Understanding the Growth Stages of Bulb Onions

Onions are a kitchen staple and a satisfying vegetable to grow in the garden. Getting acquainted with the unique growth cycle of bulb onions can help you provide the right care at each stage ensuring your crop thrives. In this article we’ll explore the major phases of the onion plant lifecycle so you can support vigorous growth on the path to harvest.

Overview of Onion Growth Stages

Bulb onions go through three main growth phases:

• Vegetative stage
• Bulbing stage
• Flowering stage

The length of each stage depends on the onion variety and growing conditions. But generally, onions take 3-5 months to reach full maturity. Let’s take a closer look at what happens during each stage.

Vegetative Growth Stage

This initial stage starts when the onion seed germinates or a young seedling is transplanted. The onion plant focuses on foliage growth, sending up slender green shoots and developing its roots.

• Plants look like scallions or chives with long straight leaves.

• Roots expand to anchor the plant and take up water and nutrients.

• No bulb formation occurs yet.

• Stage lasts 60-100 days.

• Key needs are consistent moisture, full sun, and nitrogen fertilizer.

The vegetative phase is critical for building a healthy leaf canopy to power future bulb production. Onions form bulbs in relation to the number of leaves present, so robust leaf growth now pays off later.

Bulbing Stage

This is when the “magic” happens! Bulb formation is triggered as daylight hours reach a certain duration, determined by the onion variety.

• Leaves stop growing up and fall over.

• The plant concentrates energy into the base, expanding the bulb.

• Roots continue to develop as the bulb enlarges.

• The bulb swells for 30-60 days depending on conditions.

• Needs are reduced water, phosphorus-rich fertilizer, and consistent nutrients.

Once the “bulb flip” occurs, handle the onions gently to avoid damaging the developing bulb. Support good nutrient reserves with fertilizer to fill out the bulbs.

Flowering Stage

In their second season, onion bulbs use stored energy to produce a tall central flower stalk. But when growing onions for harvest, we try to avoid premature bolting.

• A long, straight stalk emerges from the bulb.

• Eventually, a cluster of small white flowers forms at the top.

• The plant redirects energy to flowering and seed production.

• This stage means the bulb won’t enlarge further.

• Caused by overgrowth, stress, or day length disruption.

Ideally, onions should be harvested before reaching this flowering phase. But occasional bolting is normal. Just pick the flowers for garnishes!

Providing Proper Care Through the Stages

Now that you know what to expect as onions grow, you can align your care to nurture them through each stage:

Vegetative stage:

• Sow seeds or transplant seedlings early.

• Keep soil consistently damp for fast leaf growth.

• Ensure full sun exposure.

• Use a nitrogen-rich fertilizer.

Bulbing stage:

• Reduce watering as the bulb forms to prevent rot.

• Apply a phosphorus-based fertilizer to boost bulbing.

• Handle gently to avoid damaging the swell.

• Provide consistent nutrition.

Flowering stage:

• Harvest or use bolting plants promptly.

• Check growing conditions that may have triggered early bolting.

• Remove flowering plants to focus the onion’s energy on the bulb.

Understanding the onion’s growth habit makes it easier to identify issues and provide tailored care when needed. Pay close attention to the duration of daylight as bulb formation depends on it. With the right support at each stage, you’ll be rewarded with a bountiful harvest of beautiful bulbs.