Understanding the Two Growth Phases in Onions: Vegetative and Bulbing

An article by Travis Key from Lazy Dog Farm

Onions are unique among many garden vegetables because their growth is closely tied to the amount of daylight they receive. This makes growing onions both fascinating and, at times, a bit tricky. Onion plants go through two distinct growth phases—vegetative and bulbing—each influenced by specific environmental conditions, particularly day length.
Understanding how onions transition from one phase to another is key to growing them successfully, especially when considering why certain types of onions thrive in different regions. Let’s break down these two phases and why they matter for onion cultivation.

The Vegetative Phase

The vegetative phase is when the onion plant is focused on developing its green top growth—leaves and roots. This phase begins as soon as the onion seed, set, or transplant is placed into the soil and continues until the plant is ready to begin forming a bulb. During this time, the plant's energy is devoted to producing strong, healthy leaves that are vital for later stages of development. This is when we want to be focused on feeding the onions heavily, giving them frequent doses of AgroThrive General Purpose Fertilizer.

Onion leaves aren’t just foliage—they are essential for storing energy that will be used during the bulbing phase. The number of leaves an onion plant produces during the vegetative phase directly impacts the size of the bulb later. More leaves generally result in a larger bulb because each leaf corresponds to a layer of the onion. If an onion has seven leaves, for instance, its bulb will consist of seven layers.

Optimal Conditions for the Vegetative Phase

• Cooler temperatures: Onions prefer cooler weather (between 50-75°F) during this phase. Hot temperatures can stress the plant, leading to poor leaf growth.
• Adequate nutrients: Nitrogen-rich soil is beneficial for leaf development. Applying a balanced fertilizer like AgroThrive General Purpose during this phase ensures the plant has the nutrients it needs to build healthy foliage.
• Consistent watering: Onions need evenly moist soil, particularly during the vegetative phase when they are developing their root system and leaves.

During this phase, the plant is focused on gathering and storing as much energy as possible, waiting for the right environmental cue to shift gears into the next stage of growth.

The Bulbing Phase

The bulbing phase is when the onion plant transitions from leaf growth to forming the bulb, the part of the plant we harvest and eat. This phase is triggered primarily by the amount of daylight the plant receives, which is why onions are categorized as short-day, intermediate-day, or long-day varieties.
Once the plant has enough leaves and the day length reaches a certain threshold (depending on the onion type), the plant redirects its energy toward the bulb. Instead of producing more leaves, it begins swelling the base of the stem, creating the familiar onion bulb. The energy stored in the leaves during the vegetative phase is transferred to the bulb, allowing it to grow larger.

What Triggers the Bulbing Phase?

The critical factor that triggers bulbing is day length, not temperature or other environmental conditions. Each onion variety has a specific daylight requirement for bulb formation:

• Short-day onions begin bulbing when they receive 10-12 hours of daylight.
• Intermediate-day onions require 12-14 hours of daylight to bulb.
• Long-day onions need 14-16 hours of daylight before they start forming bulbs.

This is why it’s crucial to match the type of onion you plant with the right region. If an onion plant doesn’t receive the right amount of daylight at the right stage of its development, it will either bulb too early (before it has enough leaves to create a large bulb) or fail to bulb altogether.

Why You Can’t Grow Short-Day Onions in Northern States

Short-day onions are designed to start bulbing when days are still relatively short—between 10-12 hours. In southern states, where winter days are shorter and milder, this works perfectly. Gardeners plant short-day onions in the fall or winter, and as the days start to lengthen in the early spring, the bulbs begin to form.

However, in northern states, where days quickly extend beyond 12 hours in the spring, short-day onions would begin bulbing too soon, before they’ve had enough time to develop a robust vegetative phase. This early transition results in small, underdeveloped bulbs because the plant hasn't had time to grow enough leaves to support a larger bulb. For this reason, short-day onions are not suitable for northern climates, as they won’t produce the large bulbs that gardeners expect.

Why You Can’t Grow Long-Day Onions in Southern States

On the flip side, long-day onions require 14-16 hours of daylight to initiate bulbing. This makes them perfect for northern states, where the longer summer days provide the necessary light conditions. In these regions, gardeners can plant long-day onions in early spring, giving the plant ample time to grow plenty of leaves during the vegetative phase. By the time summer’s long days arrive, the plant is ready to begin forming large, healthy bulbs.

In southern states, however, the days never reach the 14-16 hours needed to trigger bulbing. If you were to plant long-day onions in the South, the plants would continue in the vegetative phase indefinitely, growing more leaves but never forming bulbs. As a result, long-day onions aren’t suited for southern regions because they won’t bulb properly under shorter daylight conditions.

The Importance of Choosing the Right Onion for Your Region

Understanding the growth phases of onions—vegetative and bulbing—and how day length influences these phases is crucial for choosing the right type of onion for your garden. Planting short-day onions in northern states or long-day onions in southern states can result in disappointing harvests, with either small bulbs or no bulbs at all. By selecting the appropriate onion variety based on your region’s day length and growing conditions, you’ll set yourself up for success and enjoy a bountiful onion harvest.

Thousands of gardeners have been tuning in to The Lazy Dog Farm YouTube channel where Travis covers a variety topics ranging from how to successfully start seedlings to how to make a flavorful hot sauce that packs a punch. Accompanied by his wife Brooklyn and their two boys, the gardens on their 2 acre homestead in southwest Georgia are always filled with a wide variety of vegetables that are enjoyed fresh or preserved for later.