Onion Growing Stages

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Onions are wonderful vegetables to grow in gardens. Understanding the growing stages can help boost your knowledge.

Each garden plant has a uniquely fascinating journey from germination to maturation, broken into individual growth stages. One of the best ways to become a better gardener is fully understanding your plants’ growth stages. This knowledge helps you know what your plant needs every step of its life.

Onions are common in vegetable gardening. Since so much growth occurs beneath the soil surface, out of sight, it makes it especially important to understand its growth stages.

Understanding onion growing stages will help you be a better gardener

How Long To Grow

Onions have a much longer growing season requirement than many other garden vegetables. This longer season is due to the time needed for plants to form bulbs. It takes 100 to 175 days for it to go from seed to a mature, dry bulb depending upon the variety and local growing conditions.

Development Stages

Like carrots, home gardeners typically grow onions as an annual crop, harvesting plants when the bulb is mature, and the vegetative tops begin to die. In reality, though, Allium cepa are biennial plants, meaning it takes two growing seasons to go from a germinating seed to a plant that produces its own seeds.

Even if you plan to plant onions yearly, let’s break the individual growth stages down into two separate seasons.

The different growth stages that an onion grows through for reproduction

Year 1

The first year in an onion’s life cycle focuses on growing the tops and bulb development. Since most gardeners harvest their bulbs in the first season, these growth stages are the only ones they know about.

Planting

As discussed in our guide about growing them, onions are considered a cool-season vegetable. They can be planted at two different times of the year, depending upon your growing zone. Gardeners in northern latitudes plant then in early spring for a late summer crop. Gardeners in southern latitudes plant them in the spring or the fall.

  • For spring planting, aim to get transplants or plant onion sets in your garden beds when the soil temperature is consistently above 28℉. This is typically late March or early April, before the last frost date of the spring. If you are growing from seed, plant the seeds 8 to 10 weeks beforehand.
  • For fall planting, aim for planting sets in August or September, giving them six to eight weeks to grow before the temperatures start to fall. As cool weather settles in, they go dormant, resuming growth in the spring.

Once you plant your seeds, keep the soil moist at all times. Seeds capable of germinating contain both an embryo and food reserves within their protective seed coat. Soil moisture softens the seed coat and enters the seed through a process call imbibition. Once inside the seed, the moisture triggers cellular respiration and metabolization of the food reserves.

You can expect your seeds to germinate in about ten days.

Germination

The first step in germination is the emergence of the radicle through the seed coat. Also known as the primary root, its job as the first emerging root is to anchor seedlings in the soil and support the plant. Once the radicle emerges, it absorbs moisture and nutrients from the soil to drive plant growth. 

Sprouting

As the radicle starts absorbing nutrients and soil moisture, the first shoot develops and emerges through the softened seed coat. Gravitational forces naturally direct the shoot to grow upward through the soil, reaching for the sun. Once the shoot breaks through the soil surface, the seedlings change focus, and plants direct their energy to developing leaves.

First leaves form as the oinon grows

First True Leaf Forms

The plant still relies on food from the endosperm, so the growth of the seedlings is relatively slow. With reserves dwindling, the seedlings are working hard to form leaves. The first true leaf appears — a smaller version of the flat green leaves seen on a mature plant. Photosynthesis begins, and seedlings start producing glucose for food.

Seedlings at this stage look like what we know as green onions and can be harvested now if you need green onions for a recipe.

“Leek” Stage

With the ability to photosynthesize, the plant’s growth picks up at a rapid pace. As the seedlings develop new, undifferentiated cells, plant growth hormones work to turn the cells into new leaves. Each new set of leaves increases the rate of photosynthesis, driving growth. At this time, the neck of the seedlings begins to thicken.

This stage encompasses the time from forming the first true leaf until the plant has four to seven true leaves. If you are growing to harvest leeks or you want a small onion, harvest now. If you are growing for bulb, allow them to keep growing.

The "leek" stage of onions

Bulb Initiation

At this time, the growing onions begin focusing on bulb initiation, with light being the primary driver behind initiation. At a minimum, the plants need four leaves for bulbing to occur regardless of climate conditions. Each new leaf that grows forms a scale that eventually develops into a ring or sheath of the bulb.

Different varieties are classified by the day length, or amount of sunlight the plants need daily to “set” bulbs.

  • Long-day varieties require 14 to 16 hours of daylight and produce the largest bulbs since these types have more time to develop leaves before bulb initiation.
  • Short-day varieties require 10 to 12 hours of daylight.
  • Day-neutral types require 12 to 14 hours of daylight.

Bulb Development

Once 8 to 12 leaves develop, the onion plant stops sending up new leaves and focuses all of its resources on the bulbing process. The previously formed leaves continue to grow and elongate, amassing more surface area for photosynthesis. The leaf sheaths swell in size to form the central storage tissue of the bulbs.

At this point, the onions begin to push the soil away and “pop” up out of the ground.

A harvested onion showing its bulb stage within its life cycle

Maturation

As bulbs approach maturity, plants demonstrate a physiological response known as “tops down.” This happens because resources are moved from the tops (leaves) to the scales, causing the bulb size to swell. When resources are depleted in the leaves, the tops fall over and dry down, signifying the bulb is ready to harvest.

Vegetative Senescence

When autumn temperatures drop and a hard frost occurs, water within the foliage freezes. The sharp edges of the ice crystals rupture the cell walls, causing fluid to leak out. With a lack of structural integrity in the cell walls and no fluid within the cells, the onion tops die back to the soil surface.

Year 2

Many home gardeners never let their onions see the second growing season, so most aren’t familiar with the plant’s formation of flowers and seeds. But if onions aren’t harvested and are allowed to overwinter in the ground, they start to grow the following spring again to set seeds and complete their life cycle.

Vegetative Growth

When springtime temperatures begin to climb, onions start actively growing again. Hormones within the bulb trigger cells to start developing and elongating, similar to when germination occurred the previous season. Since the root system fully developed the last year, the plant can focus its energy entirely on sending up a shoot to break through the soil surface.

Flowering Stalk Emerges

As the season heats up and air and soil temperatures start to climb, the plants begin to bolt. Vegetative growth halts, and the plant quickly shoots up a flowering stem to prepare for the end of its life cycle. Once the stalk is fully formed, buds grow, and a white or purple flower blooms at the top.

An onion in its flowering stage

Formation of Seeds

Pollination of the showy, round flower heads by birds, bees, or butterflies causes seeds to form. All of the nutrient reserves found in the bulbs are translocated from the onion bulbs upwards to the seeds, creating the food reserve the seeds use for germination. If harvested now, the bulbs are fibrous and woody.

Senescence

Once seeds form, the onion plant’s life cycle has come full circle. Since the onion directed so much of its resources towards developing seeds, there is little left for further plant growth. With little resources left and no need to keep growing, plant growth hormones induce the genes responsible for senescence, and the plant dies.

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About the author: Carley Miller is a horticultural expert at TheGreenPinky. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.

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