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Garlic and its allium relatives stun the growth of certain crops (Beans
Peas, Sage,Parsley).
Onions and Garlic are great against fungus related tomato plant illnesses. Some people use them as a tea to spray on their tomato plants to prevent the fungus related "early blight" for example. Just to name one of many benefits. @dfinney

Thank you!

They last a long time. The ones you buy in the store have been out of the ground at least a few weeks already. Not sure of the science behind why you don't plant them together, that's what I was taught growing up. Mother Earth News and the farmers almanac have good companion planting guides that I've used also. Thanks for the comment.