Water, Water, Water.

Q: How can I grow my onions sweet rather than strong and spicy?

A: If your onions are too strong and spicy, you haven't watered them enough. Don't be fooled by healthy-looking green stalks. The stalks will be green even if the onions are too dry. The secret to sweeter onions is water, water, water.

Courtesy of The Old Farmer's Almanac