Growing big bulb onions can often be an elusive goal for new gardeners. I struggled for the first few years because I failed to understand three things: certain types of onions grow best in certain locations, planting depth is super important, and onions need specific nutrition and specific times.
Let’s tackle the basics of each of these to get you growing onions with ease.
Types of Onions for Each Location
Onions are photoperiodic which means they start forming bulbs based on daylength. There are three different types of onions – short day, long day, and intermediate or day neutral. Most onion varieties will begin to form their bulbs when the temperature and the number of daylight hours reach a specific level. Short day onions start forming their bulbs when the daylight hours are between 10 and 12 hours; long day onions don’t start developing until the daylight is at least 14 to 16 hours per day. Day neutral onions bulb during that in-between day length of 12 to 14 hours. How do you know which one to grow?
The dividing geographic line between long- and short-day varieties lies roughly at a latitude of 36 degrees north (the 36th parallel). If you garden north of the 36th parallel, you should grow long-day onions; south of it, grow short-day onions. Just about everybody can grow day-neutral (intermediate) varieties so long as there is 12 hours of daylight.
Why does your location matter? Don’t most gardens get up to 16 hours per day of light at some point? Yes, but onions need lots of leafy top growth in order to develop those big storage bulbs. The more leafy growth you have at the top of the onion before that process starts, the larger the onion will be, so it is important to give the leafy part of the plant enough time to size up prior to bulbing.
This is where that line of latitude comes in. What are your temperature conditions where you garden when the daylight hours begin to lengthen after winter. When are your days between 10 and 12 hours and how warm is it when that happens? If it’s late winter or early spring and your temperatures are already pretty mild and your garden is taking off nicely, you likely live below the 36th parallel and you need short-day onion varieties. When planted at the right time, your onions will have plenty of time to grow those leafy green tops before starting to form their bulbs.
If, however, your temps don’t really start to warm up until your daylight hours hit at least 14 hours per day, you likely are above the 36th parallel and need to grow long-day or day-neutral onions. Your plants need more time to put on those leafy greens because the temperature conditions are colder for much longer, slowing that plant growth. If you grow short-day onions, there won’t be enough time for those onions to grow their tops before the daylength triggers the bulbing process and you’ll end up with small onions.
Proper Planting Depth
In order for onion bulbs to develop properly, they need to be able to pop up out of the soil as they mature. If they remain too deep in the soil, the soil will restrict the bulb growth and contact with wet soil for extended periods of time will encourage then to rot.
The ideal planting depth for onion seeds is ¼” deep in the soil. If planting from sets or transplants (seedlings), the ideal depth is 1”. That’s it!
And as a side note, once your onion bulbs start to pop up out of the ground, don’t cover them back up. Let the grow as far out of the ground as they like for bigger bulbs.
Feeding Your Onion Plants
Remember, healthy foliage growth ensures that the plants have enough energy to form large bulbs. How do we ensure lots of leafy growth in other plants? Nitrogen! For the best growth and highest yield, onions need to be fed right from the start. Use an amendment or fertilizer with either equal amounts of NPK (10-10-10) or with a little bit more phosphorous (the middle number higher than the other two) such as 10-20-10. This is what you want in the soil prior to planting. Either incorporate these amendments into the soil where you’ll plant or dig a trench alongside where you’ll be planting, add the amendments, and cover with a few inches of soil. Water in those amendments and plant the onions alongside the trench.
After your onions are planted, focus on amendments or fertilizers that are mainly nitrogen. Every 2 to 3 weeks after planting, side dress with that nitrogen or if you using the trench method, put it right over top of where your initial amendments went. This will encourage lots of leafy growth.
Stop fertilizing when the onions begin to bulb. Because this is triggered by day length, when this occurs after planting is going to vary based on when you planted them and the region you live in. The only sure-fire way to know is a visual inspection. When the ground begins to crack around the plants and the soil starts to push away, that means bulbing has started. Stop feeding the plants at that point. Those bulbs are now going to rely on the energy stored in those green tops to move down into the storage organ, which is what the bulb is, so no more feeding is necessary and too much could actually reduce your yield.
Follow these tips and you’ll be well on your way to growing big, beautiful onions! For more details on growing onions, check out Ep. 133 - Growing Onions of the podcast.
Your Friend in the Garden,
