Dear Gardener,

Thank you for contacting us about growing onions. There is no advantage to pruning onions except to remove dead, damaged, or diseased leaves. As long as the leaves remain green they are photosynthesizing and providing energy to to root. Cutting the tops retards this process. 

You will know that your onions are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over.

I hope this information is helpful. Please write again whenever you have a gardening question.