Mechanically undercut onions when the tops are down and dry. If windrowed with the tops on, they should be handled in such a way as to cover the onions with their tops. Greening is a risk when white onions are exposed to excessive sunlight any time after bulbs begin to mature. To avoid greening, the lower portion of the onion plants may be covered with soil after bulbs begin to mature, or approximately 4-6 weeks before harvest. Research from Texas indicates that covering bulbs 4-6 weeks before harvest did not have a detrimental effect on yield or any effect on the incidence of rots.
Special equipment is used to cover onions with soil during the growing season or at harvest.
Some of this equipment consists of special cultivators that are used to replace soil that may have washed off the edge of the plant beds. The blades remaining lift soil from between onion beds and specially installed deflectors built into the rototiller hood deflect the soil over the onions. Care must be exercised to avoid burying growing plants. Although covering white onions whose tops have fallen will help reduce greening, topping such onions becomes more of a problem and needs to be considered. These machines top the onions after they are lifted and before windrowing or loading.