The variety is the first thing, says Susan. Then you must grow it properly. You sow them in a greenhouse any time between mid-December and mid-January. Youve got to use good soil compost. Good heat, good light, keep the temperature 15c at night. Then you plant them outside around mid-April. The die is cast in the propagation prior to planting. And you must put them outside when theyre at the right stage of maturity or youll never get big onions. The other thing is the ground pre