Seedling leaves turn yellow for a host of different reasons. But from my experience, the three main culprits are overwatering, insufficient lighting, and using too much fertilizer or nutrition in the seedlings’ early stages of development.
But first, you must ensure you have a problem that needs fixing.
When yellow leaves are part of the natural growth cycle
When starting seeds indoors, you patiently wait for those first leaves, or cotyledons, to appear. These first leaves tend to be generic looking and hold none of the plant’s true characteristics.
Check out our article Photos of cotyledons from our plant garden to see photos of first or seed leaves from some of out plants. Please let us know if you could identify more than 10.
You start seeds in a lean potting soil mix. The seeds hold all the energy needed to germinate and sprout those first leaves. And these first leaves play an essential role in the seedling’s early stage of development.
But the first leaves work is done when the seedling develops its first true leaves. And it is at this stage the first leaves will yellow and wither. And this is normal and to be expected.
Let these first leaves fall off, or cut them close to the stem when they show signs of yellowing. This is no cause for alarm.
When yellow leaves are cause for concern
You should worry when mature leaves or, even worse, the first true leaves show signs of yellowing. Let’s start by looking at 3 reasons seedling leaves turn yellow and why.
1. Yellow leaves and overwatering
From my experience, overwatering is the main reason seedlings develop yellow leaves. We want our young plants to do well, and it is easy to forget that healthy growth requires enough water – but not too much water.
If you are a regular reader, you know we always promote the general rule of “moist but never wet soil” and that “an occasional mild dry out is better than constant wet soil”.
Tomato leaves can also turn white from overwatering. Check our the tools we use and recommend for proper watering and read more in this article: Tomato leaves turning white (explanation and fixes)
It can be tricky to water by touch by inserting a finger into smaller-sized starter pots. And for new gardeners just looking at the surface does not tell the whole story, especially with deeper starter pots.
3 steps to “watering by weight”
I find “watering by weight” to be extremely useful. It takes a little while to get “the feeling,” but when you have it, it will be so much easier to give enough water without risking giving too much or too little water.
- You need to use one and the same type of pots and potting soil mixes.
- Fill 1 pot with dry potting soil mix and the other 2 pots with moist and soaking wet soil respectively.
- Now feel the weight of the pot with dry soil compared to the pots with moist and soaking wet soil. Memorize the feeling and use it to guide you when you water.
And remember, you are looking for moisture not wet.
2. Yellowing leaves and insufficient lighting
All plants need light as the process of photosynthesis converts light energy into the energy required for plant growth and development.
When plants grow in your garden, you rely on the sun to provide enough light during the growing season. But when you start seeds indoors, you need to use a grow light to help your seedlings develop into healthy plants.
And it is not as simple as more light is always better. You need to find a balance.
Not enough light
If you do not provide enough light, you will end up with tall, spindly, and leggy seedlings, and leaves will turn light green to yellow.
Grow lights do not have to be expensive and can make all the difference. Check out our recommended grow light alterantives to fit all budgets and ambition levels. Learn how to fix seedlings that grow tall and leggy in our article How to fix leggy seedlings
Too much light
Plants need their rest. Overexposure to light can stress your plant and lead to leaf curl and even yellow leaves.
When you use a grow light, I recommend 14-16 hours of artificial light followed by at least 8 hours of darkness and rest.
3. Yellow leaves due to the use of fertilizer
When leaves turn yellow, it is easy to suspect nutrient deficiency and to automatically think there is a need to add nutrients and fertilize.
But it does depend on whether you are dealing with younger seedlings or more mature and even older plants.
But based on experience, there is no need to give seedlings nutrients for the first 3-4 weeks. Between the energy stored in the seed and the nutrients in your potting soil mix, there should be enough nutrients for your seedlings to grow and develop.
When the seedling matures and your seedling starts to resemble a young plant, yellow leaves could however be a sign of nutrient deficiencies. Always start by adding nutrients in low dosages and work your way up to the manufacturer’s recommended dosage.
When plant leaves turn yellow on mature plants, it could be a sign of nitrogen deficiency. But before you add a high-nitrogen fertilizer, you should ask yourself the following question:
Is it mature or young leaves that are turning yellow?
If it is just happening to older leaves, it could be a part of the plant’s life cycle, where young leaves replace older leaves.
Is it happening to many plant leaves or just a few?
If just a few plant leaves turn yellow, it is not a cause for concern.
Are the yellow leaves growing at the top or bottom of the plant? If plant leaves growing at the base of the plant turn yellow, it is more likely to be natural.
You should ask yourself these questions because too much nitrogen can cause damage to the root system or what is often called “fertilizer burn”.
And this is a vicious cycle. You add more nitrogen to cure yellow leaves and end up causing fertilizer burn that can cause brown leaves, curling of leaves, wilting, and yellowing leaves.
More reasons leaves are turning yellow
Most plants do well in a slightly acidic to neutral soil pH. And if the soil pH is too acidic or alkaline, your plants would suffer, and you could see yellowing leaves.
But if you are using a good-quality potting soil mix, your soil pH should be in the correct range.
A pH soil meter can be a way to measure or approximate the pH level of your soil.
For reference soil pH 7 is referred to as neutral whereas soil pH below 7 is referred to as acidic and a soil pH above 7 is called alkaline.
The wrong type of soil can hurt plant roots
Using potting soil that is too compact can hurt the growth of plant roots and lead to a lack of oxygen, compacted roots, and root rot.
Compact soil retains too much water, and the resulting wet soil will result in root damage and plants with stunted growth.
3 ways to deal with yellowing leaves
Yellow leaves are not necessarily a problem that needs to be addressed. But, when you see leaves turning yellow, it should signal that you need to pay attention as something could be wrong.
If your first true leaves turn yellow on your seedling you need to take action. Check your watering habits and consider exposing the seedlings to more light (12 hours per day) for the plants to recover.
1. Remove the yellow leaves
You should cut or pinch your plants when you see yellow leaves on them. There is no reason for your plant to spend its energy on supporting a yellow leaf that no longer serves a function.
Instead, cut the yellow leaf and let your plant use its energy for new healthy leaves.
2. Examine your watering habits
I may sound like a broken record, but incorrect watering habits will cause yellowing leaves, and I see it happen all the time.
If you find watering by touch (inserting a finger in soil) or by visual inspection difficult, I recommend looking into “watering by weight”. Look above for simple step-by-step instructions.
Also, remember that a good-quality potting soil mix will give you better drainage and minimize the risk of soggy or soaking wet soil.
3. Add nutrients with care
Wait 3-4 weeks before you consider giving your seedlings nutrients. Young seedlings will typically not need any external boost during these first weeks of development.
Check for white deposits around around drain holes as well as the stems if you suspect giving too much fertilizer. The cure is to flush the soil and root system with water and allow the excess to drain.
More mature seedlings and plants can, however, suffer from nutrient deficiencies. But as it is difficult to know for sure, I recommend starting by adding nutrients in low dosages and then observing reactions. Remember, you always add nutrients, but it is challenging to take them away when given.
