Time the season from your frost dates
Cool-season and warm-season crops, frost tolerance classes, soil temperature, seed-starting dates, succession sowing and the fall planting formula.
- Start here: the essentials
- Understand: two seasons, four frost classes
- Soil temperature and the soil thermometer
- Counting back to a seed-starting date, and succession sowing
- The fall planting formula, and garlic
- Why extension services disagree
- See it: a squash that does not fit, and a balcony with no frost
- Practice: date your crops
- Your garden
- Check your work
- Knowledge check and scenario
Start here Essentials
This module turns your frost dates into planting dates for your own crops. The planting calendar does the subtraction. Everything marked Go deeper is optional on a first pass.
- Enter your frost dates in setup, and note whether they are the 50 percent or the 10 percent dates. The calendar counts from them.
- Read your crops' rows on the planting calendar. Each row gives the window for the crop's frost class. From class to window.
- Check each crop's fit. A row marked "Does not fit between your frost dates" needs transplants, a variety with fewer days to maturity, or a different crop. Does a crop fit at all?
- Let the soil and the forecast overrule the calendar. Sow when the 8 a.m. soil reading, 4 inches down (6 for beans), is at the crop's minimum and the ten-day forecast shows no frost. Soil temperature.
- Mark your first three planting dates on your own calendar.
Minimum time: about 30 minutes, plus a few spring mornings with a soil thermometer. Done when: three planting dates are on your own calendar.
1. Two seasons of crops and four frost classes Understand
Module 2 gave you two dates: an average last spring frost and an average first fall frost, read as probabilities. This module turns those two dates into a date, or a window, for every crop on your plan. It is the module the planting calendar is built from, and the calendar uses exactly the rules taught here. Nothing in the calendar is hidden: each column is a published convention you can check by hand.
Timing rests on one physical fact and one convention. Physical principle Frost kills or damages plant tissue that cannot tolerate ice forming in or on it, and plants differ widely in how much cold they tolerate. Convention Extension services group crops by that tolerance and give each group a planting window counted from the average last spring frost.
Cool-season and warm-season crops
Colorado State University Extension's Vegetable Planting Guide (GardenNotes #720) divides vegetables into two seasons. A cool-season crop prefers growing temperatures of about 60 to 80 °F and loses quality in hot weather, so it is planted in early spring and often again in midsummer for a fall harvest. Peas, lettuce, spinach, cabbage, broccoli, radish, onion, beet, carrot and potato are cool-season crops. A warm-season crop prefers summer weather of about 70 to 95 °F, cannot tolerate frost and may be set back by cool spring winds. Beans, cucumbers, squash, sweet corn, tomatoes, peppers and eggplant are warm-season crops.
Frost tolerance classes
Within each season, Colorado State sorts crops into a frost tolerance class: a group of crops that tolerate the same amount of cold and so share a planting window. There are four.
- Hardy crops grow with daytime temperatures as low as 40 °F and may survive a frosty night. Colorado State lists broccoli, cabbage, onions, lettuce, peas, radish and spinach among them. Purdue Extension's Indiana Vegetable Planting Calendar (HO-186) adds kale, which Colorado State does not list.
- Semi-hardy crops grow with daytime minimums of 40 to 50 °F and are less tolerant of a frosty night: beets, carrots and potatoes among the course's crops.
- Tender crops need daytime temperatures above 55 °F and do not tolerate frost: beans, cucumbers, summer squash and sweet corn. The University of Minnesota treats basil as a tender annual planted after the danger of frost.
- Very tender crops do not tolerate frost or cool spring winds, need daytime temperatures above 60 °F and prefer 70 to 95 °F: tomatoes, peppers, eggplant and winter squash. Colorado State adds that a week of daytime temperatures below 55 °F may stunt them.
Garlic sits outside the four classes because it is planted in fall; the garlic section below covers it.
From class to window
Convention Colorado State gives each class a spring window counted from the average last spring frost. The calendar uses these windows.
| Class | Course crops | Plant outdoors | Also wait for |
|---|---|---|---|
| Hardy | Pea, lettuce, spinach, cabbage, broccoli, radish, onion; kale (Purdue) | As early as 2 to 4 weeks before the average last frost | Soil dry enough to work; minimum soil temperature |
| Semi-hardy | Beet, carrot, potato | As early as 0 to 2 weeks before | Soil dry enough to work; minimum soil temperature |
| Tender | Bush bean, cucumber, summer squash, sweet corn; basil (University of Minnesota) | From seed, around the average last frost. Cucumber and summer squash transplants without frost protection wait until frost potential is over | Minimum soil temperature |
| Very tender | Tomato, pepper, eggplant, winter squash | Typically 2 or more weeks after | Days consistently above 55 °F and breezes without a cool nip |
Converting a window to dates is subtraction. If your average last frost is April 20, hardy crops can go out from March 23 (four weeks before) to April 6 (two weeks before); semi-hardy crops from April 6 to April 20; tender crops from seed around April 20; very tender crops from May 4 onward. Every one of those dates rests on the frost date you entered. If you entered the 50 percent date, half of past years had a frost after it. Purdue's HO-186 frost maps put the point plainly: there is a 10 percent chance of a 32 °F frost after the mapped date, and about two weeks earlier that chance rises to 50 percent. Module 2's frost-date section explains the choice; entering the 10 percent date moves every window about two weeks later and lowers the risk. The choice of risk is yours.
The window opens; it does not oblige. Colorado State's own wording for hardy crops is "as early as" two to four weeks before, and it lists two more conditions before the date: soil dry enough to work, and soil at the crop's minimum temperature. Cornell Cooperative Extension's Rockland County guide gives a field test for the first: a handful of soil squeezed in your hand should crumble somewhat, not hold as a wet ball. Working wet soil makes clods.
If your area rarely or never frosts, these windows do not describe your season. Heat, not frost, usually sets the limits there, and the calendar computes no dates; Module 2's section on rarely-frosting climates covers what to use instead, and Marisol's example below shows it.
Soil temperature and the soil thermometer
The frost date is a long-run average of air temperature. A seed responds to the soil it sits in, this week. Purdue's HO-186 says that in years when cool air and rain keep the soil from warming, warm-season crops such as beans, tomatoes, squash and sweet corn should wait until the soil reaches at least 60 °F; planted into cold soil, they are likely to suffer seed and root rot and related disorders. The University of Minnesota calls soil temperature the best indicator of when to plant, and gives two bands: peas, spinach, lettuce and radish can be direct-seeded when the soil is 40 to 50 °F; warm-season crops need 50 to 60 °F.
What the numbers mean Go deeper
Physical principle Germination speed depends on soil temperature, and below a crop's minimum, seed germinates poorly or not at all. The clearest data are J. F. Harrington's laboratory measurements, published by Oregon State University Extension, which the course uses for every minimum soil temperature. Harrington measured how many days seedlings took to appear from seed sown half an inch deep at a range of constant soil temperatures. A few rows show what cold soil does:
| Crop | Minimum (°F) | 41 °F | 50 °F | 59 °F | 68 °F | 77 °F |
|---|---|---|---|---|---|---|
| Lettuce | 35 | 15 | 7 | 4 | 3 | 2 |
| Pea | 40 | 36 | 13 | 9 | 7 | 6 |
| Carrot | 40 | 51 | 17 | 10 | 7 | 6 |
| Tomato | 50 | little or none | 43 | 14 | 8 | 6 |
| Sweet corn | 50 | little or none | 22 | 12 | 7 | 4 |
| Bean | 60 | little or none | little or none | 16 | 11 | 8 |
| Cucumber | 60 | little or none | little or none | 13 | 6 | 4 |
A pea sown into 41 °F soil took 36 days to come up; at 59 °F it took 9. A seed that sits in cold, wet soil for a month is a month exposed to rot and to anything that eats seeds. The minimum is not the target. It is the temperature below which sowing is mostly wasted; warmer soil, up to each crop's optimum, brings faster emergence. Harrington's optimums are useful mainly for starting seeds indoors, where you control the temperature (Module 7).
Two sources, two sets of numbers. Colorado State's own table gives minimums too, and some differ from Harrington's: Colorado State gives snap beans 55 °F, Harrington 60 °F. The course does not mix them. The calendar and the crop guide use Harrington's minimums throughout, so one crop's figure is never compared with another source's figure for a different crop.
How to read a soil thermometer
A soil thermometer is a probe thermometer with a stiff metal stem, sold for the purpose. What matters is where and when you read it. Convention Colorado State's GardenNotes #720 gives the method in a footnote to its tables: plant when the soil reaches the crop's minimum temperature measured at 8 a.m., 4 inches deep, except beans, which are measured at 6 inches. The early-morning reading matters because soil cools overnight and warms through the day; an afternoon reading on a sunny day overstates the temperature a seed sits in overnight.
- Mark the depth. Put a piece of tape on the stem 4 inches from the tip (6 inches for beans). If the probe is hard to push in, make a pilot hole with a pencil or dowel of the same size so the stem is in contact with soil, not air.
- Read at 8 a.m. Push the stem in to the tape in the bed you will sow, in the open, not under mulch or in the shade of a wall unless that is where the seed goes. Wait until the reading stops changing; the thermometer's instructions say how long that takes.
- Read for several mornings. One warm morning after a mild night is not a trend. Write each reading in the sowing sheet's notes with the date, and sow when the readings are at or above the crop's minimum and the ten-day forecast holds no cold snap.
- Read each bed you sow. A raised bed in full sun, a shaded corner and an in-ground plot can differ. Your records show how much. Test it
A thermometer settles the question for one spot on one morning. Some states publish networks of soil temperature stations; Minnesota's, which the University of Minnesota points to, is an example. Readings from a station are a guide to the trend, not a reading of your bed.
The planting calendar computes windows from published conventions and your own frost dates. It cannot see this year's weather. A cold spring moves everything later, and a warm one can tempt you early. When the calendar says a window is open but the 8 a.m. soil reading is below the crop's minimum, or the ten-day forecast shows a frost, the soil and the forecast overrule the calendar. Wait, or protect the planting.
Counting back to a seed-starting date, and succession sowing
Transplant age
Tomatoes, peppers, eggplant, cabbage and broccoli are usually set out as transplants: young plants started indoors or bought. The planting window above is the date the transplant goes outdoors. To grow your own, you count back. Transplant age is the number of weeks from sowing a seed indoors to a plant ready to set out. Colorado State gives tomato 5 to 7 weeks, pepper 6 to 8, eggplant 6 to 9, broccoli and cabbage 5 to 7, and cucumbers and squash 2 to 3.
Seed-starting window = outdoor planting date minus transplant age. With a last frost of April 20, tomatoes go out from May 4. Counting back 7 weeks (49 days) from May 4 gives March 16; counting back 5 weeks (35 days) gives March 30. Sow tomato seed indoors between March 16 and March 30. Peppers at 6 to 8 weeks from May 4: March 9 to March 23. The calendar's "Start indoors" column does exactly this subtraction, using the earliest outdoor date minus the longest age for the start of the window and the latest outdoor date (or the earliest, for very tender crops) minus the shortest age for its end.
Vine crops are the exception that the numbers hide. Colorado State says cucumber, squash and melon roots are extremely intolerant of disturbance, and that these crops do best from direct seeding; if you use transplants, choose small young plants not more than 2 to 3 weeks from seeding. Root crops are usually sown directly too. Minnesota's beginner planting guide says radishes, carrots and beets do not tolerate transplanting, but Minnesota's guidance for vegetable farms describes growers transplanting beets from paper pots. Direct sowing is the simplest starting method for beets; young beet seedlings can also be transplanted carefully. Module 7 covers which crops to transplant and how.
| Crop | Colorado State #720 | Cornell, Rockland County | University of Maine, Cumberland County |
|---|---|---|---|
| Tomato | 5 to 7 | 4 to 6 | 6 to 8 |
| Pepper | 6 to 8 | 6 to 8 | 8 to 10 |
| Eggplant | 6 to 9 | 6 to 8 | 8 to 9 |
| Broccoli | 5 to 7 | 6 to 8 | 5 to 7 |
| Cabbage | 5 to 7 | 6 to 8 | 5 to 8 |
| Cucumber | 2 to 3 | less than 4 | 2 to 3 |
Cornell and Maine count back from the last frost date; Colorado State gives the age of the transplant, which you count back from your outdoor planting date. For very tender crops those differ by two weeks or more. The calendar uses Colorado State's ages for the crops it lists, and Cornell's for lettuce, kale and onion (6 to 8, 6 to 8 and 10 to 12 weeks), and the University of Minnesota's 6 to 8 weeks for basil. The ranges overlap more than they disagree. A transplant that is a week younger or older than the range is still a transplant; a tomato started in January for a May planting is not.
Succession sowing Go deeper
A single sowing of lettuce or beans matures all at once and then is done. Virginia Cooperative Extension's "Intensive Gardening Methods" (426-335) uses two terms. Succession planting, in Virginia's sense, is planting something new in the space a finished crop leaves, such as corn after peas, or a fall crop after a summer one. Relay planting (Virginia's "relaying") is sowing the same crop several times so the harvest continues. Many sources, including the course's calendar, call both "succession." What matters in a plan is the interval.
Convention The University of Kentucky's Franklin County extension office publishes interval groups credited to Johnny's Selected Seeds, a seed company. The calendar's succession column uses them.
| Interval | Crops |
|---|---|
| 7 days | Leaf or baby lettuce, radish, spinach |
| 10 days | Sweet corn, bush beans, head lettuce |
| 14 days | Beets, basil |
| 21 days | Cucumbers, carrots |
| 30 days | Summer squash |
Other services frame it differently. Virginia's 426-335 suggests sowing one variety about every two weeks, with more time between early sowings in cold soil and about 10 days between the last ones, or sowing two varieties with different days to maturity on the same day. Virginia's planting guide (426-331) suggests sowings two to three weeks apart within the planting window. Purdue's HO-186 says several plantings within the window raise the chance of success and spread the harvest, and that if you make only one, make it about midway through the window. These agree in substance: small, repeated sowings in place of one large one.
Succession ends somewhere. Each later sowing has less season left, and the last useful sowing is set by the fall formula below. A first-year gardener can reasonably skip succession entirely: one sowing of each crop teaches the timing, and the second year adds the repeats.
The fall planting formula, and garlic
Spring planting counts forward from the last frost. Fall planting counts back from the first fall frost, and it answers two questions with one calculation: what is the latest date a crop can be sown and still mature, and does a crop fit between your frost dates at all.
Convention Nebraska Extension's "Fall Vegetable Gardening" (G1343) gives the formula, and Virginia Cooperative Extension's "Fall Vegetable Gardening" (426-334) gives the same one:
Days to count back from the first fall frost =
days from seeding or transplanting outdoors to harvest
+ days from seed to transplant size (only if you grow your own transplants)
+ the average harvest period
+ the fall factor (about 14 days)
+ the frost-tender factor (14 days, for frost-sensitive crops only)
The two factors are what make the formula more than subtraction. The fall factor accounts for slower growth in the cooler weather and shorter days of late summer and fall; Nebraska and Virginia both put it at about 14 days. Cornell's Rockland County guide gives the same rule in plainer form: add two weeks before counting back for a summer or fall crop. The frost-tender factor is another 14 days, added only for frost-sensitive crops; Nebraska names corn, beans, cucumbers, tomatoes and squash, which must mature at least two weeks before frost to give a reasonable harvest. The harvest period is how many days you want to keep picking before frost. No source the course found publishes harvest periods per crop, so the calendar sets it to 0 days and gives you a field on the calendar page to enter your own; a longer period moves every latest-planting date earlier.
Days to maturity come from the variety. Nebraska says to choose cultivars with the fewest days to maturity for fall plantings, using the figure on the seed packet or in the catalog; the calendar uses Colorado State's typical figures (Module 5 covers what they count from). When your packet gives a different number, use the packet's.
Worked: the Okafor beans and lettuce Go deeper
The Okafor garden (fictional; its frost dates are invented) has a first fall frost of October 20.
- Bush bean, last sowing. Colorado State gives 60 days from seed. Beans are frost-tender. 60 + 0 (direct sown) + 0 (harvest period) + 14 (fall factor) + 14 (frost-tender factor) = 88 days. October 20 minus 88 days is July 24: the last date to sow beans for a fall harvest by the formula.
- With a harvest period. If they want two weeks of picking before frost, add 14: 102 days, so July 10.
- Leaf lettuce, fall sowing. 60 days, hardy, so no frost-tender factor: 60 + 14 = 74 days. October 20 minus 74 days is August 7.
- Tomato from their own transplants. 65 days from transplanting (Colorado State's 65T), frost-tender: 65 + 14 + 14 = 93 days, so the latest transplanting date is July 19. Counting back the 7-week transplant age gives a latest seed-starting date of May 31. That is far beyond what a household usually needs, but it shows how a second, late tomato planting would be dated.
To count back by hand, subtract whole months first and then days: October 20 back to September 20 is 30 days, to August 20 is 61, to July 20 is 92; 88 days is four days later than July 20, so July 24. The calendar does the same arithmetic.
Does a crop fit at all?
The same arithmetic run forward tells you whether a long crop fits between your frost dates. Take the crop's earliest outdoor date from its spring window, and its latest planting date from the fall formula. If the latest date falls before the earliest one, the crop does not fit, sown directly, as calculated. The calendar marks those rows "Does not fit between your frost dates." There are three ways to respond: grow transplants, so part of the crop's time is spent indoors before the outdoor window opens; choose a variety with fewer days to maturity on the packet; or extend the season at one or both ends with row covers or a cold frame, which Virginia's 426-331 says may extend the planting and harvest windows by two to four weeks in spring and fall (Module 12 covers season extension). The Lindqvist example under See it works through all of this with numbers.
Other services' fall rules Go deeper
Purdue publishes two shortcuts for Indiana. HO-186 says cool-season crops for fall go in at least 4 to 8 weeks before the first fall frost, and to consider days to maturity and choose faster cultivars. Purdue's "The Fall Vegetable Garden" (HO-66) gives last feasible planting dates for northern and southern Indiana; for example, leaf lettuce (40 days) by August 25 in northern Indiana and bush beans (50 days) by August 15. HO-66 also notes the fall equivalent of the spring 10 percent date: on the average first frost date the chance of frost is 50 percent, and two weeks earlier it is about 10 percent.
Johnny's Selected Seeds' succession chart gives last-sowing rules too, such as bush beans "up to 8 weeks before first frost date." For the Okafor garden that is August 25, a month later than the formula's July 24. The chart does not show its arithmetic, and a seed company's chart is written for its own varieties and growers, many of whom use row covers. The course's calendar uses the formula and labels the chart as one company's practice.
Late-summer sowing has a hazard of its own. Nebraska and Purdue both say lettuce, peas and spinach germinate poorly when the soil is 85 °F or above, and suggest shading the soil with a board or light mulch until the seedlings emerge. Virginia's 426-334 adds that presprouting seed indoors can shorten the fall factor by two to five days.
Garlic: the fall-planted crop
Garlic runs on a different clock. University of Illinois Extension's "Planting and Growing Garlic" explains why: garlic needs a cold period to form bulbs, so it is planted in fall, grows roots before winter, and is harvested the next summer. Convention Illinois's timing is 4 to 6 weeks before the ground freezes. It gives a depth of 1 to 2 inches, pointed tip up, and spacing of 4 to 5 inches in rows 15 to 18 inches apart, or 5 inches in all directions for a stand, with straw mulch to prevent frost heaving. Spring planting is possible only after about 8 weeks of refrigerating the cloves to meet the chilling requirement.
The ground freezing is not on any frost-date table, so the calendar computes no garlic date. It says "fall, 4 to 6 weeks before the ground freezes" and points to your extension service. Virginia's planting guide (426-331) shows what a local answer looks like: in its zone 6a and 6b tables, garlic goes in from September 1 to October 1. That is Virginia's window for those zones, not a rule for anywhere else.
Garlic also holds its space for most of a year, which matters in a plan. In a community garden with a fall clearing rule, like Plot 14 in Module 5, garlic planted in October conflicts with a November 1 clearing date unless the garden allows overwintering crops. That is the coordinator's decision, asked in writing before the cloves go in.
Why extension services disagree
If you read more than one planting calendar, you will find the same crop in different classes with different windows. This is not an error in one of them. Each is a convention built for one state's climate, by people who drew the lines where their gardeners' experience put them.
| Crop | Colorado State #720 | Purdue HO-186 (Indiana) | Utah State (groups A to D) | K-State MF315 (frost resistance) |
|---|---|---|---|---|
| Pea | Hardy | Hardy | A, hardy | Hardy |
| Lettuce | Hardy | Semi-hardy | B, semi-hardy | Half-hardy |
| Potato | Semi-hardy | Hardy | B, semi-hardy | Half-hardy |
| Kale | Not listed | Hardy | Fall list only | Hardy |
| Tomato | Very tender | Tender | D, very tender | Tender |
| Cucumber | Tender | Very tender | C, tender | Very tender |
| Summer squash | Tender | Very tender | C, tender | Very tender |
| Winter squash | Very tender | Very tender | D, very tender | Very tender |
| Windows relative to the average last spring frost | ||||
| Hardy | 2 to 4 weeks before | 4 to 6 weeks before | A: as soon as the soil dries | Not given as offsets |
| Semi-hardy | 0 to 2 weeks before | 2 to 4 weeks before | B: about 2 weeks before | |
| Tender | Around the date | After the date | C: on the date | |
| Very tender | 2 or more weeks after | At least 2 weeks after | D: about 2 weeks after C | |
Look at tomato: Colorado State and Utah State call it very tender and plant it two weeks after the last frost; Purdue and K-State call it tender, planted after the last frost. Cucumber goes the other way. Purdue even disagrees with itself: HO-186 lists potato as hardy, while its fall publication HO-66 lists potato as semi-hardy. Utah State says outright that its groups come from interviews with extension faculty and experienced gardeners around the state, and its city table shows how far local practice moves from the formula. For Farmington, with an average last frost of May 5, Utah State gives Group A crops March 15, about seven weeks before the frost; for some towns it lists very tender crops before the frost date only with protection.
The publications do not explain their differences, so the reasons are our reading, not theirs. Climate is one: Colorado State's class descriptions mention cool spring winds, and Indiana's springs are not Colorado's. Purpose is another: Purdue's HO-186 classes also carry minimum air temperatures, and K-State's table is a frost-resistance column, not a planting rule. A third is that a window is a judgment about acceptable risk, made by people who know local springs.
What the course does with this: the calendar uses Colorado State's classes and windows for every crop Colorado State lists, Purdue's class for kale, and the University of Minnesota's for basil. It does not average two sources, because an average of Indiana's and Colorado's windows describes neither place. It shows the disagreement, here and on the calendar page, so you know which convention you are following. Your own extension service's calendar is built for your state. Where it differs from the course's calendar, follow your extension service; you can find yours through the Extension Foundation's state directory. Ask the authority
Where extension services disagree on a crop's window, no single answer is correct everywhere. The course's calendar follows one named source and shows the others; it does not claim its windows are better than your state's. Your local extension calendar and your own soil thermometer readings carry more weight for your garden than any national rule, and a season's records (Module 12) are how you find your own garden's offsets.
2. See it: a squash that does not fit, and a balcony with no frost See it
Both cases are fictional. Their frost dates are invented for the examples and are not lookups for any real place.
The Lindqvist winter squash. Nils and Karin Lindqvist garden in a short season: last spring frost May 25, first fall frost September 15, 113 days apart. They want winter squash for storing.
- Known: frost dates May 25 and September 15; Colorado State gives winter squash 100 days from seed and classes it very tender; the fall factor and frost-tender factor are 14 days each (Nebraska G1343).
- Assumed: that a transplant started indoors 3 weeks before planting out removes about 21 days from the outdoor growing time. This is the household's working assumption, not a published figure; Colorado State says squash transplants should be no more than 2 to 3 weeks from seeding, which is why they stop at 3. Harvest period: 0 days, the calendar's default.
- Open: whether the 85 days printed on the packet are counted from sowing or from transplanting (settled by the seed company's catalog description; Module 7 explains why it matters); whether the variety matures in their garden in 85 days (settled by their own harvest log, which records the date of the first ripe squash); whether a 4-ft fence keeps the deer off it (it did not last year; settled by extension's published guidance on fence height and their own trial, covered in Module 10).
Earliest outdoor date. Very tender: 2 weeks after the last frost. May 25 + 14 days = June 8.
Direct seeding, 100-day variety. 100 + 14 + 14 = 128 days before September 15 is May 10. The latest sowing date (May 10) falls before the earliest safe date (June 8). From June 8 to September 15 is 99 days, and the crop needs 128. It does not fit, short by 29 days.
Transplants, 100-day variety. With the assumed 21 days spent indoors: 100 - 21 = 79 outdoor days, + 14 + 14 = 107 days needed against 99 available. Still 8 days short; the latest outdoor date would be May 31, a week before June 8.
Transplants, shorter variety. They find a variety whose packet prints 85 days. If the 85 days are counted from sowing, and the 21 days indoors come off that count as they assume: 85 - 21 + 14 + 14 = 92 days, against 99 available. Under those assumptions it potentially fits, with the latest outdoor planting date September 15 minus 92 days, June 15. If the 85 days are counted from transplanting, nothing comes off: 85 + 14 + 14 = 113 days against 99, and it does not fit. So before they order, they check the catalog description for which count the 85 days use. If it is from sowing, they plan to set transplants out on June 8, a week before June 15, and sow the seed indoors 3 weeks earlier, on May 18. The week of slack exists only if both assumptions hold.
What the result says. The formula did not tell them squash is impossible; it told them which changes make it fit, and how much margin each leaves. Their direct-seeded 100-day squash would have needed 29 more days than their season holds. The same arithmetic flags their sweet corn: Colorado State gives 60 to 90 days, and the calendar uses the longer figure, so 90 + 28 = 118 days before September 15 is May 20, five days before corn's tender window opens. A packet printing 60 days gives 88 days, June 19, which fits. They check the packet before ordering.
Marisol's balcony, where it rarely frosts. Marisol Vega grows cherry tomato, pepper, lettuce, basil and radish in six containers on a west-facing balcony with 5 hours of afternoon sun.
- Known: her area rarely frosts; summer afternoons on the balcony are hot; she set "rarely frosts" in setup, so the calendar computes no dates for her.
- Assumed: that heat, not frost, sets her season, which is the general pattern where frost is rare. Her own extension calendar confirms or corrects that.
- Open: when her local extension service says to sow lettuce and set out tomatoes (settled by her county extension office's planting calendar, which she finds through the Extension Foundation's state directory); whether 5 hours of afternoon sun is enough for the pepper (settled by her own season).
The frost-class windows tell her the order of things, not the dates: lettuce and radish are cool-season crops and will struggle in the hottest months; tomato, pepper and basil are warm-season crops. Two parts of this module still apply directly. Soil temperature: Harrington's minimums say when seed germinates, and Nebraska's and Purdue's warning that lettuce germinates poorly at 85 °F and above matters more on a hot balcony than anywhere; she reads a container's soil at 8 a.m. before sowing lettuce, and sows it in the cooler months her extension calendar names. Succession: radish every 7 days and lettuce every 7 days, from the interval table, through the cool part of her year. The fall formula does not apply to her, because there is no first frost to count back from.
- Nebraska Extension, Fall Vegetable Gardening (G1343): the fall planting formula in full, with the reasons for the fall factor and the frost-tender factor.
- Colorado State University Extension, Vegetable Planting Guide (GardenNotes #720): the four frost classes, their windows, transplant ages and the 8 a.m., 4-inch soil temperature method.
- Oregon State University Extension, Soil temperature conditions for vegetable seed germination: Harrington's full tables of minimum and optimum temperatures and days to emergence.
- Utah State University Extension, Suggested Vegetable Planting Dates for Utah: a state's own groups and city-by-city dates, showing how local practice departs from a general rule.
3. Practice: date your crops Practice
Minimum: Classify each crop, date the spring windows and indoor starts from the calendar, decide on any crop that does not fit, and put the first dates on your calendar; skip the hand-run fall formula, succession and the comparison with other calendars. Time: about 60 minutes at a table, plus a few minutes on several spring mornings. Materials: your two frost dates from Module 2 (and which probability they are), your crop plan from Module 5, the seed packets or catalog pages for your varieties, a paper calendar or a calendar app, and a soil thermometer if you have one.
- Classify each crop. Write each crop's frost tolerance class from the table in section 1, or the crop guide. Mark garlic as fall planted.
- Date the spring windows. From your last frost date, write each crop's outdoor window as dates. Check your figures against the planting calendar; if they differ, find which of you made the error.
- Date the indoor starts. For each crop you will grow from your own transplants, subtract the transplant age from the outdoor window. Put the first sowing date on your calendar.
- Go deeper Run the fall formula. For every crop, compute days to count back: the packet's days to maturity, plus seed-to-transplant days if you grow your own, plus a harvest period if you want one, plus 14, plus 14 more if the crop is frost-tender. Count back from your first fall frost. Note any crop whose latest date falls before its spring window opens.
- Decide on the ones that do not fit. For each, choose one of transplants, a shorter variety or season extension, or drop it this year. Write down the choice and the margin it leaves.
- Go deeper Plan successions, if any. For lettuce, radish, beans or others you want over a longer period, list sowing dates at the interval from the table, stopping at the fall formula's latest date.
- Go deeper Compare with your extension service. Find your state's or county's planting calendar and note any crop where its window differs from the course's by more than a week. Where they differ, the local calendar is the one to follow.
- Record it. Enter each crop's target planting date in the crop plan sheet. When spring comes, log each soil thermometer reading and each sowing in the sowing sheet.
4. Your garden Your garden
5. Check your work Check your work
Criteria
- Does every crop have a frost tolerance class, with the source named where your classes differ from your extension service's?
- Do your spring dates follow from your own frost date, and do you know whether that date is the 50 percent or the 10 percent date?
- Are indoor sowing dates the outdoor date minus the transplant age, not the frost date minus the age (for very tender crops these differ by two weeks or more)?
- Does each crop have a latest planting date from the fall formula, with the frost-tender factor added only for frost-sensitive crops?
- For each crop that does not fit, have you written down what you will do and how much margin it leaves?
- Do you have a way to read soil temperature at 8 a.m. at the right depth, or a plan to wait for your extension service's guidance?
Worked example
The Lindqvist squash meets the criteria. The class (very tender) and days (100) come from a named source; the earliest outdoor date follows from their own last frost; the fall formula has both factors, because squash is frost-tender; the transplant assumption is labelled as theirs and kept within Colorado State's 2 to 3 weeks for squash transplants; and the decision (an 85-day variety from transplants) is stated as potentially fitting, with a 7-day margin that depends on two things they have not yet confirmed: that the packet's 85 days count from sowing, and that 3 weeks indoors saves about 21 days outdoors. The plan names who settles the first (the seed company's catalog) before any seed is ordered. Their harvest log will show whether the first squash ripens in time, and next year's plan can adjust the assumption.
Common mistakes
- Treating the frost date as a safe date. A 50 percent date is a coin toss. If you entered it, every window carries that risk; watch the forecast or enter the 10 percent date.
- Counting indoor starts from the frost date for very tender crops. Tomatoes go out two weeks after the frost; seven weeks before the frost date is nine weeks before planting out, and the plants outgrow their pots.
- Forgetting the frost-tender factor. Beans counted at 60 + 14 = 74 days from an October 20 frost give August 7, two weeks too late by the formula.
- Using days to maturity from the wrong starting point. A transplant figure (Colorado State's T) used for direct seeding understates the season the crop needs.
- Reading soil temperature in the afternoon. An afternoon reading in sun can be well above the overnight temperature the seed sits in. Read at 8 a.m.
- Averaging two calendars. A window halfway between Indiana's and Colorado's belongs to neither. Follow one named source and note the other.
What is still unresolved
The calendar cannot tell you this year's weather; the soil thermometer and the ten-day forecast do, week by week. How much your own garden differs from the averages (a frost pocket, a warm wall, a raised bed that warms early) shows only in your records over a season or two. Starting the seeds the calendar dates is Module 7, and protecting crops at the ends of the season is Module 12. Your harvest period for each crop is your own preference; enter it on the calendar page when you know it.