Lesson guide · Lesson 14 · Grades 3–8 · 7 min

When things go wrong: reading a sick plant

How to read a sick plant: check the clues, make a best guess, and change one thing.

Open the lessonPrintable worksheet

Students will

  • Name the four clues a grower checks: pH, food strength, air, and light.
  • Match a common symptom to its most likely cause.
  • Explain why a grower changes one thing at a time.

New words

symptom
A sign you can see that something is wrong, like yellow leaves or brown tips.
wilt
When leaves and stems go soft and droop because the plant can't get enough water.

Talk about it

When something goes wrong at home or at school, how do you find out why before you fix it?

Check questions and answers

  1. The newest leaves are yellow. The pH reads high. What is your best guess?

    • The pH is too high (answer) Right. High pH locks up iron, and new leaves need iron.
    • The roots need more air Not quite. The clue that stands out is the pH.
    • Not enough light Not quite. Look for the reading that is not OK.

  2. You think the plant food is too strong. What do you do next?

    • Change one thing, then watch (answer) Right. One change at a time shows you what helped.
    • Change everything at once Not quite. Then you can't tell which change helped.
    • Throw the plant away Not quite. Find the cause first. Then try one fix.

  3. One old leaf at the bottom is yellow. Every reading is OK. What do you do?

    • Nothing for now. Keep watching. (answer) Right. Old bottom leaves can yellow as a plant grows.
    • Raise the pH Not quite. The pH reads OK. Don't fix what isn't broken.
    • Add lots more plant food Not quite. The food reads OK. Too much food can hurt the plant.

Sources

  • Mattson, N., and T. Merrill. Symptoms of Common Nutrient Deficiencies in Hydroponic Lettuce. e-GRO Research Update no. 2015.09, October 2015.
  • 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.
  • Moore, K. A., and L. K. Bradley, eds. "3. Botany." North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022.
  • 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.
  • 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.
  • 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.
  • Hoidal, N., Reardon, A., Worth, L., and Rogers, M. Small-scale hydroponics. University of Minnesota Extension.
  • 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.
  • Common Cultural: Fertilizer burn. Hortsense, Washington State University Extension.
  • Help with yellow leaves. Ask Extension, answered by Michigan State University Extension, 2024.
  • Royal Horticultural Society. Leaf damage on houseplants. RHS Advice.