Growing Onions in a Raised Bed
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.