The rule, stated plainly
Below roughly ten hours of daylight, most vegetable crops effectively stop putting on new growth. Not slow down — stop, or near enough that the difference does not matter to you. They stay alive, they stay green, they hold whatever size they had reached, and they wait.
Warmth does not fix it. You can heat a greenhouse to a pleasant working temperature in December and your winter lettuce will sit there doing nothing at all. The limit is light, and there is no thermostat for the sun.
The idea is often called the Persephone period, a term popularised by grower Eliot Coleman for the stretch of the year when day length falls below that threshold. Whatever you call it, it is the single most useful concept in winter greenhouse growing, and it is the reason so much fall planting advice quietly fails.
What this changes.
Anything you want to eat in winter has to reach usable size before the threshold arrives. Anything sown after it is being sown for spring. That pushes your real deadline weeks earlier than the frost date most people plan to — and it means a heater buys you far less extra growing than you would expect.
When it arrives where you are
Day length depends on latitude, and the threshold arrives correspondingly later the further south you are. Rough guide, for the northern hemisphere:
| Approx. latitude | Falls below 10 hours | Returns above 10 hours |
|---|---|---|
| 50°N (southern UK, Vancouver) | Early November | Early February |
| 45°N (Maine, Minneapolis, Milan) | Mid November | Late January |
| 40°N (New York, Denver, Madrid) | Late November | Mid to late January |
| 35°N (Tennessee, Oklahoma, Tokyo) | Early December | Early January |
| Below ~30°N | Never falls below | — |
Check the exact dates for your location with any sunrise and sunset table — find the day where the gap between the two first drops under ten hours, and note it. It is the same date every year, which is the one genuinely convenient thing about this whole subject.
Two things worth noting from the table. First, the window is symmetrical around the winter solstice, so if you know when it starts you know roughly when it ends. Second, it is shorter than most people assume — even in the north it is around twelve weeks, not the whole winter.
Why light and not temperature
Photosynthesis is powered by light, and a plant has a daily running cost — respiration — that continues whether the sun is up or not. Growth is what is left over after the day's photosynthesis has paid the day's respiration.
As days shorten, two things happen at once. The photosynthetic day gets shorter, and the sun sits lower, so the light that does arrive is weaker and passes through more atmosphere. Below a certain total, the day's production barely covers the day's costs and there is nothing left over to build new tissue.
Now add the greenhouse. Your glazing takes a share of the incoming light. A layer of bubble insulation takes another share. Condensation on the glass takes more. A heavy fleece over the bed takes more still. Inside a lined, covered greenhouse in December, plants can be operating at a fraction of the already-scarce outdoor light.
Which produces the central irony of winter growing: every layer you add to keep plants warm makes it darker, and darkness is the actual constraint. This is why we argue against fully lining a greenhouse and in favour of covering plants rather than heating air.
What to do about it
Front-load the calendar
Work backward from the threshold date, not the frost date. If your ten-hour date is mid November and a crop needs eight weeks to reach cutting size, it has to be sown by mid September — regardless of the fact that frost will not arrive until late October.
Size the plants, do not grow them
The aim is to have plants standing at a good size on the threshold date. They will hold there through the dark weeks and you harvest from that standing crop. This is why the fall sowing calendar is organised around windows: each one targets a different size at the same deadline.
Pick crops that hold well
Some crops sit through the dark in good condition — mâche, claytonia, land cress, winter lettuce, chard, spring onions. Others deteriorate while they wait. Choose for holding quality rather than growth rate at this end of the season.
Harvest differently
Cut lightly, take outer leaves rather than whole heads, and give the plant time to recover — which in the dark weeks means several weeks, not several days. A bed harvested at summer intensity in December will be stripped and will not come back until February.
Prioritise light over heat
Clean the glazing before winter, insulate the north side and end walls rather than the south, and lift bed covers on bright days. Every one of those is worth more than the equivalent spend on heat.
Where supplemental lighting fits
Adding light does work, and it is the only thing that does. But be clear about what it takes: extending the day past ten hours means running lights through the darkest quarter of the year, at an intensity that actually contributes rather than just looking bright, over the whole growing area.
For most hobby growers that is not a sensible trade against buying a bag of salad. Where it does earn its place is on a small propagation bench — a couple of square feet of seedlings under a light in February, getting a head start on spring, rather than a whole house lit through December.
The kit that helps
Compact grow light for a propagation bench
$ to $$ · Small areas only · not whole-house lighting
Where supplemental light genuinely pays is a small bench of seedlings in late winter, not a lit greenhouse in December. Match the fixture to the bench area rather than the room, hang it close, and run it on a timer to a set day length rather than leaving it on continuously.
Propagation grow lights on AmazonPropagation grow lights on eBay
Light meter or lux app-compatible sensor
$ · Diagnosis · find out what your glass is costing you
Worth borrowing or buying once. Measuring outside, then inside, then inside under the bed cover shows you exactly how much light each layer is taking — and it usually changes how much insulation people are willing to leave up through midwinter.
If you are further south
Below roughly 30 degrees of latitude the threshold is never crossed, and winter growing is limited by temperature and rainfall rather than light. The rest of the fall sequence still applies — cleaning, sealing, pest management, soil work — but the sowing calendar opens up considerably, and a greenhouse becomes a tool for moderating heat and wet rather than for extending a light-limited season.
Between 30 and 35 degrees the window is short, a few weeks either side of the solstice, and easy to work around by sowing a fortnight earlier than instinct suggests.
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Common questions
What is the Persephone period?
The stretch of the year when daylight falls below roughly ten hours, during which most vegetable crops effectively stop putting on new growth regardless of temperature. The term was popularised by grower Eliot Coleman. It runs symmetrically around the winter solstice and is shorter than most people expect.
Will heating my greenhouse make plants grow in winter?
Not once day length is below the threshold. Heat keeps plants alive and prevents frost damage, but growth is powered by light, and below about ten hours a day there is not enough of it left over after the plant's own running costs. Heating for growth in midwinter is largely wasted fuel.
When does my greenhouse hit the 10-hour threshold?
It depends on latitude: roughly early November around 50 degrees north, mid November at 45, late November at 40, and early December at 35. Below about 30 degrees it never falls under ten hours. Check a sunrise and sunset table for your location — the date is the same every year.
Does insulation make the light problem worse?
Yes, and it is the central trade-off of winter growing. Glazing, bubble film, condensation and bed covers each take a share of already-scarce light. This is the argument for lining the north side and end walls rather than the whole house, and for covering plants rather than heating air.
Are grow lights worth it in a hobby greenhouse?
Rarely for a whole house through winter, because you are lighting a large area through the darkest quarter of the year at an intensity that actually contributes. They earn their place on a small propagation bench in late winter, giving seedlings a head start on spring.