Understanding Onion Growth Phases

An article by Travis Key from Lazy Dog Farm

Onions are one of the easiest crops to grow in the backyard garden once you understand how they grow and what types you should be planting in your growing zone. This blog and the next few blogs will be addressing some of the more misunderstood details about the types of onions and how they grow. Before we talk about onion growing tips and how to maximize your harvest, I think it’s important to explain how an onion grows. Once you understand the distinct growing phases of an onion, you’ll be well on your way to growing big onions every year in your backyard garden.

I think it’s important to note that this discussion refers to bulbing onions — the kind that you would buy in a bag at the grocery store. These growing phases and growing tips don’t apply to bunching onions (also known as spring onions), Egyptian walking onions, or multiplying onions.

How Many Growth Phases Do Onions Have? 

Depending on what you read, onions have two or three distinct growing phases. These would include the vegetative phase, the bulbing phase, and the flowering phase. In most cases you’ll want to harvest onions prior to any flowering or seed head production, so I often don’t mention the flowering phase when talking about growing onions in a home garden. This phase would only be beneficial if you were wanting to save onion seeds from the bloom so that you could replant them the following year. As such, we’ll primarily talk about the vegetative and bulbing phases below. 

The Vegetative Growth Phase of Onions

Once an onion seedling emerges from the soil, it will begin to produce green leaves. These leaves will elongate and thicken during the vegetative phase. New leaves will also continually form throughout this growing stage. The onion stem may thicken during the vegetative phase, but there will be no evident bulb formation while the onion plant is in this phase.

Each leaf that forms will represent a ring on the onion bulb that eventually develops during the bulbing phase. Onions will typically produce 8-12 leaves throughout the vegetative phase, but this can vary slightly depending on the quality of the soil, fertility, and growing conditions. Some of the younger leaves will tend to dry and wither as the onion plant grows, so it’s not likely that you’ll see all 12 leaves on an onion plant at any given time.

As a backyard gardener your goal should be to maximize the green growth on the onion plant during the vegetative phase. The more leaves you have and the larger those leaves are, the more energy the plant will be able to generate in the bulbing phase. I recommend never trimming the tops of bulbing onions. Doing this will compromise the leaves and likely result in a smaller bulb during the bulbing phase.

It’s also important to feed and water your onions well during this phase of leaf production. As we’ll discuss more in future blogs, onions are heavy feeders. This means that they like plenty of water and especially nitrogen during this vegetative phase. Feed them frequently with a biologically active fertilizer like AgroThrive General Purpose to keep them happy. The more vegetation the onion plant produces, the larger the eventual bulb will be. 

The Bulbing Growth Phase of Onions

While the vegetative phase is simple to understand, the bulbing phase is a little more complex and will differ depending on where you live. The initiation of the bulbing phase is easy to identify because you’ll see the soil start to crack around the base of the onion plants. This is in response to the bulb enlarging and pushing soil out the way to make room for the enlarging bulb. When the bulbing phase begins, the onion plant is no longer producing green leaves. It is now devoting most of its energy into producing the large bulb that you’ll eventually harvest.

The bulbing phase in onions is triggered by day length or the number of sunlight hours in each day. This obviously varies depending on where you live. During the winter months, the northern part of the country will have shorter days as compared the southern states. But during the summer months, the northern states will have longer days compared to the south. As such there are three distinct categories of bulbing onion varieties — short-day, intermediate-day, and long-day.

Short-day onion varieties will be initiate bulbing when the sunlight hours per day reaches 11-12 hours in the late winter or early spring months. These onions should only be grown in the southern states. They perform best when planted in the fall and overwintered for a late spring harvest. This allows the southern grower to maximize leaf formation and growth in the vegetative phase throughout the mild winter. The result is a large, delicious onion that is formed in the bulbing phase.

Intermediate-day onion varieties begin to bulb when the sunlight hours per day reaches 13-14 hours in the late spring or early summer months. These varieties work well for those living in the middle of the country. They’re usually planted in late winter or early spring and harvested in the early summer months.

Long-day onion varieties will start to bulb when the sunlight hours per day reaches 15-16 hours per day. These varieties should be grown by gardeners in the northern states and are usually planted in early spring once the risk of freezing temperatures has passed. They’ll then be harvested in the mid to late summer months depending on how far north you are.

If you plant the wrong “day-length” type for your area, you likely won’t produce a very large onion or may not produce a bulb at all. If you plant a short-day onion in the northern states, the sunlight hours per day will already be near or more than 11-12 hours a day by the time the weather allows you to plant. As such, the onion plant wouldn’t spend much time in the vegetative phase and would quickly start bulbing. Because there wouldn’t be much vegetation on the plant, the onion bulb would be small and underwhelming.

If you plant a long-day onion in the southern states, it may never bulb at all. As mentioned above, southern states don’t receive as much sunlight per day as northern states in the summer months. Depending on how far south you live, you may never experience the 15-16 hours of sunlight per day required to initiate bulbing in the long-day onion varieties. As a result, you’ll just have an onion plant with a bunch of leaves but no bulb.

If you live on the border of the short-day and intermediate-day boundaries indicated in the figure above, you can likely plant either type. Similarly, if you live on the border of the intermediate-day and long-day boundaries, you can probably grow either and be successful. But if you live in the deep south, you should stick with short-day varieties. And if you live in the far north, you should only grow long-day varieties. 

Using the Onion Growth Phases to Your Advantage

Now that you understand the two distinct growing phases of an onion and how they differ depending on your latitude, you can now use this information to select the right varieties for your area. When buying onion seeds or plants, always make sure you know whether you are buying a short-day, intermediate-day, or a long-day variety. On the following blog, we’ll dig deeper into the types, colors, and shapes of onions that you can grow in your backyard garden.

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 hotsauce 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.