The fall garden's one rule
Count backward from frost. Each crop needs its full days-to-maturity before the first fall frost, plus a cushion because September growth is slower than June growth — which is why fall sowing starts in high summer and why the window closes earlier than instinct says. The zone pages list every crop's window in order; the succession planner computes your personal last-viable-sowing date for any crop and town.
And then there's garlic
Garlic inverts the whole calendar: planted in fall, harvested next summer, and better for it — the winter chill is what makes bulbs divide into fat cloves. It's the single highest-reward fall planting in nearly every zone. Find your window in the table above, then see the garlic guide for varieties (hardneck for cold winters, softneck for mild ones), depth, and mulch.
What actually improves with frost
Kale, spinach, carrots, Brussels sprouts, collards, parsnips, and turnips all convert starch to sugar when lightly frosted — the botanical reason fall harvests out-eat spring ones. Tender crops get no such grace; the zone pages mark which is which, and your city's first-frost date marks the deadline.