This year I have compared growing seed onions sown directly in the ground with some sown 4 to a module and planted out.
This came about because I sowed the seed in the ground and then decided that I needed a backup so sowed some more in modules. The direct sown were OK so I decided to plant the modules also.
The variety was F1 Hytech from Suttons. A 10 ft row direct sown and 10ft planted with modules. Rows 12 inches apart
The seed was sown at about 1 inch spacing in the ground on February 23rd under a polythene hoop cloche. My soil is very light so I can do this.
The modules were sown on 26th February with 4 seeds to a module on a window sill, and were put outside in the daytime once germinated if the weather was kind.
Those in the ground appeared towards the end of March. The modules were planted out at 12 inch spacings on 7th April, by which time they mostly had 3 leaves and looked slightly ahead of the direct sown ones.
The direct sown were thinned to 3 inch spacing. Some thinnings were transplanted to fill the odd gap but the majority were eaten as spring onions. A bonus crop.
Yields were as follows
Direct sown 38 onions weighed 4.550Kg. Average weight 119g.
Modules 34 onions weighed 4.132Kg. Average weight 110g
Although raising plants in modules and planting them out was extra hassle they were easier to manage after they were planted. This was because by then many of the weeds had germinated and been killed and clumps of 4 onions at 12 inch spacing are more easily hoed than a line of tiny seedlings in the direct sown ones.
By harvest many of the direct sown ones were touching each other. They were very even, but this would be expected as they had been thinned to 3 inch spacing.
The clumps of 4 pushed each other aside and mostly developed OK . 2 plants were lost, hence 34 onions from 12 modules, and there were two or three that were small that had not competed with the other 3 in the clump.
On any but very light soil I would go for modules. On the other hand the there were no spring onions to be thinned from the modules.