Planting onion bulb sets
Onions are quite possibly one of the easiest vegetables to grow inside containers or outside in the garden. They do not require much of anything after the bulbs have been planted in good soil. As their green tops grow, I use them as needed when cooking by cutting the tops off with kitchen shears. I grow the majority of my onions in containers in my greenhouse.
Prior to planting, add compost to your soil. Loosen up the soil by adding sand to it (if needed). If you're planting your onion sets in containers, add loose gravel to the bottom of them. Then fill the containers with your fertilized soil. Having good drainage is so important. It will keep the onion bulbs from rotting and getting diseased.
Next, plant the onion bulbs in the fertilized soil. Make sure that the pointed end is placed in an upward position and the root end is placed downward.
The onion bulbs need to be placed about an inch and a half deep. Then water the bulbs and cover the tops of them with additional soil.
When spacing the onion bulbs in containers, I plant them close about one and a half inches apart (to harvest small onions). The spacing all depends on how soon you will be planting and harvesting them.
For full-sized, mature onions, allow four to six inches between bulbs when you're planting them. Doing this allows the onions to have plenty of room to grow into full maturity.
Once the bulbs have grown green tops, add about four inches of mulch around each onion. Doing this will help to keep the soil from drying out as fast and to keep weeds from taking them over.
I water my onion bulbs every couple of days for the first few weeks of sprouting. After that, I water them as needed. I never add any more fertilizer to the soil.