Question
can i grow onions,cabbages,radishes and beetroot alongside seetcorn or will it restrict the light too much

Answer
Dear Martin,
Radishes should be mature before the corn gets very tall, as well as green onions.  My recommendation would be to plant the corn north or east of these other vegetables, depending upon the row direction in your garden.  That way, the other vegetables will receive full south and west sun and the corn will be tall enough very quickly to obtain enough sunlight, as well.
I hope this helps.
Kind Regards,
Kindred
www.eingedi.us