i'd say 8" and i doubt that is adequate. when i pull my onions i then dig the area to prep it for planting small kale plants. i dig down 12" and i still see roots from the onions. not sure about garlic but i'd think they are similiar.
as to a follow up crop in may, i have no idea where in california you live or the temps/weather at that time of the year. swiss char does well in summer here just water it every day. cukes? bush or pole beans? broccoli will do amazingly well in the heat of summer (i used to say oh broccoli is a cool weather crop) if you water it every other day. 1/2 gallon for small plants 1 gallon for large. i never saw such high output as this summer even tho we had upper 90's and even into 100's with heat indexes above 100 with ultra high humidity for long periods of time. but i did something different this summer - i watered every other day due to the heat. cabbage, carrots, eggplant loves the heat, peas, peppers, zucchini. all these should produce by the time you need to use it again.
tom
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