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In a deep-winter greenhouse in a cold northern climate, the most dependable starting group is lettuce, herbs, brassicas, Asian greens, and sprouts. That is the crop-group list in the University of Minnesota Extension guidance on deep winter greenhouses. For a steady harvest, pair those crops with short sowing intervals: the Minnesota crop-planning figures use a new planting of leaf lettuce or spinach every 7 days and kale every 14 days. Treat both lists as a starting point. Light, heat, variety, and sowing date decide whether a given crop keeps pace in your structure.
Choose crops by the light and heat your structure actually provides
A greenhouse does not give every winter crop the same conditions. Before picking crops, find out what your structure can hold on its coldest and darkest weeks. A simple unheated or lightly heated house will favor hardy leafy crops; a heated house with supplemental lighting can hold a wider range.
- Lettuce: the most forgiving leafy option in the University of Minnesota group list, and the one most often harvested leaf by leaf.
- Herbs: listed alongside lettuce for deep-winter houses; growth slows sharply in low light, so expect small yields in the darkest weeks.
- Brassicas: including kale, which the Minnesota planning table treats as a longer-maturing crop with a wider interval between sowings.
- Asian greens: a fast-cycling group that suits frequent small sowings.
- Sprouts: listed in the same source. They are grown without soil and are not a substitute for a field-style planting schedule.
- Spinach: Penn State Extension discusses spinach and other greens as winter high-tunnel crops, and spinach is covered in its own cool-season guide (see the spinach section below).
Penn State Extension’s high-tunnel guidance is written for a structure with a different heat profile from a heated greenhouse, so use it for cool-season logic rather than as a setpoint list. The reverse also holds: a heated house does not make a warm-season crop suitable for winter.
Stagger sowings so the harvest arrives in steps
A single planting of a crop will mature in one burst. Repeated sowings spread that burst across weeks. The University of Minnesota crop-planning tools, reviewed in 2026, give the following regional planning figures for Minnesota. They are not a universal winter schedule, and they do not predict growth inside any particular greenhouse.
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| Crop | Days to maturity (UMN planning figure) | Planting interval (UMN planning figure) | What the interval does for you |
|---|---|---|---|
| Leaf lettuce | 40–60 days | Every 7 days | Weekly sowings keep a steady succession of leaves ready to cut. |
| Spinach | 35–45 days | Every 7 days | Same weekly rhythm; spinach is more sensitive to temperature, so check the section below. |
| Kale | 40–65 days | Every 14 days | A slower cycle, so fewer overlapping plantings are needed to keep cutting. |
The table is the source of the interval figures. Source: University of Minnesota Extension, crop and field planning tools for vegetable farmers.
How to work out a sowing calendar
- Pick the crop and take its maturity range from the table. Use the longer end if your house is cold or dark, because growth will be slower than the planning figure.
- Choose the planting interval. Leaf lettuce and spinach at 7 days give a new row every week; kale at 14 days gives a new row every two weeks.
- Count the overlap. Dividing the maturity range by the interval gives a rough number of plantings maturing at once. For leaf lettuce at 40–60 days and a 7-day interval, that is about 6 to 9 overlapping sowings, which is more than most small greenhouses have bed space for. Cut the number to what your beds can hold.
- Write the dates on a calendar and record the actual maturity day for each row. Adjust the interval after one winter of records, not after one week.
The overlap number is arithmetic on planning figures. It is a way to size your beds, not a yield forecast.
Spinach needs its own temperature plan
Spinach is the crop most likely to disappoint in a cold house, because its temperature needs are narrow. Penn State Extension’s spinach guide, updated July 16, 2026, gives an optimum growing temperature of 50°F to 60°F. Young seedlings may tolerate 15°F to 20°F. Those are crop-specific figures for spinach. They are not a setpoint for the whole greenhouse, and other winter crops will differ.
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The same guide makes three practical points:
- Spinach grows best in cool weather, so a winter house that overheats on sunny days can work against it.
- Keep the soil consistently moist. Uneven watering stalls growth more quickly than a cold night does.
- Use fresh seed. Germination rate declines as seed ages, which matters when you are sowing every week and holding a reserve in storage.
Source: Penn State Extension, growing spinach, a cool-season vegetable.
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Light is the first limit, and it does not improve with warmth
Protection from cold does not fix a shortage of light. Penn State Extension notes that high tunnels may not receive enough sunlight to support crop growth until about mid-February or early March, in its regional context, regardless of how warm the structure is. In practice, that means sowings made in December and January may sit nearly still even in a house that never freezes.
Plan around this in two ways. Sow the most cold-tolerant crops first, so their slow start is absorbed by the early weeks. Then time larger or more demanding sowings for late winter, when day length and sunlight are increasing. Check your own local sunlight data rather than relying on a regional date; a structure in a shadier site or a more northerly latitude will run later.
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Protect the crop, not just the house
Penn State Extension describes floating row covers and low tunnels placed over the crop as a way to trap heat around the plant canopy. The same guidance describes a tunnel-within-a-tunnel arrangement for cool-season production. These measures are options for crop protection. They do not show that every greenhouse needs the same configuration, and they add labor each time the cover is lifted for harvest or watering.
Venting cold frames
University of Minnesota Extension’s guidance on extending the growing season says that lettuce, kale, cabbage, cauliflower, and broccoli can perform well in cold frames during cool months, and that the cover should be vented during the day. This advice is written for a cold frame, not a heated greenhouse. If you apply it to a greenhouse, treat it as a ventilation rule for sunny days, and monitor temperature so the cover does not overheat the crop.
Source: University of Minnesota Extension, extending the growing season.
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Heat, ventilation, and power
Penn State Extension’s greenhouse production guidance, updated September 15, 2025, says winter production needs a heat source, along with ventilation and uninterrupted electricity. The same guidance says a backup generator is worth considering when storm-related outages are a risk. Whether winter crops pay for their heating depends on your structure, your fuel costs, and your local climate; the source does not give a general cost figure, and this article does not offer one.
- Heat: confirm the heat source can hold a night-time minimum your chosen crops tolerate before you commit to a sowing schedule.
- Ventilation: plan vents or fans for sunny days, since a closed winter house can overheat quickly even when it is cold outside.
- Power: any fan, heater, or temperature alarm depends on electricity. Decide in advance what happens during an outage.
Source: Penn State Extension, greenhouse production.
A checklist before the first sowing
- Know your structure’s coldest night temperature and your sunniest midday temperature over a full week.
- Choose two or three crops from the group list that match those readings, rather than a long list you cannot manage.
- Set a sowing interval for each crop and mark the dates on a calendar.
- Buy fresh seed and keep a record of germination and maturity dates for each row.
- Decide how you will vent, cover, and power the house before the coldest weeks arrive.
What these figures can and cannot tell you
The sources in this guidance support crop groups, maturity ranges, sowing intervals, and environmental limits. They do not give a yield for winter greenhouse crops that applies across regions, structures, and varieties, so this article does not promise one. If you need a production figure for your own site, record it from your own beds for a full season and compare it with the planning figures above.
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