Onions can be one of the tougher crops to master for beginner or even experienced gardeners. We often hear from gardeners who say their onion plants look great, but don't make a very big bulb. I think much of this has to do with the lack of clarity on which types of onions should be planted based on your climate. It is not uncommon to see onion seeds being sold online without any mention of "day-length" (explained below) for that specific variety.
Another source of confusion is introduced when you mention planting onion seeds, plants, or "sets." And while you can certainly have some level of success planting any of those, you should know that there are differences and what to expect from each. Here we'll try to explain different types of onions and help you make an educated decision on what to plant in your garden. And once you understand the growth cycle of an onion plant as we'll explain below, they're pretty easy to grow.
DIFFERENT TYPES OF ONIONS
Let's start with the different types of onions. Although some of these may have different names regionally, the three main types are:
1) Bunching Onions
Also called "spring onions," these are the small onions that are sold in bunches at the farmers market or grocery store. And while the small bulbs are edible, the greens are the most often used portion on this type of onion. They make a great garnish or flavoring for soups, salads, and more. These are typically direct-seeded very thickly and harvested as a "bunch." The bulbs don't get large, and so they can be planted densely without any issues. We currently have the Parade Bunching Onion variety planted in our garden and it's doing well.
2) Multiplying Onions
These are planted from small bulbs and the bulbs multiply into clusters as they grow. You'll also see them called "perennial onions" as they can grow year round in some climates. Once they form a cluster of bulbs, you can split the clusters and replant the single bulbs to make more and more onions. They tend to have a spicier flavor than most standard bulbing onions, but they are a great sustainable food source. The Shirley Hammond Multiplier Onion is a popular multiplying onion variety.
3) Bulbing Onions
These are the big onions you see sold individually or in bags at the grocery store. They come in a wide variety of shapes, colors, sizes, and flavor profiles. They may have a white, yellow, or red skin and can be flattened or round in shape. The flattened, or granex-style, onions tend to be sweeter while the round onions tend to have a more pungent flavor. Red onions have a bit more spice, especially when eaten raw.
*For the remainder of this article, we will be talking about "bulbing onions" exclusively.
SEEDS, PLANTS, OR SETS?
If you want to grow large bulbing onions, should you be planting seeds, onion plants, or "sets?" We'll explain the advantages and disadvantages of each below.
1) Onion Seeds
Planting onions from seed is pretty easy if you have a greenhouse or indoor seed starting setup. You don't want to direct-seed bulbing onions in your vegetable garden. Onion seeds can take a while to germinate and can be slow to grow initially. As a result, weeds will often overtake them before they're able to start growing well.
You'll want to plant these in seed starting trays and grow them much like you would a broccoli or cabbage transplant. Once the seedlings have a solid root system and 3-4 leaves, they can be transplanted to the in-ground garden, raised bed, or container garden where they'll mature.
Growing your own onion plants from seeds is great because you have total control of the varieties you want to plant and the timing of your planting. You don't have to wait on onion plants to ship from a particular online vendor, and you're not at the mercy of having to choose from only the varieties they have available. As mentioned above, you will need some type of seed starting setup though (greenhouse or indoor).
2) Onion Plants
Purchasing onion plants from an online vendor or a local feed and seed store is a great option if you don't have a greenhouse or indoor seed starting setup. Just be sure to specify your ship date when ordering online or over the phone. Often times these online vendors will ship the onions later than the ideal planting date for your area. But if you contact them to specify your desired ship date, most companies are usually willing to comply.
The disadvantage to buying plants, as mentioned above, is that you can only grow the varieties for which the grower has plants. The plants you buy will also tend to have more transplant shock when putting them in the ground. Because they've been pulled from the ground (or tray) at least a couple weeks when you receive them, it takes them a little longer to start growing in their new soil.
3) Onion Sets
Although some people will use the term "sets" to refer to onion plants, sets are the small onion bulbs that you'll find at big box stores or garden centers. These look very similar to the multiplying onion bulbs mentioned above. If you're wanting to grow onions with big bulbs, I would discourage purchasing sets. I've never heard many people having great results with them. Grow your own onion plants from seed, or purchase onion plants instead.
3 MOST COMMON ONION MISTAKES
Before we get into the most common onion planting/growing mistakes, it will be helpful to explain the growth cycle of a bulbing onion plant.
The onion plant has two distinct growing phases -- the vegetative phase and the bulbing phase. During the vegetative phase, the onion produces lots of green leaves that it will use for energy production in the subsequent bulbing phase. During the bulbing phase, the onion plant uses all those green leaves to enlarge the base of the plant, making the big onion bulb that we all want.
All onion plants start in the vegetative phase, and the bulbing phase is then triggered by day length. When the sunlight period within a day reaches a certain length, this will trigger the onion plant to start bulbing. Once the bulbing phase begins, the plant will stop producing leaves and devote all its energy to making the onion bulb.
1) PLANTING THE RIGHT ONION VARIETIES FOR YOUR AREA
As mentioned above, the bulbing phase is triggered by day length. But day length can be quite variable depending on your growing lattitude. In the winter months, day length is longer in the southern states and shorter in the northern states. In the summer months, day length is longer in the northern states than the southern states. This is why there are different varieties of onions that differentiated by the day-length at which they start bulbing.
- In the southern states, we plant short-day onion varieties. These varieties start bulbing when the day length reaches 10-12 hours in the late winter months.
- If you live in the middle of the country, you'll want to plant intermediate-day onion varieties. These varieties start bulbing when the day length reaches 12-14 hours.
- In you live in the northern states, you should be planting long-day onions which start bulbing when day length reaches 14-16 hours.
If you plant a short-day onion variety in the northern states, it will start bulbing before much vegetative growth is developed. As a result, you won't make a very big onion bulb because there's not enough leaves to help produce a large bulb.
If you plant a long-day onion variety in the southern states, it will likely never enter the bulbing phase because the day length isn't long enough in the south.
If you live on the edge of the short-day and intermediate-day growing zones, you can likely plant either type. Similarly, if you live on the edge of the intermediate-day and long-day growing zones, you can plant either.
2) PLANTING AT THE RIGHT TIME
If your goal is to grow big onion bulbs, you'll want to maximize the growth of the onion plant during the vegetative phase. This means that you want the vegetative phase to be as long as possible before the bulbing phase is triggered by day length. If you plant onions late, you won't have much time to make a substantial amount of vegetation prior to bulbing.
Planting times will differ based on climate. Onions can tolerate temperatures down to 20°F as long as those temperatures aren't sustained. So you'll want to plant as early as you can without exposing them to temps below 20°F. Here in the south, we plant short-day onion plants in the fall and let them "overwinter" for a spring harvest. In the middle and northern states, onions should be planted in late winter or early spring as soon as the risk of sub 20°F nights is gone.
3) FEEDING THEM PROPERLY
Onions are heavy-feeding plants. They benefit from having plenty of water and a substantial amount of nitrogen. Per our fall garden fertilization schedule, we feed onions a balanced fertilizer initially to promote root development. But once the plants are established and growing, we'll feed them a nitrogen-only fertilizer until the bulbing phase begins.
Giving them plenty of water and nitrogen will ensure that you maximize the green growth of the onion plant during the vegetative phase. This will in turn help the plant to produce a large bulb during the bulbing phase. It is important to note that fertilization should be ceased once the bulbing phase begins. Fertilizing onions during the bulbing phase can lead to some disease issues, especially in more humid climates.
We hope this onion-growing information has been helpful. It can seem certainly seem complicated initially. But once you find the right varieties for your climate, you're well on your way to great success. Then you'll just need to decide if you're going to grow your own plants or will be buying ground-ready plants. That's the easy part!