Every September the same list goes around: broccoli, cabbage, Brussels sprouts, kale — plant your fall garden now. It is a good list. It is also, for a lot of us, about three weeks out of date by the time we read it.
Here is the part that never makes the headline: a fall planting date is not a date, it is a countdown. You don't plant a fall crop when the weather finally turns pleasant. You count backwards from your first frost, and if the number lands before today, the window shut while you were still thinking about it.
The countback, and the bit everyone forgets
Start with the days to maturity on the seed packet. Add about two weeks for the length of time you actually want to keep picking — you want a harvest, not a single meal. Then add another two weeks, and this is the one people skip: the fall factor. The number on that packet was measured in spring, when days are lengthening and the soil is warming. In autumn your crop grows into shortening days and cooling ground, and it takes visibly longer to finish. Virginia Cooperative Extension puts that adjustment at roughly 14 days; Maryland calls it the short-day factor and uses 10 to 14. Either way, leaving it out is the most common reason a fall garden goes in "on time" and still doesn't finish.
One more adjustment. If frost kills the crop outright, add another two weeks so it matures safely before the freeze instead of racing it. Hardy crops skip that step — they're allowed to finish at or after the first frost, and several of them taste better for having been through one.
What that actually means in September
Run those numbers against my own garden — Middle Tennessee, first frost around November 3 — and the answer is blunt. Broccoli, cabbage, cauliflower and kale from seed: that window closed in August. Brussels sprouts closed in July. Carrots, beets, chard, peas, bush beans, cucumbers — all closed, some of them a month ago.
What's still open in the second week of September comes down to one crop: radishes, with a few days to spare. The rest of the fast, cold-tough end of the catalog closed just before it — spinach around September 1; leaf lettuce, turnips and dill around August 27; cilantro, the slowest of them at 45 to 70 days, around August 22. Radishes are the most forgiving thing on the list — twenty-odd days from sowing, which means you could still be eating them in October if you sow this week. And if one cutting of baby leaves will do instead of two weeks of picking, drop the harvest fortnight from the sum: spinach and leaf lettuce then stay open until about September 10–15.
And one crop isn't late at all, it's early. Garlic doesn't want to go in until October, once the soil has cooled enough that it puts down roots without pushing up top growth. Shallots are the same. Plant garlic now because it feels like fall and you'll get leaves before winter and a smaller bulb next summer.
Those are my dates, not yours. Move a hundred miles north and the whole thing shifts a fortnight. We built a planting calendar for every location so you can read your own first-frost date and your own fall windows instead of borrowing mine — or start from the crop if you already know what you want to grow.
Where the moon comes into it
Honestly? Further down the list than the frost date, and we'd rather say so than pretend otherwise. If the countdown says the window has shut, no phase of the moon reopens it.
But when you do have a real window — and radishes give you a couple of weeks of one — the old rule is worth keeping: sow root crops as the moon wanes, leafy things as it waxes. It costs nothing. And it quietly makes you do something good for reasons that have nothing to do with the sky: it splits one sowing into two batches a week apart, which is succession planting, which is how you get three weeks of salad instead of one enormous Saturday of it.
That's the honest version of this whole idea. The moon sets a rhythm. The frost sets the deadline.
Sources: Virginia Cooperative Extension 426-334, Fall Vegetable Gardening; University of Maryland Extension, Fall Vegetable Gardening. Frost dates are 1991–2020 NOAA climate normals at the 50% probability level.
Want your own countdown instead of ours? Moon Phase Garden keeps your frost dates, your planting windows and tonight's phase in one place, works offline, and will tell you when a window is about to close. Free and ad-free.