There are a couple of weeks every year when the garden looks finished and isn't. The tomatoes are going over, the light has changed, the whole thing has the feel of something closing down. It's easy to read that as the end of the season and let the greenhouse coast into autumn.
It isn't the end of anything. What you do in the next three weeks decides whether you're cutting spinach in January or looking at an empty greenhouse from the kitchen window.
Four jobs. The first one is a calculation, and most people run it with the wrong number.
1. Count backwards - from the light, not the frost
Every seed packet tells you to work from your first frost date: count back the days to maturity, add a couple of weeks for autumn's slower growth, and don't bother sowing anything that won't finish in time.
That's sound advice if you're growing in open ground. In a greenhouse, frost is the problem you've already solved - a few degrees of overnight warmth and no wind push your effective frost date weeks past your neighbour's.
What a greenhouse can't do is make daylight. Sometime between early November and early December, depending on how far north you are, the day drops below ten hours and growth simply stops. Nothing dies. Plants sit there, green and perfectly healthy, and stop getting any bigger until the light comes back at the end of January.
That's your actual deadline. Anything you want on the table in winter needs to be more or less full-grown by early November, and most cold-hardy crops take eight to ten weeks to get there - which puts the sowing date somewhere in the next three weeks.
You can add lights, of course, and commercial growers do it as a matter of course. But there's a real difference between lighting that keeps a plant ticking over and lighting that actually drives growth. The second kind means genuine intensity, for long hours, across the whole footprint, through the darkest and most expensive months of the year - and it means having power run out to the greenhouse in the first place. For most backyard growers it's an expensive answer to a problem you can sidestep entirely by sowing three weeks earlier.
2. Start your fall crops
Spinach, lettuce, radishes and Asian greens all fit comfortably inside that window. They're also the crops that actually want what's coming - cool nights, softer light, no heat stress - and several of them taste better after a cold snap than they ever do in summer.
- Spinach - the most reliable winter green there is, and genuinely sweeter after cold
- Winter lettuce - choose hardy varieties; summer types sulk badly in low light
- Radish and turnip - fast enough that you can sow at the end of the month and still get a crop
- Asian greens - mizuna, pak choi, mustard - quick, cut-and-come-again, forgiving if you're late
- Kale and chard - slower, but they stand all winter and start again in February
- Cilantro and parsley - both prefer the cool, both bolt the moment it turns hot
If you're filling beds rather than trays, it's worth thinking about what goes next to what while the beds are empty - easier now than once everything is up.
Sow a little thicker than you would in spring. Germination gets less certain as the soil cools, and thinning a crowded row is a much better problem than staring at a bare one in October.
3. Ease off the heat lovers
At some point in the next few weeks your tomatoes and peppers stop being productive and start being optimistic. They'll go on making flowers and leaves right up until the cold gets them, and none of it will come to anything. The job now is to stop encouraging it.
Start with the feed. Nitrogen tells a plant to grow, which is the opposite of what you want - either stop feeding altogether or switch to something potassium-heavy that pushes ripening instead. Ease back on the water at the same time. A plant under slight stress puts its energy into finishing what it's carrying rather than making more, and the flavour concentrates as a bonus.
Then be ruthless with the plant itself. Pinch off new flowers, because nothing setting now is going to ripen. Cut the growing tip two leaves above the highest truss you realistically expect to get. Strip the leaves off the bottom third - more light onto the fruit, more air around the base, less mildew.
Green fruit still hanging when you finally clear the plant isn't a loss either. Cut whole trusses and ripen them indoors in a paper bag with a banana; it works far better than leaving them on a dying plant in an October greenhouse. And when the plants do come out, take the fallen leaves with them. The debris that collects in the corners and under the staging is exactly where next year's aphids and mildew spores spend the winter.
4. Cure and store
When the tops of the garlic and onions flop over, they're telling you they're finished. Lift them - but don't put them straight into storage. The drying that happens between the bed and the shelf is the entire reason they keep. Skip it and they'll be soft within a month. Do it properly and they'll see you through to spring.
Curing wants three things: warmth, moving air, and no direct sun. An early-September greenhouse gives you all three, which makes it about the best curing shed you'll ever have. Lay the bulbs out on staging or hang them in bunches somewhere out of the midday glare, leave the vents open, and give them two or three weeks. They're ready when the necks are papery and dry the whole way through.
Then move them out. The greenhouse is the right place to dry them and the wrong place to keep them; after curing they want somewhere cool and dark. Winter squash and pumpkins go through the same routine - a week or two of warmth to harden the skins, then storage.
The next three weeks may feed you all winter
None of this takes long. An afternoon for the sowing, twenty minutes with the tomatoes, and the garlic mostly just sits there doing its own thing.
The catch is that none of it feels pressing. There's no frost in the forecast, the greenhouse is still full, and it's easy to assume you've got until October. You haven't. You've got until the light goes, and that deadline arrives without any of the drama that would make you notice it.
Do the work now and January looks like spinach, chard and a handful of parsley. Leave it and January looks like glass.
All of it assumes you have somewhere to do it - and not every greenhouse holds up through a real winter.
Take a look at our polycarbonate greenhouse collection
Frequently asked questions
I've already missed the window. Is it too late to bother?
No, but change what you're aiming for. Sowing in October won't give you full-size heads of anything, so stop trying - sow thickly and harvest as baby leaf instead, which needs a fraction of the growing time. Radishes and mizuna will still make it. And a greenhouse that's too dark to grow in is still warm enough to overwinter tender perennials, root cuttings and hold pots that would die outside.
Do these dates change if I'm in a warm zone like 8 or 9?
Less than you'd expect, and this catches a lot of people out. Hardiness zones describe cold, not light - and it's light that sets this deadline. Phoenix and Atlanta sit at similar latitudes to parts of the Mediterranean and lose daylight on much the same schedule as anywhere else at that line. A warm zone buys you a longer growing season at both ends and a milder winter, but it doesn't move the ten-hour threshold. Your frost date shifts; your light date barely does.
Will the greens survive if the greenhouse drops below freezing?
Spinach, kale, chard and mustard will, and several of them get sweeter for it - the plant converts starch to sugar as antifreeze, which is why winter spinach tastes better than summer spinach. What they won't tolerate is being frozen and then thawed repeatedly, or being walked on and handled while frozen. Harvest in the afternoon once the leaves have softened, not first thing on a cold morning.
