Diagnose seedling problems from the stage, pattern, and growing conditions before adding fertilizer or water. Poor germination points first to seed, depth, moisture, temperature, or dormancy. Collapse at the soil line suggests damping off. Tall pale growth points to inadequate light or excess warmth. Yellowing, scorching, and stalled growth require checking roots, moisture, temperature, light, and only then fertility.
Fast symptom guide
| Symptom | Check first | Immediate response |
|---|---|---|
| No germination | Seed age, depth, moisture, temperature, pretreatment | Confirm packet directions before reseeding |
| Patchy tray | Uneven fill, watering, heat, seed placement | Map the pattern and correct the uneven factor |
| Collapsed stem at soil line | Damping off and saturation | Remove affected plants and correct moisture and airflow |
| Tall, thin, leaning growth | Light coverage and warmth | Improve usable light and spacing |
| Yellow lower leaves | Roots, saturation, temperature, true-leaf fertility | Correct root conditions before feeding |
| Bleached or scorched tops | Light intensity, heat, spray or fertilizer injury | Reduce the damaging exposure and observe new growth |
| Stalled seedlings | Root space, moisture, temperature, salts, nutrition | Find the limiting factor before potting up |
Poor or delayed germination
Check the expected germination period before deciding a sowing failed. Confirm seed age and storage, sowing depth, medium moisture, root-zone temperature, and any required light, scarification, soaking, or moist chilling. Dig up only a sample: a firm swollen seed differs from a dry seed, and both differ from a soft decayed seed.
Use the Seed Starting 101 sequence and the pretreatment guide.
Damping off and stem collapse
Seedlings that pinch, darken, or collapse near the medium surface may have damping-off disease. Dense sowing, contaminated containers or media, persistent saturation, and stagnant humid conditions increase risk. Remove affected plants and nearby debris; improve drainage, watering, spacing, and airflow. Do not expect a product to revive a collapsed seedling. Follow Damping Off: What It Is and How to Prevent It.
Leggy, leaning seedlings
Insufficient usable light, uneven coverage, excess warmth, and crowding cause stretching. Move seedlings under suitable light, center the tray within the footprint, thin crowded cells, and adjust fixture height from the manufacturer guidance. Do not bury every stretched crop deeply; transplant depth is species-specific. See Grow Lights for Seed Starting.
Yellow, purple, or pale leaves
Color changes have multiple causes. Saturated cold roots may be unable to function normally; small cells may be congested; high salts or extreme pH can impair uptake; and low-fertility mixes may need feeding after true leaves develop. Inspect roots and conditions before applying fertilizer. Seed leaves naturally decline as true leaves take over, so note which leaves are affected.
Scorched, curled, or bleached leaves
Sudden intense light, a hot fixture, dry roots, excessive fertilizer, spray injury, or abrupt outdoor sun can damage leaf edges and tops. Remove the damaging exposure, restore moderate conditions, and judge recovery from new growth. Harden indoor seedlings gradually before full sun and wind.
Algae, fungus gnats, and surface growth
Green algae signals a wet, light-exposed surface; it does not by itself identify the root problem. Allow appropriate surface drying, improve airflow, and remove standing water. Fungus gnats also favor moist organic media. Adjust watering first and use only labeled controls when needed.
Roots, potting up, and stalled growth
Slide out one representative plug. Healthy roots vary by species but should not smell rotten or be surrounded by constantly saturated mix. Pot up when roots hold the plug and the plant needs more time, not simply because a larger pot is available. Use Seed Starting Trays, Cells & Pots to match volume and schedule.
Transplant shock
Wilting, bleaching, or stalled growth after planting can follow damaged roots, dry wind, sudden sun, cold soil, or inadequate hardening. Water the root ball before planting, minimize disturbance, transplant at the crop’s correct depth, and provide gradual exposure. Check the soil-temperature guide.
Troubleshoot systematically
- Identify the first affected stage and plant part.
- Compare affected cells with healthy cells in the same tray.
- Measure temperature and check moisture within the root zone.
- Inspect light coverage, drainage, roots, and recent treatments.
- Change one likely cause and record the response.
Return to the complete Seed Starting guide for the stage-by-stage system or compare Seed Starting Supplies when a setup component needs replacement.