Growers cover seed flats with plastic film or a humidome to engineer a controlled microclimate for optimal seed germination. A seed flat is a shallow tray, often partitioned, used for starting multiple seedlings indoors. The clear plastic film or rigid lid (humidome) acts as a temporary barrier. The primary purpose is to maintain the high-humidity and warm-temperature conditions seeds require to break dormancy and begin growth. This controlled environment significantly increases the rate and success of germination compared to uncovered trays.
How Covers Create Optimal Moisture Levels
The primary function of the plastic cover is to prevent the rapid evaporation of water from the seed-starting mix. Seeds require sustained moisture uptake to trigger the internal metabolic processes of germination. If the soil surface dries out during this initial phase, the embryonic structure can quickly desiccate, resulting in failure.
The cover creates a closed system, trapping water vapor that evaporates from the soil and growing medium. As this warm, moist air touches the cooler plastic surface, it condenses back into liquid droplets. This condensate drips back onto the soil, creating a self-watering, miniature water cycle that maintains a near-saturated atmosphere, often close to 100% relative humidity.
This high humidity is crucial for seeds sown shallowly, ensuring the seed coat remains consistently hydrated for the embryonic root (radicle) to emerge. By minimizing moisture loss, the grower does not need to water or mist the seeds as frequently, reducing the risk of disturbing small seeds or over-saturating the medium.
The Effect of Plastic on Soil Temperature
Beyond moisture retention, the clear plastic covering serves a thermal function, acting as an insulator and passive solar collector. The plastic creates a small-scale greenhouse effect, trapping warmth from ambient room air or supplemental grow lights. This insulation slightly elevates the temperature of the seed-starting mix above the surrounding environment.
Higher soil temperatures accelerate the complex enzymatic reactions that initiate germination. For many warm-season crops, such as tomatoes and peppers, optimal soil temperatures (75°F to 85°F) are significantly higher than typical indoor air temperatures. A clear plastic cover can raise the soil temperature by several degrees, substantially increasing both the speed and percentage of successful germination.
This heat retention is often used in conjunction with a seedling heat mat, where the plastic cover maximizes the upward transfer of warmth into the soil. The cover prevents the heat from dissipating too quickly into the cooler surrounding air.
Managing the Transition After Germination
The highly humid, still-air environment becomes counterproductive and harmful the moment the first seedlings emerge. Once a seed sprouts, the young plant needs increased airflow and a reduction in humidity to develop strong stems and avoid disease.
The most immediate and serious risk of keeping the cover on is a group of fungal diseases collectively known as “damping off.” Damping off is caused by soil-borne pathogens that thrive in the exact conditions created by the cover: high humidity, warm temperatures, and poor air circulation. These fungi attack the seedling stem at the soil line, causing the tissue to collapse and the young plant to abruptly topple over and die.
The cover must be removed immediately upon seeing the first sign of emergence to prevent this rapid spread of fungal infection. Leaving the cover on also risks etiolation, where the seedling stretches excessively to find light, resulting in a thin, weak, and spindly stem.
After removing the cover, the seedlings should be placed directly under strong light and introduced to gentle air movement, often with a small fan, to strengthen their stalks. The final step is a gradual process called “hardening off,” where the seedlings are slowly acclimated to lower humidity, direct sunlight, and outdoor wind before being planted into the garden.