Garlic goes in the ground in fall. Alabama cooperative extension puts the window at October through November, and extends it from October into February in the southern half of the state.1 Georgia extension recommends late September through November.2 The Central North Carolina planting calendar runs September 15 to November 30.3
Three neighboring states, three windows that disagree by months. That is not carelessness on anyone's part. It is what happens when one temperature requirement gets translated into calendar dates for places whose cold arrives at different times.
What the date is for
Garlic needs a stretch of cold after planting before it will form bulbs at all. Georgia extension puts the requirement at 6 to 8 weeks of weather below 40°F.2 Fall planting is how the crop collects it.
The other half of the work happens underground and out of sight. Roots and shoots emerge from the clove by the first hard freeze, but the shoots usually stay below the soil surface until the following spring.4 A clove planted with time ahead of hard cold roots in and then waits. A clove planted late goes into winter without that root system underneath it.
Placing your own date
A state-level window has to cover the whole state, which is why it is wide. Your own first frost date tells you where inside it you sit. In the upper South, where frost arrives in late October or early November, the front half of the window is the useful half. Along the Gulf, where the first frost comes weeks later or barely comes at all, the back half is, and Alabama extension carries the southern part of the state all the way into February.1
The practical version: plant while the soil is still workable and the hard cold is still ahead of you, not behind.
Getting it in the ground
Alabama extension calls for cloves about 1 inch deep and 4 to 6 inches apart, in full sun and a well drained bed with organic matter worked in.1 Georgia extension gives 1 to 3 inches deep and 6 inches apart, with rows 12 to 14 inches apart.2 Both are describing the same crop. The range is the room you have.
Mulch after the shoots come up. Georgia extension suggests 2 to 3 inches over the bed, kept 2 to 3 inches back from the stems.2 Minnesota extension, working in a much colder winter, uses 3 to 4 inches of leaf or straw to hold the soil temperature steady.4
Through the winter
Frost is not the enemy here. As NC Cooperative Extension puts it, garlic can withstand cold weather, and requires it to grow successfully.3 A bed that looks like it is doing nothing in January is usually doing exactly what it should.
Harvest lands in early summer. Alabama extension expects fall plantings to mature in late May or early June.1 Georgia extension gives a wider range of late May to mid-July depending on where in the state you are, and says to pull when one third to one half of the leaves have died back.2