Don't give up on growing onions. Big firm bulbs are simple once you know the facts about how onions grow:
1) Onions are not roots.
The true roots emerge from the little plate on the bottom surface of the onion. The bulb itself is made from the swollen bases of the leaves. Each "ring" is a leaf. So, the more leaves your onion plant grows, the bigger the bulb will be. To grow lots of leaves, you need moisture and fertile soil. Onions need more fertility and organic matter than true roots like carrots. Compost will give them what they want.
Onions need a long time to grow. So it is very important to start these slow-growing plants very very early. Now is the time--January and February sowings produce the best onions.
2) Timing is everything.
Once the plant has grown lots of leaves, it is day length, (not the age of the plant or the temperature), that makes the leaf bases swell into a big bulb. You need the plants to size up before they get the cue to start bulbing. Sow in January or Febuary for big bulbs.
The most important effect of day length sensitivity is that you need to choose a variety adapted to your latitude. If the onion plant doesn't get the right amount of light and dark, it will not bulb up. The further north you go, the longer the days are in summer. In the south, days and nights are more equal in length. If you are north of a line drawn from San Francisco to Washington DC, you can use long-day or intermediate-day onion varieties. South of that line, choose short-day varieties. There are a few varieties that don't care; these are called day-length neutral and can be grown anywhere.
This work by Bountiful Gardens is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.