harder for the roots to absorb the water. When that happens, leaves start yellowing.
Water your Calathea with tepid water and until you see water pour out the drainage holes. An all-too-common watering mistake is to top it up, which will mean the soil isn’t being flushed of excess minerals.
2.Low Light Conditions
As with all plants, they grow best when their natural habitat is replicated. For the Calathea, that’s the jungle floor so the preferred lighting is dappled sunlight – bright and indirect.
Bright and direct light can cause the leaves to discolor.
Grown indoors, the best place for Calathea plants is up to 2-meters away from an east-facing window.
There, it will get a few hours of daily sunlight that isn’t as warm as a west-facing window therefore, there’s less risk of it overheating.
Calathea plants are finicky about the temperatures they grow in. Preferable is to maintain a consistent temperature that’s between 62ºF and 81ºF (16ºC to 27 °C).
When temperatures sway away from their preferred range, the plant will be stressed and it can cause the leaves to yellow.
When temperatures are too high, the leaves on Calathea plants start to curl before discoloring. Too low a temperature is worse as that will stunt growth.
4.Insufficient Humidity Levels
In their native environment, Calathea plants are used to humidity levels in the 90% range. Indoors, that’s near impossible to replicate.
They can tolerate humidity levels between 50% and 70%, but never lower than 50%. When the humidity drops under 50%, that’s when you’ll start to see the leaves on Calathea plants turn yellow.
A room humidifier is beneficial for growing tropical plants indoors, but if you struggle to maintain a relative humidity level of 50%, there are some things that can be done to increase local humidity around the plants.
Grouping similar plants together is ideal because it creates a micro-growing climate. Humidity is increased through plant transpiration – the water that evaporates through plant leaves. The more plants you have, the more localized humidity rises.
For maximum effect, place a bowl of water in the middle of the collection of plants.
You can either buy a humidity tray to sit your plant pots on, or make one yourself by lining a tray with gravel or pebbles.
Pebbles and gravel hold water and release it slowly, helping to increase humidity. By resting your plant pots on top of a tray lined with pebbles, they benefit from increased humidity while being protected from the plant pot standing in sitting water.
Misting will slightly increase humidity but given the high humidity Calathea plants need, it won’t be practical. You’d be constantly misting the leaves daily, possibly multiple times each day.
Also, consider how you heat your home affects humidity. Heating systems produce dry air so if you’re noticing your Calathea plants leaves yellowing in the winter, it might be a good idea to move it to a room with higher humidity such as the kitchen or the bathroom.
The only thing to remember is that the leaves could still discolor due to having to acclimatize to new growing conditions.
As Calathea plants are grown indoors for their lush foliage, they don’t need a heavy feeding of fertilizer . The thing with fertilizers is that they change the soil pH.
Calathea plants prefer slightly acidic soil of 6.5. Too much can increase the pH and that’s the most likely reason for yellowing leaves. Too much fertilizer, rather than not using enough, or at all.
If you are feeding your plant with fertilizer, it’s beneficial to flush the soil every few months to prevent minerals from accumulating in the soil.
Too much mineral accumulation in the soil can lead to compaction, and that deprives the roots of oxygen.
The only time you really need to use a fertilizer with Calathea plants is if you’re watering with distilled water.
- Rainwater will have more beneficial nutrients.
- Tap water will have detrimental additives.
- Distilled water has nothing the plant can use.
If you using fertilizer, they do best with a liquid fertilizer applied every other watering or monthly during the growing season. Be careful not to overdo it though as too much fertilizer can cause the leaves of Calathea plants to turn yellow.
Of all the usual indoor plant pests, spider mites are the most likely to infest Calathea plants.
Others are mealybugs, fungus gnats, aphids, and scale insects, but the spider mite is the most attracted to tropical plants and they love the Calathea.
The problem is they are only about 1mm in size and they live in colonies. By the time you see them, you’ll have an infestation on your hands.
They look like tiny white dots and you’ll see them on the underside of the leaves of your Calathea. The other thing you’ll notice is a fine webbing coating the leaves.
The yellowing on the leaves of Calathea is a result of mites sucking the sap out of the leaves. It’s the equivalent of drought because the insects steal the nutrients the plant needs to survive.
There a number of ways to kill spider mites , however, it needs to be done with care to avoid damaging the leaves. If you’re something strong like rubbing alcohol, it’s safer to dilute it.
Preventing Yellow Leaves on a Houseplant
- The most common scenario is just forgetting to water the plant. If that is your problem, then you may want to make yourself a watering schedule (I suggest weekly) to check the plant for moisture and water accordingly.
- You may need a saucer to hold a little excess water to get the plant through the week, especially if it is in an area with high light or heavy air flow.
- Beware of the danger of over-correction—that is, turning from an under-waterer to an over-waterer. There is a balance to be met in plant care as with many things in life, there really is some skill involved in caring for a houseplant.
- If all else fails and you have the funds, there are watering services available. Consider hiring someone to take care of your plants for you.
Remember this: Some yellowing of leaves is normal, especially if you have a new plant acclimating to its new, lower light environment. If you see yellow, it does not necessarily mean your plant is doomed. It merely means that the plant is sending out some signals that something has happened or changed. Let the signals teach you.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
Last but not least, your plants could have an infection or disease that is causing the yellowing. Typically, diseases are more likely to cause yellow spots and patches on the leaves rather than the general yellowing of an entire leaf.
Here are three common diseases and infections that orchids encounter.
If you encounter any diseases, chances are it’ll be root rot because it is most common.
Root rot is a fungal infection of the roots, which typically happens if you overwater, use a pot without drainage holes, or poorly draining medium.
The primary issue with root rot is that it will take over fast and kill your plant quickly. So, if you notice yellowing leaves, check the roots.
You'll know your plant has root rot if the roots are brown or black, soft, and fragile.
If the plant has some healthy roots, it's possible to save the plant, but you need to use sharp, sterile scissors to remove all rotten roots.
Fungal Leaf Spot
This infection is known for causing yellow areas that start on the bottom and underside of the leaves.
When left untreated, fungal leaf spot will cause the spots to become larger, turning to brown or black.
For mild infections, you can spray or wipe the leaves with a fungicide. It’s typically advised to remove all infected leaves and then treat the healthy leaves.
Bacterial Brown Spot
If you notice wet-looking yellow or brown spots on the leaves, chances are you have the bacterial brown spot.
If you have your orchid in a hot and humid area, this is more common. As it gets worse, it leads to the generalized yellowing of the leaves, which is a sign of the stress the plant is under.
The best treatment plan is to remove all infected parts of the leaves or the entire leaves. Always use sterile scissors!
After removal, you can try a broad spectrum of bacterial spray or fungicide to prevent the fungus spores from infecting more of the plant.