The common onion, scientifically known as Allium cepa, is a subterranean bulb, which is a specialized storage organ that forms beneath the soil surface. This underground growth pattern stores the energy and nutrients the plant needs. The visible green tops above ground are the leaves, which collect solar energy to fuel the development of the edible bulb below.
The Onion’s Structure Relative to the Soil Line
The edible portion of the onion is a bulb, a modified, fleshy structure composed of concentric layers. These layers are the swollen, nutrient-storing bases of the plant’s leaves, known as leaf sheaths, which develop entirely underground. The base of the bulb is a compressed stem plate from which fibrous roots extend downward into the soil for anchorage and nutrient absorption.
While the bulb grows beneath the surface, the hollow, cylindrical green leaves push up above the soil line. These leaves perform photosynthesis, channeling the carbohydrates they produce down into the forming bulb. The junction where the green tops meet the subterranean bulb is called the neck, and as the plant matures, the bulb often pushes the surrounding soil away and becomes partially visible above ground.
The Stages of Onion Cultivation
Onions are started primarily from true seeds or from small, immature bulbs known as “sets.” Planting sets is the quickest method, as these pre-grown bulbs begin their growth cycle with a head start. Starting from seeds is a longer process, but it offers a wider variety selection and is typically initiated indoors before the outdoor planting season.
Successful onion growth requires specific environmental conditions to encourage proper bulbing. The plants need full sun exposure and loose, well-drained soil to prevent the bulb from being restricted as it expands. Onion development is photoperiod-sensitive; bulb formation is triggered by the number of daylight hours. Varieties are categorized as long-day, short-day, or intermediate-day types depending on the region. The entire growth cycle generally takes between three to five months.
Knowing When to Harvest
Determining the time to harvest relies on specific visual cues signaling the completion of the underground growth cycle. The primary indication of maturity is the softening of the neck, the area where the leaf stem meets the top of the bulb. This softening shows that the transfer of carbohydrates from the leaves to the bulb has concluded.
Following the softening of the neck, the green tops will naturally begin to weaken, bend, and fall over toward the ground. When approximately 80 to 90 percent of the tops have fallen, the bulb is considered fully mature and ready to be pulled from the soil. After harvest, the bulbs are typically left in a warm, dry location to undergo “curing,” which allows the neck to dry completely and the outer skins to tighten for long-term storage.