hello again
The term onion sets refers to small bulbs that are planted and either harvested as green onions or( scallions ) or left to mature into a larger onion bulb. The mature onions from sets are about 2 to 2 and a half inches in diameter. This is typical size.These sets are planted in early spring about 1 and a half inches apart. They can be harvested as they develope green tops and nice clean bottoms.If you wish to allow them to mature simply harvest every other one as you need them . They can be , red , yellow or white . However they are not the large sweet spanish onions about the size of softballs. The large spanish onions come from tiny plants the size of a large blade of grass or a pencil size plant.The tiny ones are sold in small plastic trays and have been started in early spring from seed. The larger pencil size plants are grown in the south during winter. They are harvested , bundled and stored for spring sales in the north.they come in bundles secured with rubber bands.The ones intended for northern summer growth are " long day " onions .The short day onions are intended to be grown in the south during the more temperate cooler weather hence shorter daylight hours.This can be started early in the north. Again be wary of hard freezes regardless of what seed catalogs say.
The large sweet spanish also are red , yellow or white.They should be planted about 6 inches apart.Lots of sun.They are heavy feeders , needing phosphorus and potash and lesser amounts of nitrogen. There is another onion familiar to most of us. It is what we commonly call cooking onions. They are smaller and usually are very long storage oniopns. Most people do not grow them because they are readily available and relatively inexpensive. There is seed available for growing them though. Another onion is a periennial type called egyption or top onions. They develope a cluster of bulblets at the top of the green in fall. these can be stored or planted at that time. Simply break off , divide and plant for a very early spring crop. Or one can plant the stored bulblets in early spring.
Garlic is also very easy to grow.I am starting to Ramble I hope this helps . Happy gardening Johnny

This is going to be a really stupid question, but how deep in the soil should the onion sets be planted?

The varieties that were sold locally were red and yellow onion sets, so I'm assuming that both can be grown in northern Michigan?