"Sowed peas into moist coco - four days later nothing, seven days later a few scattered sprouts, most never emerged." Seeds don't "just grow" when moisture is present. Each seed is a dormant embryo with its own activation requirements: temperature, moisture, oxygen, and for some species - darkness or, conversely, light. Fail even one condition and germination rate drops below 50%, or seedlings emerge unevenly and deliver inconsistent product at harvest.
Quick glossary: Germination rate - the percentage of seeds that germinate under standard conditions within a defined period; stated by the seed producer; declines with improper storage and in old seed lots. Soaking - pre-moistening seeds in water before sowing; accelerates hydration of the seed coat and shortens time to emergence; essential for seeds with a hard or thick coat. Germination temperature - the substrate and ambient temperature at which enzymatic activity of the embryo is maximized; differs from the optimal growing temperature and is typically higher for most crops.
What Soaking Actually Does
Seeds are in a state of dormancy. To break dormancy, the embryo must absorb a certain amount of water - hydration activates enzymes that break down stored reserves and initiate cell division.
For seeds with a thin coat (basil, most lettuce types, radish microgreens) - a moist substrate provides sufficient rapid hydration. Soaking reduces time to emergence by 6-12 hours but is not critical.
For seeds with a thick or hard coat (pea, sunflower, corn, beet) - without pre-soaking the coat hydrates slowly. Soaking for 4-12 hours in room-temperature water accelerates germination and evens out emergence. How much pea yield soaking adds, and whether a biostimulant beats plain water, is covered in a 2026 trial on pea and arugula microgreens.
Water temperature for soaking: 64-77°F (18-25°C). Hot water (above 104°F / 40°C) damages the embryo. Ice-cold water slows hydration.
Soaking time: do not over-soak. Pea - 4-12 hours depending on the variety. Sunflower - 6-8 hours. Beet - 4-6 hours. Thin-coated seeds (radish, arugula) - no soaking needed, or 30-60 minutes maximum. Excessive soaking (beyond 12-16 hours for most crops) causes anaerobic stress and the embryo begins to rot.
Temperature: A Narrow Window and Why It Matters
Enzymatic activity during germination is temperature-dependent. Each crop has an optimal germination temperature and a minimum below which activation does not occur.
Reference ranges for common crops:
- Most microgreens (basil, arugula, radish, sunflower): 72-79°F (22-26°C)
- Pea and beans: 64-72°F (18-22°C) - more cold-tolerant at germination
- Beet: 68-77°F (20-25°C)
- Cilantro: 68-75°F (20-24°C) - germination rate drops sharply above 82°F (28°C)
Below the minimum temperature: seeds sit without germinating or sprout very slowly and unevenly. Typical winter scenario: cold substrate (57-61°F / 14-16°C), radish germination rate drops from 90% to 30%, the remaining seeds rot.
Above the maximum temperature: enzymes denature or thermally induced dormancy activates. Basil at 90°F (32°C)+ frequently fails to germinate or produces highly uneven seedlings.
Substrate vs. air temperature: the critical measurement is substrate temperature where the seed sits - not the air above it. Substrate on a cold metal shelf in a room at 72°F (22°C) air temperature may be only 63°F (17°C).
Oxygen: Why Overwatering Kills Germination
The embryo requires oxygen for aerobic metabolism during germination. When substrate is excessively wet, inter-particle aeration disappears, oxygen is consumed, and CO₂ accumulates. Result: even at correct temperature and moisture, seeds rot instead of sprouting.
Typical symptom: some seeds emerged normally, others appear swollen and soft with no sprout - these are seeds that underwent anaerobic stress.
Optimal substrate moisture during germination: moist but not wet. When you squeeze a handful - moisture is felt but no water drips out. For microgreens on trays - bottom-water or carefully top-mist to "evenly moist" with no puddles.
Dark phase: when growing microgreens, trays are covered for the first 2-4 days to create darkness and retain moisture. The cover must not be airtight - oxygen must enter. Small gaps or partial coverage maintain the balance between darkness and aeration.
Germination Problems: Where to Look
Low germination rate evenly across the entire tray: seed problem or germination conditions - temperature, old lot, improper storage. Test: germinate 10-20 seeds on a moist paper towel at 72°F (22°C) and count after the standard time for that crop.
Uneven germination (good in the center, poor at edges or vice versa): uneven conditions - temperature or moisture distribution across the tray area. On a metal shelf - edges are colder. With top watering - center is wetter.
Seedlings present but weak and etiolated: either too warm (seeds rush to emerge in search of light), or the dark phase is too long (for crops where darkness is needed for 2-3 days, not 5-7).
Microgreens Did Not Come Up: Where to Start Looking
Dig the seed up. Until you open the tray, "did not come up" covers two different diagnoses with different causes, and from above they look identical.
Seed swollen, coat intact, no sprout. The embryo took up water and never started. Look at substrate temperature and oxygen, both covered above.
A sprout emerged and turned brown or slimy. The seed germinated and died after the start. The cause is moisture, and whatever lived in the mat before sowing.
Seed dry and hard, the same size it went in. Water never reached it. That is uneven wetting of the mat, or a top layer dried out by venting too early.
Seed hollow, or crumbling between your fingers. The lot is dead. A paper-towel test confirms it in two days and saves the next tray.
Keep the order exactly as listed: dug-up seed first, then a thermometer in the substrate, then the germination test. The reverse order costs a week - the lot gets written off, a new one goes in, and the result repeats, because the cold shelf never went anywhere.
Three Mistakes That Cost the Most
Soaking thin-coated seeds for more than 2-3 hours or leaving them in water overnight. Radish, arugula, basil - do not need extended soaking. At 12 hours, anaerobic stress kills part of the seed lot before sowing. Extended soaking is only appropriate for hard-coated seeds where it is justified.
Not checking substrate temperature in winter or in cold rooms. "The room is at 72°F (22°C)" - but on a cold metal shelf the substrate may be 59-63°F (15-17°C) and germination rate is half as high. A thermometer in the substrate during germination, or a heat mat under the trays, solves the problem.
Covering trays airtight and forgetting about aeration. An airtight cover during microgreen germination causes CO₂ accumulation, oxygen depletion, and mass seed loss from anaerobic stress within the first 24-36 hours. Darkness and moisture retention - yes, but always with air access.
How to Know Germination Is Going Correctly
- Even emergence across the full tray area with no bare patches
- By the expected time for the crop (radish 1-2 days, pea 2-3 days, sunflower 2-4 days) - 80-90%+ germination rate
- Seedlings stand upright, neither etiolated nor drooping
- Substrate smells fresh - no mold or sour odor
If you want to go deeper: Transplanting and Adaptation: Transplant Stress and How to Minimize It explains what happens to the root system and the water relations when germinated seed moves from a cube or tray into a substrate system, and how to keep the gap between those phases small.