When starting seeds indoors, many gardeners turn to household plastic wrap to cover their trays, aiming to replicate the warm, humid conditions of a greenhouse. This technique is a simple way to create a self-contained microclimate directly over the seed starting medium. The goal is to provide a consistent, stable environment that encourages the dormant seeds to initiate the germination process. Understanding the science behind why this cover works, and when to remove it, is paramount to successfully growing strong, healthy seedlings.
The Role of Humidity in Seed Germination
A seed requires three main things to awaken from dormancy: water, oxygen, and a favorable temperature. Covering a seed tray with plastic wrap directly addresses the need for consistent moisture and warmth by significantly reducing the rate of evaporation. This creates a near-100% relative humidity environment above the soil surface, which is a necessary condition for water uptake by the seed, a process called imbibition.
The trapped moisture is absorbed through the seed coat, signaling to the embryo inside that conditions are right to begin metabolic activity and growth. The plastic also acts as insulation, helping to stabilize the soil temperature. This stability prevents fluctuations that can slow or halt the delicate biochemical reactions required for germination. Providing this greenhouse effect helps speed up the initial sprouting phase, especially for seeds that naturally germinate in warm conditions.
Deciding When to Use the Cover
The decision to cover a seed tray depends heavily on the type of seed and the conditions of the indoor growing space. Small seeds, particularly those that are surface-sown or barely covered with soil, benefit greatly from the plastic wrap because their shallow placement makes them highly susceptible to drying out. The cover ensures that the soil surface remains consistently moist, preventing the tiny seeds from being prematurely dehydrated. This method is also useful when the ambient air in the home is particularly dry, such as during the winter months when central heating is used.
Conversely, the cover should be skipped when planting large seeds or those that naturally prefer cooler germination temperatures, such as spinach or lettuce. Seeds that are planted deeper into the medium are already protected from rapid surface drying and do not require the high humidity environment. Furthermore, if the seed starting medium is already overly saturated with water, adding a cover will only exacerbate the issue by trapping excess moisture and preventing the necessary gas exchange that roots require.
Crucial Next Step: Removing the Cover
The single most important step when using a plastic cover is knowing exactly when to remove it. The cover must be taken off immediately once the first seedling emerges from the soil and sheds its seed coat. Leaving the cover on for too long can have severe and rapid consequences for the young plant’s health. The high humidity and low air circulation create an ideal breeding ground for fungal pathogens that cause damping-off disease.
This fungal infection attacks the seedling stem at the soil line, causing it to weaken, rot, and collapse. A covered seedling that has sprouted also begins to suffer from etiolation (weak, pale, and stretched growth). This stretching occurs as the seedling searches for light, resulting in a fragile plant that cannot support its own weight. Once the cover is removed, the seedlings must be immediately transitioned to a location with bright, direct light and greatly increased airflow to encourage sturdy stem development.
Practical Tips for Preventing Mold and Disease
Preventing mold and fungus requires careful attention to hygiene and environmental controls.
Hygiene and Medium
Pathogens thrive in cool, wet, and still conditions, so sterilize all pots, trays, and tools with a diluted bleach solution before use. Always use a fresh, sterile seed-starting mix rather than garden soil, as the latter almost always contains fungal spores.
Environmental Control
Ensure the soil is moist but never soggy, often by watering from the bottom to avoid wetting delicate stems. To maintain air exchange and prevent stagnation, lift the plastic cover daily for a brief period or poke small ventilation holes in the material. Introducing a small fan to circulate the air for a few hours daily will strengthen emerging stems and discourage surface mold.