What to Sow in September and October: A Zone-by-Zone Greenhouse Sowing Calendar

Two limits decide everything you sow now

Spring sowing has one gate: is it warm enough yet. Fall sowing has two, and they close in opposite directions. Soil temperature is falling, which slows germination and eventually stops it. Day length is falling, which slows growth and eventually stops that too. A seed sown in early September gets warm soil and eight useful weeks of growing light. The same seed sown four weeks later gets neither.

This is why fall sowing advice keyed to calendar dates is close to useless. "Sow spinach in September" is right in Vermont and a month early in Georgia. What travels is the logic: sow to hit a target size before the light goes, and pick crops whose germination temperature matches the soil you actually have.

Every window below is measured against your average first frost date, the same anchor used in the fall changeover sequence. Find yours, then read the three windows as "six weeks before", "three weeks before" and "at or after".

Window one: six to four weeks before first frost

Soil is still warm, days are still over twelve hours, and this is the only window in the fall calendar where you can genuinely grow something to eating size before winter. Treat it as the last of summer rather than the first of winter, and do not waste it on crops that would be happier sown later.

What goes in now

• Fast salad leaves — loose-leaf lettuce, mizuna, mustards, rocket. Cut-and-come-again in five to six weeks, then again in spring.
• Spinach — the classic fall crop, and the one most people sow too late. Spinach germinates poorly in warm soil, so the front end of this window is often better than the back.
• Chard and leaf beet — slower, tougher, and will stand all winter under glass once established.
• Turnips and radish — genuinely fast, genuinely worth the bench space, and forgiving of imprecise timing.
• Overwintering brassicas for planting out — spring cabbage and similar, raised in modules under glass and moved to their final position before the ground goes cold.
• Coriander, chervil, parsley — herbs that resent summer heat and are far happier started now.

The one thing to get right

Sow into warm soil but keep it damp. A greenhouse bed in early September can be well over the temperature at which lettuce and spinach seed goes dormant rather than germinating, particularly in the top inch where the seed actually sits. Water the drill before sowing, sow into the cool damp, and shade the bed for the first few days if the house is still running hot. That single habit fixes most of the "my fall spinach never came up" complaints.

Window two: three to one weeks before first frost

Growth is slowing measurably now. What you sow in this window is being sown to overwinter, not to eat before Christmas. Sowing density goes up slightly, expectations go down, and you switch to crops chosen for cold hardiness rather than speed.

What goes in now

• Mâche (corn salad) — probably the single most reliable crop in this window. Tiny, slow, unbothered by cold, and productive in the depths of winter when nothing else moves.
• Claytonia (winter purslane) — the same argument as mâche, with better yield and a mild succulent leaf.
• Land cress and winter purslane — hardy, sharp-flavoured, and effectively unkillable under glass.
• Winter-hardy lettuce varieties — specifically the winter types, not summer varieties sown late. The difference in survival is stark.
• Spring onions — sown thickly in a module or drill, they sit through winter and bulk up early.
• Pak choi and tatsoi — fast enough to still make something, hardy enough to hold.

Sow these where you can protect them twice: under glass and then under a layer of fleece on the coldest nights. Two light layers beat one heavy one, and the second layer costs you almost nothing when it is not in use.

Window three: at first frost and into October

This is the allium and legume window, and it works on completely different logic from the leaf crops above. These are not being grown now. They are being positioned now, so that root development happens in cold soil over winter and top growth starts the instant conditions allow.

What goes in now

• Garlic — the headline crop of October. Many varieties genuinely need a cold period to split into cloves properly, so late sowing is not a minor timing slip, it is the difference between a head of garlic and a single fat round.
• Overwintering onion sets — started in modules under glass and planted out, or grown on under cover in mild areas.
• Broad beans — the hardy varieties only. Sown in root trainers now, they make a strong root system and crop weeks ahead of a spring sowing.
• Sweet peas — not edible, but the single best argument for an October sowing. Autumn-sown plants outperform spring-sown ones by a wide margin.
• Hardy annual flowers — calendula, cornflower, ammi and similar, overwintered as small plants under glass.

Why October sowings stop growing, and why that is fine

Somewhere in the back half of October, most temperate locations drop below roughly ten hours of daylight. Below that threshold, most crops effectively stop putting on new growth regardless of temperature. You can heat a greenhouse to a pleasant working temperature in December and your salad will still sit there doing nothing.

This is not a failure and it is not something you fix with a heater. It is a hard limit set by the sun, and it is the single most important thing to understand about winter growing. It has two practical consequences. First, anything sown after roughly mid-October is being sown for spring, not for winter. Second, everything you want to eat in the dark months has to reach usable size before the wall arrives — which pushes the real deadline weeks earlier than the frost date most people plan around.

The full explanation, including how to work out when the wall hits your latitude, is in the 10-hour light rule.

The same three windows, shifted by zone

Crop reference at a glance

Four ways fall sowings fail

Sowing summer varieties late

A summer lettuce sown in October is not a winter lettuce. Variety selection is doing most of the work in fall sowing, and seed packets label winter-hardy types clearly. This is the cheapest mistake to avoid and the most commonly made.

Sowing into soil that is too warm, then too cold

Warm soil suppresses germination in lettuce and spinach; cold soil suppresses it in almost everything. The workable band is narrower than people think, and it moves down through the fall. If you are sowing at the tail of a window, start in modules on a bench rather than in a cold border, where you can control the conditions and move the trays.

Watering like it is still July

The most common killer of a fall sowing is not cold, it is damping off in cool, wet, still air. Sow into damp soil, then water sparingly, in the morning, at soil level. Keep some air moving. This is the same discipline that keeps gray mold out of your standing crops — see autumn condensation and gray mold.

Leaving no protection margin

Glass alone is a few degrees. Glass plus a fleece layer over the bed is meaningfully more, and it is the difference between losing and holding a young sowing on a sharp early frost. Have frost cloth cut and ready before you sow, not ordered after the forecast changes.

What you need on hand before the first window

Fall sowing is cheap, but it is unforgiving about timing, so the kit needs to be sitting there in August. Tiers rather than prices, since seed and hardware pricing moves constantly: $ is a few dollars, $$ is a mid-range purchase, $$$ is a serious one-off.

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Common questions