Onion seeds are a popular and cost-effective way for gardeners to grow bulbs or scallions. Successfully starting onions from seed involves a careful balance of environmental factors, as these small seeds are highly sensitive to their surroundings. The timing of emergence relies on providing consistently favorable conditions, and the germination period can fluctuate widely depending on how well the growing environment is controlled.
The Expected Germination Timeline
Under ideal circumstances, onion seeds germinate swiftly, with the first sprouts appearing within four to ten days of planting. This quick emergence is typically seen when seeds are started indoors where conditions are stable. The visual sign of successful germination is a small, pale green loop, known as the cotyledon, breaking the soil surface. If the soil temperature is cooler than optimal, the germination window can extend significantly, sometimes taking two weeks or more. While seeds can germinate as low as \(40^\circ \text{F}\) (\(4^\circ \text{C}\)), emergence will be much slower and less consistent.
Essential Conditions for Successful Germination
The most rapid germination occurs when the soil is held at a consistent temperature between \(70^\circ \text{F}\) and \(75^\circ \text{F}\) (\(21^\circ \text{C}\) to \(24^\circ \text{C}\)). Temperatures below \(50^\circ \text{F}\) (\(10^\circ \text{C}\)) drastically slow the seed’s metabolic processes, extending the timeline to two or three weeks. Utilizing a seed heat mat is the most effective method for indoor starts to maintain this optimal temperature zone.
Consistent moisture is important for triggering the seed to sprout, but the soil must never be saturated. The planting medium should feel like a well-wrung sponge—consistently moist but not soggy—to prevent the seeds from rotting. Misting the surface or watering from the bottom of the tray helps maintain this moisture balance without displacing the shallowly planted seeds.
Onion seeds should be sown shallowly, ideally at a depth of \(1/4\) to \(1/2\) inch. Planting them deeper expends too much of the seed’s stored energy before the seedling can push through the soil to reach the light. The combination of warmth, steady moisture, and shallow planting provides the necessary cues for quick emergence.
Next Steps After Sprouting
Once the seedlings have emerged and straightened from their initial loop, providing strong light is the immediate priority to prevent weak, spindly growth. Indoor seedlings benefit from being placed directly under a grow light source for 14 to 16 hours daily. If the light source is weak or too far away, the young plants will stretch excessively toward the light, resulting in “legginess.”
Thinning the seedlings is necessary once they develop sufficient leaf growth to prevent overcrowding and competition for resources. If multiple seedlings sprouted closely together, remove the weakest ones, leaving the strongest plants spaced about \(1\) to \(2\) inches apart. This ensures the remaining seedlings have adequate air circulation and space for root development.
Before moving the young onion plants outdoors, they must undergo a process called hardening off. This involves gradually acclimating them to outdoor conditions, such as direct sunlight, wind, and temperature fluctuations, over seven to ten days. Starting with a couple of hours in a shaded area and slowly increasing the exposure prepares the indoor starts for the garden environment.