Growing Onions in a Raised Bed
Raised beds are popular among gardeners with limited space or poor naturally occurring soils. Onions can be successfully grown in raised beds with certain precautions taken. Remember that onions are shallow-rooted, but the majority of the roots are intensely located within 8" of the plant. If you were to plant the lines of the bed closer than 8" it would double the competition for nutrients within that area.
The roots of the plant extend horizontally for up to 1' so it is important to have loose soil at least down to that level. It is fine to space the plants 4" down the row, but do not put the lines of onions any closer than 8" apart. The additional consequence of placing the lines of onions too close together is that it impedes the airflow through the onions and will keep the tops from drying out as quickly. This will lead to more disease pressure in the foliage.
As always, remember to spray fungicides on your onions to prevent the disease formation.