Garlic in USDA zone 7b (average min temp -17.78 C) can grow alright as an understory, but only during the fall-early spring months before your trees leaf out. We grew three garlic plants each under a willow oak (dense shade) and river birch (moderate shade) along with our main plantings in full sun. As soon as shade sets in the garlic retreats to bulb form, but will return in the fall. Although I have to add that the overall temperature underneath even the oak can pass 30 C, so it is probably a combination of shade and high heat!
My favorite understory plant this year were volunteer tomatoes that grew under the same oak tree. But then again, we just started this past year so our experience is very limited.