Most greenhouse owners I know run two seasons, not four. Tomatoes from May to September, then the door shuts and stays shut. The house sits idle through the months it could be feeding them salad. I did exactly that for three winters before I worked out what I was throwing away.
A greenhouse is a light trap with a roof. In winter the light is weak and short, and the cold is only half the problem. Once that lands, the calendar rearranges itself. You stop asking what will survive the frost and start asking what can make usable growth on eight hours of grey December daylight. Different question. Different answers.
What follows is the rotation I have settled into in an unheated house, plus the extension research I lean on when my own notes are thin. The mistake I made in my first greenhouse winter was sowing in November and expecting a crop by February. Nothing came. Sowing date decides almost everything under glass.
The greenhouse year is shaped by light, not the calendar
Greenhouses do not experience four tidy seasons. They experience phases. From the winter solstice to late June, day length roughly doubles across temperate latitudes, and that curve drives leaf expansion, germination, and whether a lettuce hearts up or simply sits.
Purdue Extension describes the winter high tunnel season in the lower Midwest as running from September to May, around nine months, and notes that for roughly three months in the middle, plant growth is likely to be very slow because of low light, low temperature, or both. That middle stretch is the part most growers never plan for. They sow for the shoulders, then wonder why January gives them nothing.
I plan five phases instead. September and October build the crop. November through January holds it. February and March restart growth. May to August is a heat management job. Late August hands the house back to the second autumn. I keep a cheap max-min thermometer, because the overnight low in a closed house on a clear January night decides whether my winter greens keep growing or sit and wait.
Day length also picks your varieties. Onion types are classified by the daylight hours that trigger bulbing, with long-day varieties needing 14 or more and short-day types under 12. A variety bred for a southern latitude will not bulb properly in a northern one.
Winter is about holding a crop, not pushing it
Nothing in an unheated house grows much in December. That is fine once you stop fighting it. The job in winter is to have a crop standing that is already big enough to eat, then keep it clean and alive until the light comes back. Cold-hardy plants do best when soil temperatures stay above about 40°F, and acclimatised plants can survive freezing and still be harvestable once thawed.
My winter house carries spinach, winter lettuce, mizuna, claytonia, land cress, and a row of kale I cut leaf by leaf. Kale and spinach are the workhorses. Purdue notes that bok choy, tat soi, and kale hold their quality through repeated freezing better than most baby greens, which matters if you cut and come again rather than lifting whole plants. I sow kale in late August now, not September. A fortnight of extra growth in warm soil beats anything I can do in January.
Two habits do more for me than any heater. I water in the morning, once the soil has thawed, because wet foliage through a cold night is how grey mould starts. And I throw a second layer of fleece over the hardiest crops when the forecast drops below 25°F, the threshold Purdue’s scheduling work assumes for row cover. I do not heat this house. I insulate it and accept slow growth.
Soil temperature decides when a sowing is worth making
Seed packets quote air temperatures. Soil temperature decides whether a seed does anything. The University of California Master Gardener programme publishes a table from the California Master Gardener Handbook giving minimum, optimum, and maximum soil temperatures for germination, and the gaps between crops are wide. Lettuce has a minimum of 32°F and an optimum of 60 to 75°F. Cucumber will not germinate below 60°F. Tomato germinates at 50°F, but takes over 40 days there, against six to eight days at optimum.
That one fact runs my propagation bench. In a cold house in March, bench soil can sit around 50°F. Fine for lettuce. Hopeless for cucumber. Colorado State Extension draws the line from the other end, describing hardy vegetables as growing with daytime temperatures as low as 40°F and semi-hardy types as wanting 40 to 50°F.
| Crop | Minimum soil temperature | Optimum range |
|---|---|---|
| Lettuce | 32°F | 60-75°F |
| Spinach | 32°F | 65-75°F |
| Onion | 32°F | 65-85°F |
| Peas | 40°F | 65-75°F |
| Carrot | 40°F | 65-85°F |
| Radish | 40°F | 65-85°F |
| Beet | 40°F | 65-85°F |
| Cabbage and broccoli | 40°F | 60-85°F |
| Swiss chard | 40°F | 65-85°F |
| Tomato | 50°F | 65-85°F |
| Cucumber | 60°F | 65-95°F |
| Pepper | 60°F | 65-75°F |
| Squash | 60°F | 85-95°F |
Sources differ by a few degrees. Colorado State lists lettuce at 35°F and spinach and peas at 40°F, a little more conservative than the California figures. Neither is wrong. What matters is the pattern. Cold-house crops germinate in cold soil. July crops sulk in it. I read a probe thermometer in the bench each morning instead of trusting the forecast.
Spring turns the house into a furnace by mid-morning
Spring is the season people get wrong in the other direction. Everyone worries about frost and nobody worries about the ten o’clock sun. The RHS puts the damage threshold at around 27°C (81°F), advises some shading from mid-spring until early autumn, and warns against over-shading because light drives growth. Keep an edible house below 25 to 27°C and let in as much light as you can while you do it.
Ventilation is the first tool, and size matters. The RHS notes that in larger glasshouses, ridge vent area equal to about 20% of floor area gives a complete air change every two minutes. UMass Extension quotes the ASAE recommendation that sidewall vent area should equal ridge vent area, each at 15 to 20% of floor area, and that a fan moving one air change per minute to a height of eight feet still leaves an 8 to 10°F rise between the intake louvre and the fan. That number explains why a house cooks with every vent wide open.
In my own house the temperature jump between nine and eleven on a clear April morning is the sharpest of the day, so I open the door early rather than waiting until I feel the heat. Hardening off matters just as much. The RHS advises moving plants outdoors on overcast days, because sudden strong sun scorches leaves even when the nights are mild. I lost a tray of courgettes learning that one.
Summer is a survival exercise, and shade is the cheapest fix
UMass Extension says plainly that in many parts of the United States, cooling a greenhouse is a greater problem than heating. Shading is the low-cost answer, and it belongs outside the glazing rather than behind it, where fabric re-radiates the heat it caught. Research they cite from North Carolina State found that 50% shade cut radiation by about 10°F. Fabrics run from 10% to 90% shade.
Heat drags humidity with it, and humidity drags disease. Penn State Extension reports that tomato leaf mould prefers humidity of 85% or higher and temperatures between 50 and 90°F, which describes a closed house on a warm July afternoon. Air movement is the fix. UVM Extension’s agricultural engineering guidance is 8 CFM per square foot of growing space in the hot season and 2 CFM in the cool season.
Long seasons under one cover create a problem of their own. Penn State’s high tunnel soil testing found that soluble salts accumulate in stationary four-season tunnels, because the plastic keeps rain out and drip irrigation limits leaching, and that average levels in their conventional tunnels landed in the strongly saline range. Beets tolerate that. Peppers do not. If a house has been covered for years and yields are drifting down, a soil test beats another feed. I ran one after four seasons and it explained a pepper crop I had blamed on the weather.
Watering changes too. I damp the paths in the late morning, water deeply and less often, and vent hard before midday. The site’s guide to keeping a greenhouse from overheating covers the rest of that routine, and there is a companion piece on summer greenhouse crops if you are choosing what to fill the beds with.
Autumn decides what you eat in January
This is the part that took me longest to accept. Autumn sowing dates do not shift harvests by a few days. They shift them by months. Purdue’s scheduling trials in Indiana and Ohio show kale transplanted on 30 August reaching first harvest on 27 September, 28 days later. Move that planting to 29 September and harvest lands on 9 November, 41 days. Move it to 14 October and it arrives on 20 January, 98 days from transplant. Plantings on 28 October and 11 November never reached a harvestable stage. They bolted first.
Spinach behaves the same way. Direct seeded on 31 August it was ready on 28 September. Sown on 31 October it took 120 days and arrived in late February. Lettuce heads transplanted in mid-October cropped on 1 December, while a February planting took 70 days to reach the same point in mid-March. Early autumn sowings do the work. Late ones borrow light they never receive.
The RHS recommends planting lettuce into beds or growbags in autumn to crop through winter, sowing hardy lettuce cultivars and pea shoots for spring cutting, and starting broad beans and peas to overwinter and go out once the soil is workable. Their spring guide adds corn salad and land cress sown in mid to late summer under a cloche, and oriental salad leaves, which germinate well in autumn. My own rule is blunt. If it is not up and growing by the middle of October, it will not feed me in December. The wider winter gardening guide covers frost protection in more detail.
What I sow, and when
Kale and spinach repay attention above all others, because they hold quality through hard frost and keep cropping as the light improves. Radish and salad turnips fill gaps fast. Carrots and beets take fifty days or more and are usually better value grown outside and stored, which is Purdue’s own assessment.
Cold-house crops for a late summer sowing
Kale, spinach, mizuna, claytonia, land cress, winter lettuce, radish, salad turnip, and chard. Sow from the third week of August through mid-September for a genuine winter crop. Use modules and transplant if the soil is still hot, because spinach germinates poorly when the ground stays warm into early September.
Warm crops for the summer house
Tomatoes, cucumbers, peppers, aubergines, and melons. Sow peppers and aubergines in February, because they are slow producers with the RHS suggesting a February to early March sowing for a long season. Time tomatoes so they are ready to plant when the first flower opens, which the RHS recommends as the signal, and bury any leggy stem. Cucumbers want warm roots and sulk if the night temperature drops near 10°C. UF/IFAS gives 80 to 85°F by day and above 62 but below 72°F at night for tomatoes, and warns against nights below 60°F, because grey mould and blotchy ripening both get worse. Lycopene, the pigment that reddens a fruit, stops forming above 86°F. A hot August house gives you pale, uneven tomatoes.
None of this needs expensive equipment. A thermometer, a shade net, a roll of fleece, and a willingness to sow earlier than feels natural will carry a house through twelve months. The kit matters less than the timing. Most winter failures I see, including my own early ones, were decided in August.
Sources and further reading
UC Master Gardener Program: Seed Germination Temperature and Timing: the germination table above, and the 40-day tomato figure at 50°F.
Colorado State University Extension: Vegetable Planting Guide: cool-season and warm-season germination minimums, and the 40°F hardy threshold.
Purdue Extension: Scheduling Fall and Winter Vegetable Production: the September to May season, the 25°F row cover threshold, and the harvest date figures.
Penn State Extension: Soluble Salts Levels in High Tunnels: why salts accumulate in four-season tunnels, and the tolerance gap between beets and peppers.
Penn State Extension: Managing Leaf Mold in High Tunnels: the 85% humidity and 50 to 90°F conditions that favour tomato leaf mould.
RHS: Vegetables, growing in your greenhouse: the seasonal cropping sequence and autumn plantings for winter harvest.
RHS: Greenhouse, ventilation and shading: the 27°C damage threshold, vent area at 20% of floor area, and damping down.
RHS: Vegetables, growing for spring: corn salad and land cress under cloches, and overwintered broad beans and peas.
UMass Extension: Ventilation for Greenhouses: the ASAE vent area guidance, and the 8 to 10°F rise per air change.
UMass Extension: Shade for Cooling Greenhouses: shading outside the glazing, and the 10°F radiation cut at 50% shade.
UVM Extension: Ventilation in Greenhouses and High Tunnels: the 8 and 2 CFM per square foot seasonal rates.
UF/IFAS: Production of Greenhouse Tomatoes: the day and night temperature range and humidity targets.
Oregon State University: Types of Onions and Varieties: the short-day, intermediate-day, and long-day classes, and their bulbing day lengths.