September Planting Deadline: 6 Crops to Sow This Week or Miss Fall Harvest

September 1 is the real fall-sowing deadline, and it has nothing to do with frost. Spinach, kale, lettuce, radish, arugula, and turnip can all still go in the ground this week and reach a real fall harvest — but each one is running on a days-to-maturity clock that’s about to run out. Wait past this week and at least a couple of these won’t size up before growth stalls in October.

If you were hoping to still direct-seed bush beans, summer squash, or a fresh round of broccoli, that window closed weeks ago in most zones — those need 55 to 70+ days from seed, which meant sowing by mid-August. The six crops below are all fast enough to still make it, which is exactly why they’re worth your next hour outside.

Why September 1, Not Your First Frost, Is the Real Cutoff

All six of these crops are cold-hardy — University of Maryland Extension notes that kale, arugula, and turnips can overwinter with frost protection [1], so a light frost isn’t what kills your planting window. The real constraint is that fall growth is slower than spring growth, because daylight is shrinking instead of lengthening. UMD’s horticulturists account for this with what they call a Short-Day Factor: add 14 days on top of whatever days-to-maturity your seed packet lists [1]. A “25-day” radish is really a 39-day radish in September.

On top of that slowdown, leafy growth mostly stops once daylight drops below roughly 10 hours a day — market farmer Eliot Coleman named this the “Persephone Period,” and for most of the continental US it starts in mid-November, not October [4]. So September’s cutoff isn’t the daylight wall itself — it’s the runway you need to reach harvest size before that wall arrives. Sow too late and a crop doesn’t die; it just stalls at a few small leaves once the light budget runs out. That’s the mechanism behind every date in the table below.

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The Six Crops’ Sowing Windows

NC State Cooperative Extension charts these exact windows for its zone, where the average first frost lands around October 30 [2]. If your frost date is earlier — mid-October instead of late — shift every date roughly two to three weeks earlier.

The single biggest takeaway: lettuce has the tightest window of the six — it doesn’t open until September 1 — while radish and spinach have the most room to work with through mid-September [2].

My Zone 6a Countback

I garden in zone 6a, where the average first frost lands around October 15 — about two weeks earlier than NC State’s chart above [3]. Radish is the fastest of the six at roughly 25 days for a standard variety. Add the 14-day Short-Day Factor and I need 39 days of growing time before frost — counting back from October 15, that puts my own cutoff at September 6. As of today, that’s still a real week of room, so radish, spinach, lettuce, and arugula are all on schedule for me if I sow this week.

Kale and turnip are the tight ones up here. Baby kale (30 days + 14) needs to be in by roughly September 1, and fast turnip types (40 days + 14) needed to go in around August 22 — already past for a zone this cold. That doesn’t rule turnip out; sown now, it won’t size up into full roots, but it will still produce usable greens before the light runs out. If you’re in zone 7 or warmer, ignore this paragraph — NC State’s own windows above already have you covered into mid-September.

Sow This Week, Not This Weekend

Direct-seed all six rather than starting transplants — there isn’t time left for a transplant stage. Spinach and lettuce seed struggles to germinate once soil runs above 70–85°F [2], so water the row first and sow in early morning or evening if your soil is still warm from August. Radish tolerates repeat sowing better than any of the other five — put in a short row every four to five days through mid-September for a rolling harvest instead of one big flush; our radish succession planting guide covers spacing for that. Turnip is worth sowing even on the tight end of its window, since the greens alone justify the seed packet; see our turnip fall planting guide for greens-only timing. For the full fall lineup beyond these six — broccoli, carrots, peas — our fall vegetable garden guide has the complete calendar.

None of this requires new infrastructure. A cleared 3-foot row, a raked-smooth seedbed, and 20 minutes this evening is enough to get all six started before the window narrows any further. Keep the seedbed damp, not soaked, for the first week — fall rows dry out faster than they look like they should, since warm soil and shortening days trick you into watering less than a germinating row actually needs.

If this week genuinely gets away from you, radish and arugula still forgive a few extra days better than the other four; both tolerate sowing right up through mid-September in most zones with a slightly smaller harvest, not a failed one. Kale, turnip, spinach, and lettuce are the ones where a missed week costs you the crop, not just some size.

Sources

1. University of Maryland Extension. Fall Vegetable Gardening. University of Maryland Extension
2. N.C. Cooperative Extension, Wayne County Center. Time to Plant a Fall Vegetable Garden. NC State University
3. SoilStack. USDA Zone 6A Planting Calendar (NOAA 1991–2020 climate normals)
4. UC Cooperative Extension, Napa Master Gardener Column. The Persephone Period. University of California Agriculture and Natural Resources