Greenhouse Growing Calendar: How to Build One That Fits Your Location

Almost every greenhouse growing calendar you will find is written in months. Sow in February, transplant in April, harvest through October. It is comfortable to read and close to useless, because the person reading it might be in coastal British Columbia or in Kansas or in northern Alberta, and those three places do not share a single one of those dates.

The underlying schedule is real, though. It just is not expressed in months. It is expressed relative to two things that vary by location and can be looked up precisely for yours, and once you have those two anchors the whole year falls into place with very little guessing.

This page covers what those anchors are, how to find them, what a greenhouse genuinely shifts and what it does not, and how to build a calendar for your own site that will be right rather than approximately right.

The Two Anchors Everything Hangs On

The first anchor is your frost pair: the average date of the last spring frost and the average date of the first fall frost. In the United States these come from NOAA climate normals, and the current normals cover 1991 to 2020. They are probabilities rather than promises, usually published at several confidence levels, and any given year can run one to three weeks either side of the average.

The second anchor is less familiar and matters more in winter. It is the pair of dates when your day length drops below ten hours in autumn and climbs back above ten hours in late winter. Below roughly ten hours of daylight, most vegetable growth slows to almost nothing regardless of temperature. Eliot Coleman named the interval between those dates the Persephone period, and the term has stuck because the effect is real and sharply defined.

Both anchors are latitude and location specific, and both are free to look up. Any sunrise and sunset calculator will give you the ten-hour dates for your coordinates. The gap between them is what defines your winter, and it can run from a few weeks in the southern United States to well over two months in the northern prairies.

What a Greenhouse Actually Shifts

This is where most calendars quietly overpromise, so it is worth being precise about what the structure does.

During the day an unheated greenhouse is dramatically warmer than outside. It captures sunlight, warms the soil and surfaces, and on a clear day the interior runs far above ambient, which is why ventilation matters even in cold weather. Overnight the picture is very different. Cornell’s high tunnel research found unheated structures running only about four to five degrees Fahrenheit above outside air, because once the sun is down the same thin glazing that let the heat in lets it back out.

So the greenhouse alone is not what protects a crop from a hard freeze. What does that is a second layer inside it. Coleman’s account of his own unheated system describes each layer of cover moving the effective climate about one and a half hardiness zones south, so that outdoor Zone 5 becomes something like Zone 8 under an inner layer of row cover within the structure. Two layers, not one, is the arrangement that makes cold-season harvest work in a genuinely cold place.

For season length, Cornell’s New York work put the practical range at planting out around mid-April and growing until around mid-November, which is a meaningful extension at both ends rather than a transformation of the calendar. The structure moves your effective frost dates. It does not remove them, and it does nothing at all about day length.

What the Anchors Look Like in Practice

The daylight anchor varies with latitude in a way that is worth seeing before you look up your own.

Growers in central Virginia, at about 38 degrees north, report a period running roughly from 21 November to 21 January, a little over nine weeks. Johnny’s, working in Maine at around 44 degrees north, gives an end date of 6 February for their latitude, which makes for a noticeably longer dark stretch. A grower in Santa Fe describes theirs running from around Thanksgiving to mid-January. The pattern is consistent: further north means the window opens earlier, closes later, and lasts longer, and the difference between 38 and 44 degrees is several weeks at each end.

With both anchors in hand the year resolves quickly. Suppose, for the sake of the arithmetic, that your last spring frost averages 10 May, your first fall frost 5 October, and your day length crosses ten hours on 5 November and again on 5 February.

Spring falls out of the first date. Peppers and eggplant, at eight to ten weeks before last frost, get sown between 1 and 15 March. Tomatoes, at six to eight weeks, follow between 15 and 29 March. Brassicas go in from late March to around 12 April, and cucurbits wait until mid to late April. Nothing here requires judgement, only subtraction.

Autumn works from the November date instead. A spinach variety listed at 45 days to maturity needs about 34 of those days to reach three quarters grown, which counting back from 5 November lands at the start of October. That is the answer before any allowance for slow autumn growth, and the allowance is not small, so a sowing in the middle of September is the realistic target. This is exactly the calculation that catches people out, because early October feels like the obvious moment to plant a winter crop and it is already too late.

Spring: Counting Backward From Your Last Frost

Spring scheduling is arithmetic. You find your last frost date, then count backward by the number of weeks each crop needs to reach transplant size.

Crop group	Weeks before last frost	Notes
Onions, leeks, celery	10 to 12	Slowest to reach transplant size
Peppers, eggplant	8 to 10	Slow, erratic germination; want warm soil
Tomatoes	6 to 8	Started earlier they go leggy under weak light
Brassicas	4 to 6	Tolerate transplanting out before the frost date
Lettuce and greens	4 to 6	Also happily direct sown
Cucurbits	2 to 4	Resent root disturbance; often better direct sown

Published ranges differ by a couple of weeks between sources, and that is not sloppiness on their part. Transplant size depends on variety, on how much light the seedlings get, and on the temperature they are grown at, so the same crop can genuinely need six weeks in one setup and eight in another. Treat the table as a starting point and keep notes on what your own conditions produce.

The most common scheduling error in this whole list is starting cucurbits alongside tomatoes. Cucumbers and squash need two to four weeks indoors, so an eight week head start leaves them root-bound and stressed at transplant, and they spend most of the summer recovering rather than producing.

How the Greenhouse Changes the Spring Numbers

A greenhouse gives you two separate advantages in spring and they work differently.

The first is a better place to raise seedlings. Natural light in a greenhouse beats a windowsill comfortably, which means sturdier plants and less legginess, and it makes the earlier end of each range in the table more achievable than it would be indoors.

The second is an earlier transplant date, since the greenhouse soil warms sooner and the frost risk inside is lower. How much earlier depends on your climate and whether you add an inner layer, which is why the honest answer is to shift your effective last frost date by an amount you establish over a season or two with a minimum and maximum thermometer, rather than by a number someone published for a different latitude.

Soil temperature is the constraint people forget. Warm-season crops transplanted into cold soil sulk regardless of air temperature, so a soil thermometer is worth more in early spring than the calendar is.

Summer: The Season the Greenhouse Works Against You

From roughly the summer solstice onward the structure stops being an advantage and starts being a hazard, because the same efficiency that captures warmth in March overheats the space in July.

The calendar work in summer is mostly about ventilation, shading and the next season. Fall brassicas get started in midsummer, which feels wrong while the summer crop is still going and is the single most commonly missed window in the whole year.

Autumn: Counting Backward From the Ten-Hour Date

Autumn scheduling does not count backward from the first frost. It counts backward from the date your day length drops below ten hours, and this is the part that separates a working winter greenhouse from a disappointing one.

The rule from Johnny’s winter growing guidance is to seed so that plants are roughly three quarters of the way to mature size when the ten-hour date arrives. After that point they will hold, and you can harvest from them through the dark weeks, but they will not put on meaningful new growth until day length recovers. A crop that is only half grown when the light goes is a crop that stays half grown.

Working backward, that means taking the days to maturity on the seed packet, adding a substantial buffer for slow autumn growth, and sowing early enough that the crop is nearly finished before the date rather than nearly started. Successive sowings at seven to ten day intervals, tightening to a few days apart as the date approaches, give you a staggered supply rather than everything arriving at once.

Why Autumn Timing Is Less Forgiving Than Spring

In spring, a sowing made two weeks late will partly catch up, because every week that follows is warmer and longer than the one before. Growth is accelerating, and the calendar works with you.

In autumn the opposite is true. Every week is colder and shorter than the last, so a late sowing does not catch up. It falls further behind, and if it is late enough it gets caught by the ten-hour date at a size it will hold until February. This asymmetry is why experienced winter growers are far more careful about their August and September dates than their March ones, and it is worth building a week of margin into the autumn schedule that you would not bother with in spring.

Winter: A Holding Period, Not a Growing Season

Between your two ten-hour dates the greenhouse functions as a living store rather than a growing space. Hardy crops sit in the soil in good condition and you harvest from them as needed.

Soil temperature adds a lag at both ends of this period. Growers working at around 38 degrees north have observed roughly three weeks at the start of winter before the soil cools enough to actually stop growth, and another three weeks in January before it warms enough for growth to resume, so the practical dead period runs somewhat later than the daylight dates alone would suggest.

What you can grow through this window is a short list of genuinely hardy things: spinach, mâche, claytonia, kale, chard, leeks and hardy salad greens. Tender crops need heat and supplemental light, at which point the economics change completely and you are running a heated glasshouse rather than extending a season.

Late Winter: The Restart

When day length climbs back through ten hours, overwintered crops resume growth on their own, often weeks before anything outside shows signs of life. This is the most productive and least appreciated moment in the greenhouse year, because the plants are already established and simply need light.

It is also when spring sowing begins again, and the two overlap. Gaps opened by winter harvesting can be resown as they appear, and cold-hardy seed will germinate in soil in the low forties Fahrenheit, slowly at first and faster as it warms.

Building Your Own Calendar

Four steps produce a schedule that is right for your site rather than approximately right for a continent.

1. Look up your average last spring frost and first fall frost dates from NOAA climate normals, using the most recent set.
2. Use a sunrise and sunset calculator to find the dates your day length crosses ten hours in autumn and again in late winter.
3. Count backward from the frost date for spring sowings, and backward from the autumn ten-hour date for anything intended to overwinter.
4. Record what actually happens each season, including the real frost dates and the greenhouse minimums, and adjust the following year.

The fourth step is the one that turns a generic schedule into a good one. Two seasons of notes will tell you more about your particular structure and site than any published calendar can, because published calendars cannot know your latitude, your glazing, your aspect or whether you run an inner layer.