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Frost dates and frost-free days by USDA zone

The short answer

USDA zones describe average annual minimum winter temperature, not frost dates, and the two are often confused. Zone 6a averages a winter low of minus 10 to minus 5 degrees Fahrenheit and typically gets 150 to 180 frost-free days. An unheated polycarbonate greenhouse behaves roughly like one and a half zones warmer for season length, adding about 4 to 6 weeks at each end.

Zones listed
3a to 10b
What a zone is
Average winter minimum
Unheated greenhouse
about 1.5 zones
Cold frame
about 1 zone

Researched from published specifications and verified owner reviews · Updated 2026-08-16

A USDA plant hardiness zone is a single statistic: the average annual extreme minimum winter temperature over a thirty year period, in 5 degree Fahrenheit bands. That is all it is. It says nothing directly about frost dates, growing season length, summer heat or rainfall, and using it as a proxy for any of those is where planning goes wrong.

This distinction matters for greenhouse growing more than for outdoor gardening. Two places in the same zone can have frost-free seasons differing by six weeks, because one is coastal with a long cool shoulder season and the other is continental with a short sharp one. The zone tells you what your greenhouse must survive; the frost-free day count tells you what it must extend.

Figure 1. USDA zones, winter minimums and typical season length · 16 rows
Figure 1. USDA zones, winter minimums and typical season length
ZoneAverage annual minimumTypical last spring frostTypical first autumn frostFrost-free days
3a minus 40 to minus 35 degF Very late spring Very early autumn 90 to 110
3b minus 35 to minus 30 degF Very late spring Very early autumn 100 to 120
4a minus 30 to minus 25 degF Late spring Early autumn 110 to 130
4b minus 25 to minus 20 degF Late spring Early autumn 120 to 140
5a minus 20 to minus 15 degF Mid to late spring Early to mid autumn 130 to 150
5b minus 15 to minus 10 degF Mid to late spring Mid autumn 140 to 165
6a minus 10 to minus 5 degF Mid spring Mid autumn 150 to 180
6b minus 5 to 0 degF Mid spring Mid to late autumn 160 to 190
7a 0 to 5 degF Early to mid spring Late autumn 180 to 210
7b 5 to 10 degF Early spring Late autumn 190 to 220
8a 10 to 15 degF Early spring Late autumn 210 to 240
8b 15 to 20 degF Very early spring Early winter 225 to 260
9a 20 to 25 degF Late winter Early winter 250 to 290
9b 25 to 30 degF Late winter Midwinter 270 to 320
10a 30 to 35 degF Rare frost only Rare frost only 320 to 365
10b 35 to 40 degF Essentially frost free Essentially frost free 365
Frost windows are described in seasonal terms rather than dates because the same zone spans a wide range of longitudes and elevations, and a fixed date would be wrong for most readers. Get your own local averages from your regional extension service or a national climate normals dataset, then use the shift figures below.

How much does a greenhouse shift your zone?

This is the question the chart exists to answer, and the honest response has two parts, because a structure does different things to the two halves of the problem.

For season length, an unheated structure is genuinely effective. It traps solar gain during the day, and it blocks the radiative heat loss to a clear night sky that causes most light frosts. A polycarbonate greenhouse typically holds 6 to 10 degrees Fahrenheit above outside on a still clear night, which converts a light frost into a non-event and adds four to six weeks at each end of the season.

For winter survival, an unheated structure does far less than people expect. On a cold night with a hard freeze lasting many hours, an unheated greenhouse converges toward outside temperature, and by pre-dawn it may be only 2 to 4 degrees warmer. It is not a substitute for hardiness. A plant rated to zone 7 will not survive a zone 4 winter in an unheated greenhouse.

Figure 2. Effective shift from each structure type · 8 rows
Figure 2. Effective shift from each structure type
StructureNight lift on a clear still nightSeason length shiftWinter hardiness shift
Row cover, 0.55 oz fabric 2 to 4 degF about 1 to 2 weeks each end Negligible
Row cover, 1.2 oz fabric 6 to 8 degF about 2 to 4 weeks each end A fraction of one zone
Cold frame 5 to 10 degF about 3 to 5 weeks each end about 1 zone
Low tunnel with fabric 5 to 8 degF about 3 to 4 weeks each end about half a zone
Unheated film tunnel, walk-in 6 to 9 degF about 4 to 5 weeks each end about half to 1 zone
Unheated twin-wall greenhouse 8 to 12 degF about 4 to 6 weeks each end about 1 to 1.5 zones
Unheated greenhouse plus inner row cover 12 to 18 degF about 6 to 8 weeks each end about 2 zones
Frost-protected greenhouse at 38 degF Held at setpoint Year round for hardy crops Effectively zone 9
Night lift figures assume a clear, still night, which is when radiative frost occurs and when a cover helps most. On a windy night with a cold air mass the lift is substantially less. The double-cover row, a fabric layer inside an unheated greenhouse, is the single most cost-effective winter measure available.

Field tip

The double cover is the cheapest two zones you will ever buy

A layer of heavy row cover fabric draped over hoops inside an unheated greenhouse adds roughly as much protection again as the greenhouse itself, for the price of the fabric. The mechanism is that it blocks radiative loss from the crop to the cold glazing, which is where the heat actually goes on a clear night. This is the technique behind winter greens production in cold climates.

The other date that governs winter growing

Zone and frost date determine what survives. A third figure determines whether anything grows, and it is not a temperature at all. Below roughly ten hours of daylight, most crops effectively stop adding growth regardless of temperature, because the daily light integral falls under the threshold discussed in the DLI targets chart.

The length of that dark window depends only on latitude, and unlike temperature you cannot heat your way out of it. At 40 degrees north it runs roughly ten weeks. At 45 degrees north, roughly thirteen. At 50 degrees north, about sixteen. Crops sown before it starts will hold through it and resume growth on the far side, but crops sown into it will germinate and then sit. The practical rule is to have winter crops at harvest size before the window opens.

Figure 3. The under-ten-hour daylight window by latitude · 6 rows
Figure 3. The under-ten-hour daylight window by latitude
LatitudeExample locationsWeeks under 10 hoursPractical consequence
30 degrees N Central Florida, Houston, Phoenix 0 No dark window. Winter growing is temperature limited only.
35 degrees N Memphis, Albuquerque, Raleigh 4 to 5 Brief pause. Most hardy greens keep growing slowly.
40 degrees N Denver, Philadelphia, Columbus 9 to 10 Real pause. Sow winter greens by late summer.
45 degrees N Minneapolis, Portland, Toronto 12 to 13 Substantial pause. Harvest-size plants before it starts.
48 degrees N Seattle, Winnipeg, Quebec City 14 to 15 Long pause. Winter production is holding, not growing.
52 degrees N Edmonton, northern Europe 16 to 17 Supplemental light is the only route to winter growth.
Daylength depends only on latitude and date, so these figures are exact rather than climatic averages. This is why two growers in the same USDA zone but at different latitudes have very different winter greenhouse experiences.

Shifting your effective zone

The four items that produce the shifts in the table above, in order of cost per degree gained.

Turn your own frost dates into a sowing calendar with the frost date sowing calculator, check accumulated heat with the growing degree day calculator, and see which crops survive which minimum in the temperature tolerance chart.

Related on this site

Common questions

6 answers

+ What does a USDA hardiness zone actually tell you?

Only one thing: the average annual extreme minimum winter temperature over a thirty year period, in 5 degree Fahrenheit bands. It does not tell you frost dates, growing season length, summer heat, humidity or rainfall. Two locations in the same zone can have frost-free seasons six weeks apart. Use the zone to decide what a plant must survive and local frost averages to decide when to sow.

+ How many zones warmer is a greenhouse?

For season length, an unheated twin-wall greenhouse behaves like about one and a half zones warmer, adding four to six weeks at each end. For winter hardiness it is worth less, roughly one to one and a half zones, because on a long hard freeze an unheated structure converges toward outside temperature. A layer of row cover inside the greenhouse adds roughly another half to one zone for very little money.

+ How much warmer is an unheated greenhouse at night?

On a clear, still night, typically 8 to 12 degrees Fahrenheit above outside, because the structure blocks radiative heat loss to the sky. That is enough to turn a light frost into a non-event. On a windy night with a cold air mass moving through, the difference can fall to 2 to 4 degrees by pre-dawn, which is why an unheated greenhouse is not a substitute for plant hardiness.

+ Can I grow year round in an unheated greenhouse?

You can harvest year round in most of zones 6 and warmer, but you will not grow year round. Hardy greens sown by late summer will hold through winter and can be cut repeatedly, but they add very little new growth during the weeks when daylength falls under ten hours. The practical target is to have plants at harvest size before that window opens rather than expecting them to grow through it.

+ What is the ten hour daylight rule?

Below roughly ten hours of daylight, most crops stop adding usable growth because the daily light integral falls below their threshold, regardless of temperature. The length of that window depends only on latitude: about nine to ten weeks at 40 degrees north, twelve to thirteen at 45, and sixteen or more at 52. Heating does not shorten it, and only supplemental lighting does.

+ Should I use my zone or my frost dates to plan sowing?

Frost dates, always. Sowing schedules are counted backward from the average last spring frost and forward to the average first autumn frost, and the zone number does not encode either. Get local averages from your regional extension service, then subtract the greenhouse or cold frame shift for anything you are growing under cover.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.