Thermal Dormancy in Lettuce Seed - And Why it is Important.

by Paramount Seeds on Aug 25 2026

Lettuce seed won’t germinate when it’s too warm — a response called thermal dormancy. It’s one of the most common causes of poor summer germination, and it’s easily missed because the seed looks fine but simply won’t sprout.

The Basics

Lettuce germinates best at 15–20°C (59–68°F) substrate temperature. Above 25°C (77°F), germination slows. Above 28–30°C (82–86°F), many varieties enter thermal dormancy — a protective block that prevents germination in conditions too hot for seedling survival. The seed isn’t dead; it’s waiting. But once dormancy is triggered during the first water uptake (imbibition), it can persist even after temperatures drop.

Why It’s a Bigger Problem with Pelleted Seed

Pelleted lettuce seed is precision-seeded one per cell — there’s no backup seed to compensate for a miss. That makes germination reliability critical. A few things to know:

The pellet coating causes the seed to imbibe quickly, so it’s exposed to substrate temperature conditions immediately after seeding
Pelleted seed is typically primed, making it more sensitive to temperature extremes than raw seed
If the substrate dries out before the pellet fully dissolves, germination stalls and secondary dormancy can set in

See: How Is Seed Pelleted? and Seed Priming: What It Is and Why It Matters.

How to Overcome It

Control substrate temperature

Keep germination substrate at 15–20°C (59–68°F) for the first 24–48 hours. Use a temperature-controlled germination chamber if possible. Don’t rely on air temperature alone — measure the substrate directly. Warm benches and direct sun can push substrate temps well above ambient.

Germinate in the dark

Light at high temperatures compounds thermodormancy in some varieties. Germinating trays in darkness at controlled temperature — then moving to light once radicles emerge — is standard practice in many commercial operations during summer.

Pre-chill the substrate

If a germination chamber isn’t available, seed into pre-chilled substrate (cooled to 15–18°C / 59–64°F). This gives the seed a favorable window during imbibition even if ambient temperatures rise afterward.

Keep moisture consistent

For pelleted seed, the substrate must stay consistently moist from seeding through radicle emergence. Drying out stalls pellet dissolution and can trigger secondary dormancy. Mist lightly if needed — don’t saturate.

Choose the right variety

Thermal dormancy sensitivity varies by variety. Summer-specific varieties are bred for better germination at elevated temperatures. Check datasheets before seeding in warm conditions.

Quick Reference

✅ Optimal: 15–20°C (59–68°F) substrate
⚠️ Reduced germination: 22–25°C (72–77°F)
❌ Thermodormancy risk: >25°C (77°F); high risk above 28°C (82°F)
🌱 Pelleted seed: Keep moist and cool from seeding through radicle emergence
🌙 Summer tip: Germinate in darkness at controlled temperature; move to light after radicle emergence