Yellowing on leaves of pepper seedlings - Ask Extension
Hello, I started these pepper seedlings indoors almost 10 weeks ago, on a germination mat, using potting mix and seed starter on the top. The cultivar...
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Yellowing on leaves of pepper seedlings #924427
Asked February 10, 2026, 3:17 PM EST
Hello, I started these pepper seedlings indoors almost 10 weeks ago, on a germination mat, using potting mix and seed starter on the top. The cultivar is CA Wonder Gold bell pepper, though this also affected another Bell pepper variety called red impact. The leaves on the seedlings get yellow between the veins, usually on the older leaves. And as you can see by the size, these aren't exactly thriving. Other plants in the same 1020 tray and under the same LED full spectrum grow lamp on a timer are not showing these symptoms. Tomatoes, herbs, cucumber and kale plants are all fine, no yellowing. You can see there's a little bit of dry brown spot on a couple leaf margins. What could be causing this?
Charles County Maryland
Expert Response
Are the plants intended to be grown year-round indoors, or put outside after frost? We ask because 10 weeks ago was very early to have started seed for summer vegetables, as typically even the crops needing the earliest start get sown later this month.
Have the plants been fertilized, and if so, how? Chlorosis, or a yellowing of leaves while the veins stay greener, indicates a deficiency of either iron, manganese, magnesium, or perhaps zinc. A fertilizer formulation that contains micronutrients (all of those four nutrients, plus some others) should prevent that, unless the acidity level (pH) of the potting mix is notably outside of the pH range it should be. Occasionally, mixes using, for example, coconut coir, have measured quite acidic, or have enough salt contamination that plant roots become stressed or damaged. A pH tester (probe or paper strip kit) might reveal if the pH level is grossly outside of the ideal range, though such testers don't provide as fine a measurement as lab testing. (In this case, lab testing is probably not practical or worth the expense. Standard soil testing is not useful for soilless media like potting mix, so it would need to be a different lab test.)
If the potting mix isn't nutrient-deficient and the pH is okay, the culprit tends to be related to root health. They may be getting over-watered or under-watered, as both circumstances can lead to poor root function or dieback. If you happen to have any fungus gnat issues, over-watering would be more suspect. Drying leaf margins could have a range of causes, including over- or under-watering, root damage or dieback, and insufficient ambient humidity.
Is the potting mix a commercially-sold potting mix, or does it contain some composted manure? The latter sometimes contains still-active herbicide residues (they can survive the animal's digestion) that can affect the roots of seedlings grown in that medium. Usually, we see different symptoms in those cases, like foliage distortion and stunting, but it's a factor that could be ruled-out if you didn't add composted manure to the mix.
Over-fertilization can also stress or damage roots, so depending on what your fertilization regime has been, reducing the dose or frequency might be helpful. Sometimes, excesses of some nutrients (collectively called "salts") interfere with the roots' ability to absorb other nutrients, or they "burn" sensitive root tips, causing them to die back. Since a compromised root system isn't functioning well, that's why foliage can develop deficiencies.
Miri
Have the plants been fertilized, and if so, how? Chlorosis, or a yellowing of leaves while the veins stay greener, indicates a deficiency of either iron, manganese, magnesium, or perhaps zinc. A fertilizer formulation that contains micronutrients (all of those four nutrients, plus some others) should prevent that, unless the acidity level (pH) of the potting mix is notably outside of the pH range it should be. Occasionally, mixes using, for example, coconut coir, have measured quite acidic, or have enough salt contamination that plant roots become stressed or damaged. A pH tester (probe or paper strip kit) might reveal if the pH level is grossly outside of the ideal range, though such testers don't provide as fine a measurement as lab testing. (In this case, lab testing is probably not practical or worth the expense. Standard soil testing is not useful for soilless media like potting mix, so it would need to be a different lab test.)
If the potting mix isn't nutrient-deficient and the pH is okay, the culprit tends to be related to root health. They may be getting over-watered or under-watered, as both circumstances can lead to poor root function or dieback. If you happen to have any fungus gnat issues, over-watering would be more suspect. Drying leaf margins could have a range of causes, including over- or under-watering, root damage or dieback, and insufficient ambient humidity.
Is the potting mix a commercially-sold potting mix, or does it contain some composted manure? The latter sometimes contains still-active herbicide residues (they can survive the animal's digestion) that can affect the roots of seedlings grown in that medium. Usually, we see different symptoms in those cases, like foliage distortion and stunting, but it's a factor that could be ruled-out if you didn't add composted manure to the mix.
Over-fertilization can also stress or damage roots, so depending on what your fertilization regime has been, reducing the dose or frequency might be helpful. Sometimes, excesses of some nutrients (collectively called "salts") interfere with the roots' ability to absorb other nutrients, or they "burn" sensitive root tips, causing them to die back. Since a compromised root system isn't functioning well, that's why foliage can develop deficiencies.
Miri
