When things go wrong: reading a sick plant
Your plant looks sick. How do you find out why?
This plant needs help. Where do we look first?
When you feel sick, a doctor does not guess right away.
First they look for clues. They check your throat and your temperature.
You can be a plant doctor too: clues first, guess second.
A plant can’t tell you what hurts. Its leaves, stem, and roots show you instead.
Brown tips, yellow leaves, a droopy stem. Each one can be a sign that something is wrong.
A sign like that is called a symptomA sign you can see that something is wrong, like yellow leaves or brown tips.
See it in the glossary.
Different problems can make leaves look alike.
So a grower checks four clues: pH, food strength, air bubbles, and light.
Roots need air, or they turn brown and can’t drink well.
Then the leaves go soft and droop. Growers call this wiltWhen leaves and stems go soft and droop because the plant can't get enough water.
See it in the glossary.
- A · net pot & seedling
- B · roots
- C · air stone
A grower checks the clues, makes a best guess, changes one thing, and watches.
Why only one thing? If you change two, you can’t tell which one helped.
Your turn: be the plant doctor.
Tap each clue to check it. Then pick the most likely cause.
Case 1 of 6
Yellow at the top
The newest leaves are yellow. The old leaves are still green.
Something's off. Let's look for clues.
Clues · tap to check each one
What is the most likely cause?
Read each case. Check the clues. Then read the most likely cause.
Case 1: Yellow at the top
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone on the bottom sends up bubbles. A dashed line marks the top of the water. The newest leaves at the top are yellow. The older leaves lower down are still green. The newest leaves are yellow. The old leaves are still green.
- pH7.6High
- Plant food strengthJust rightOK
- Air bubblesOnOK
- Light14 hoursOK
Most likely cause: The pH is too high
Iron is locked. New leaves can't turn green without it.
A grower would: Lower the pH to about 6.
Case 2: Pale all over
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone on the bottom sends up bubbles. A dashed line marks the top of the water. The whole plant is pale and small. The whole plant is pale and small. It grows very slowly.
- pH6.0OK
- Plant food strengthWeakLow
- Air bubblesOnOK
- Light14 hoursOK
Most likely cause: The plant food is too weak
There is not enough food in the water. The whole plant goes hungry.
A grower would: Add a little more plant food. Then test the strength again.
Case 3: Droopy leaves, brown roots
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone sits on the bottom, but no bubbles come out. A dashed line marks the top of the water. The leaves droop. The roots are brown instead of creamy white. The leaves droop. The roots are brown and soft.
- pH6.0OK
- Plant food strengthJust rightOK
- Air bubblesNoneOff
- Light14 hoursOK
Most likely cause: The roots have no air
Roots need air to stay healthy. Without it, they turn brown and the leaves droop.
A grower would: Turn the air pump back on. Check that bubbles rise.
Case 4: Tall, thin, and leaning
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone on the bottom sends up bubbles. A dashed line marks the top of the water. The stem is tall and thin, and the plant leans over. The stem is tall and thin. The plant leans to one side. Its leaves are small.
- pH6.0OK
- Plant food strengthJust rightOK
- Air bubblesOnOK
- Light5 hoursLow
Most likely cause: Not enough light
The plant stretches to reach more light. Its stem grows long and weak.
A grower would: Give it more hours of light, or move the light closer.
Case 5: Crispy brown tips
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone on the bottom sends up bubbles. A dashed line marks the top of the water. The leaf tips are brown and crispy. The leaf tips are brown and crispy. The rest of each leaf is green.
- pH6.0OK
- Plant food strengthStrongHigh
- Air bubblesOnOK
- Light14 hoursOK
Most likely cause: The plant food is too strong
Too much food in the water burns the leaf tips.
A grower would: Add plain water to make it weaker. Then test the strength again.
Case 6: One yellow leaf
A cutaway of a deep water culture tub. A seedling sits in a net pot in the lid. Its roots hang down into the water. An air stone on the bottom sends up bubbles. A dashed line marks the top of the water. The plant is green, except for one old leaf at the bottom that is turning yellow. One old leaf at the bottom is yellow. The new leaves are green.
- pH6.0OK
- Plant food strengthJust rightOK
- Air bubblesOnOK
- Light14 hoursOK
Most likely cause: Nothing is wrong
Old leaves at the bottom yellow as a plant grows. Every reading is OK.
A grower would: Change nothing. Keep watching the new leaves.
Check 1 of 3
The newest leaves are yellow. The pH reads high. What is your best guess?
Show the answer
The pH is too high. Right. High pH locks up iron, and new leaves need iron.
Check 2 of 3
You think the plant food is too strong. What do you do next?
Show the answer
Change one thing, then watch. Right. One change at a time shows you what helped.
Check 3 of 3
One old leaf at the bottom is yellow. Every reading is OK. What do you do?
Show the answer
Nothing for now. Keep watching.. Right. Old bottom leaves can yellow as a plant grows.
You just learned how to check the clues before you fix a sick plant.
Next up: Project: grow lettuce in a jar.
Go deeper: why symptoms overlapOpenClose
Some nutrients can move inside a plant, and some cannot. When a plant runs short of a nutrient that moves, it pulls it out of old leaves, so old leaves show the problem first.
Nitrogen and magnesium move easily, so their shortages show in older leaves. Iron and calcium stay put, so their shortages show in new growth.
That is why the same yellow leaf can mean different things. Where the yellow shows up matters as much as the color.
Even so, many nutrient problems look alike. A lab test of the leaves or the water can help a grower be sure.
Food strength is measured with a meter that reads how well the water carries electricity, called EC. More dissolved food means a higher reading.
Roots in still water with no air can rot. Rotting roots turn soft and brown, and the plant wilts even though its roots are wet.
After a fix such as a pH change, growers check the water again a few days later. Then they decide whether to change anything else.
Sources
- Mattson, N., and T. Merrill. Symptoms of Common Nutrient Deficiencies in Hydroponic Lettuce. e-GRO Research Update no. 2015.09, October 2015. Link
- Gatiboni, L. "1. Soils & Plant Nutrients." In Moore, K. A., and L. K. Bradley, eds. North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022. Link
- Moore, K. A., and L. K. Bradley, eds. "3. Botany." North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022. Link
- Kratky, B. A. A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce. Vegetable Crops VC-1. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, 2010. Link
- Kratky, B. A. Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65-72, 2009. Copy hosted by the University of Hawaii CTAHR. Link
- Sharma, A., K. Leonberger, N. Gauthier, and Q. Ying. Hydroponic Lettuce Production in Controlled Environments. CCD-CP-63. Center for Crop Diversification, University of Kentucky, 2025. Link
- Hoidal, N., Reardon, A., Worth, L., and Rogers, M. Small-scale hydroponics. University of Minnesota Extension. Link
- Baxendale, F. P., S. N. Wegulo, D. E. Lott, and D. T. Lindgren. Guide to Growing Houseplants. NebGuide G2205. University of Nebraska-Lincoln Extension, 2013. Link
- Common Cultural: Fertilizer burn. Hortsense, Washington State University Extension. Link
- Help with yellow leaves. Ask Extension, answered by Michigan State University Extension, 2024. Link
- Royal Horticultural Society. Leaf damage on houseplants. RHS Advice. Link