10 Common Seedling Problems in the Greenhouse (And How to Fix Them)

Seedlings are the most vulnerable stage of any greenhouse crop. They have small root systems, minimal energy reserves, and low tolerance for environmental stress. Most seedling problems are caused by the same handful of environmental factors: insufficient light, incorrect temperature, overwatering, poor air circulation, and contaminated growing media. Identifying the cause quickly and correcting it saves the crop.

1. Leggy, Stretched Seedlings

Cause: insufficient light. Seedlings stretch toward the nearest light source, producing weak, elongated stems. Solution: move trays to the brightest bench in the greenhouse, add supplemental LED lighting (150 to 250 µmol/m²/s at canopy height), and lower the light source closer to the seedlings (12 to 18 inches above the canopy).

2. Damping Off

Cause: fungal pathogens (Pythium, Rhizoctonia, Fusarium) in the growing media, triggered by overwatering and poor air circulation. Seedlings collapse at the soil line — the stem appears pinched and water-soaked. Solution: use sterile seed-starting mix, avoid overwatering, improve air circulation with a small fan, and sterilize trays between uses. Once damping off appears, affected seedlings cannot be saved — remove them to prevent spread.

3. Yellowing Lower Leaves

Cause: nitrogen deficiency (most common) or overwatering (second most common). Nitrogen is mobile in plants — the plant moves it from older leaves to new growth, causing lower leaves to yellow first. Solution: apply a dilute liquid fertilizer. If the growing medium is waterlogged, reduce watering frequency and improve drainage.

4. Purple or Reddish Leaves

Cause: phosphorus deficiency, usually caused by cold soil temperatures. Phosphorus uptake requires warm roots (above 60°F). Solution: add bottom heat with a heat mat set to 70 to 75°F. Once soil warms, the purple color resolves as phosphorus becomes available.

5. Leaf Curl or Cupping

Cause: multiple possible causes — excessive heat (leaf edges curl up), overwatering (leaf edges curl down), or herbicide contamination (irregular curling and distortion). Solution: check temperature (keep seedling area below 85°F), adjust watering, and if herbicide contamination is suspected, discard the growing medium and start with fresh, uncontaminated media.

6. Slow or No Germination

Cause: soil temperature too low (most common), old or improperly stored seed, or planting too deep. Solution: use a heat mat with thermostat set to the crop's optimal germination temperature. Test seed viability with a damp paper towel germination test. Plant seeds at the depth specified on the packet — most small seeds need light to germinate and should be surface-sown.

7. Uneven Emergence

Cause: inconsistent soil moisture or temperature across the tray. Solution: ensure even watering (bottom-water for most consistent results), use a heat mat to maintain uniform temperature, and use fresh, uniform seed-starting mix without lumps or dry pockets.

8. White Crust on Soil Surface

Cause: mineral salt accumulation from fertilizer or hard water. The crust itself does not harm seedlings but indicates that EC in the growing medium is high. Solution: flush the tray with clean water to leach excess salts. Reduce fertilizer concentration or use lower-EC water.

9. Fungus Gnat Larvae Damage

Cause: small black flies whose larvae feed on root hairs in moist growing media. Seedlings wilt and fail to thrive despite adequate watering. Solution: let the soil surface dry between waterings (gnat larvae need constant moisture), apply Steinernema feltiae nematodes as a drench, and place yellow sticky traps above trays to monitor adult populations.

10. Transplant Shock

Cause: abrupt change in temperature, light, or humidity when moving seedlings from the propagation area to the growing bench or outdoors. Solution: harden off gradually — expose seedlings to the new conditions for 1 to 2 hours on day one, increasing exposure time by 1 to 2 hours daily over 7 to 10 days. Avoid transplanting on extremely hot, cold, or windy days.

Building Resilience From the Start

Strong seedlings start with strong seed. Use fresh, high-quality seed from reputable suppliers — seed stored improperly (hot, humid conditions) degrades rapidly, producing weak seedlings that are vulnerable to every problem on this list. Store unused seed in a sealed container with a desiccant packet in the refrigerator for maximum viability across seasons.

Provide mechanical stress to strengthen stems. Brush your hand gently across seedling tops once or twice daily, or run a fan on low for 2 to 3 hours. This simulates wind exposure and triggers thigmotropism — the plant's natural response to mechanical stress, which produces shorter, thicker, stronger stems. This simple practice reduces legginess and improves transplant survival rates significantly.

Harden off gradually before transplanting to the growing bench or outdoors. Abrupt transitions from the warm, sheltered propagation area to the brighter, windier, and more variable growing environment cause transplant shock that can set seedlings back by a week or more. Over 7 to 10 days, gradually increase light exposure, decrease temperature protection, and reduce watering frequency to acclimate seedlings to their new conditions. This transition period is an investment in future plant performance — well-hardened seedlings establish faster and produce sooner than shocked transplants.

Finally, record your observations. Keep a simple log noting sow dates, germination dates, problems encountered, and solutions applied. Over two to three seasons, this log becomes your most valuable growing reference — a record of what works in your specific greenhouse under your specific conditions. Problems that recur despite correction suggest an underlying systemic issue (water quality, media contamination, or environmental control) that deserves deeper investigation.