Photo: Oliver Dixon · CC BY-SA 2.0, via Wikimedia Commons.
How to read a planting calendar
A planting chart that says "sow in April" is wrong for most of the country. The skill worth having is reading any calendar as an offset from your own frost dates - here's how, at both ends of the season.
What you need
Your last spring frost and first fall frost dates (the planner gives them from your ZIP)
Each crop's cold-hardiness - hardy, half-hardy, or tender
Each crop's days to maturity, printed on the seed packet
Open any planting calendar and you'll see instructions like "sow peas in March" or "set out tomatoes in May." Those months are someone else's, not yours - a March that's safe in Georgia is deep winter in Minnesota. The skill that makes every planting chart usable is learning to read it not as months but as offsets from your own frost dates. Do that and one generic calendar works everywhere.
Everything counts from your frost date, not the month
The real clock isn't the calendar month - it's how many weeks a crop can stand before or after your last spring frost, and that depends only on how much cold the crop tolerates:
Promising
Cold-hardy annuals can be sown or transplanted before the last spring frost — hardy crops about three weeks before, half-hardy about two weeks before. Tender crops wait for the frost itself.
suboptimal if ignored → advise
Mechanism: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.
Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.
Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.
Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.
So a "cold-hardy" crop like peas, spinach, or onions goes out about three weeks before your last frost; a "half-hardy" one about two weeks before; and a tender crop - tomato, bean, squash - waits until the frost has passed. When a chart says "April," translate it: find your last frost date, then count backward or forward by the crop's tolerance. "Three weeks before my last frost" is a real instruction for your garden in a way "April" never is.
Read both ends: spring tells you when, fall tells you whether
A planting calendar has two edges - your last spring frost and your first fall frost - and the season between them is your frost-free window. Spring answers when you can start. Fall answers something calendars handle badly: whether a crop has time to finish at all.
Well established
A species must reach maturity within the site's frost-free window.
fatal if ignored → block
Mechanism: Accumulated GDD vs gdd_to_maturity, or DTM vs frost-free days.
Evidence: settled without a trial - GDD models.
remedy
Transplant, choose a shorter-DTM cultivar, or refuse the species for this site.
Every crop has a days-to-maturity number on the packet. To know if there's still time to plant one - especially for a fall crop - count that many days back from your first fall frost (add a couple of weeks, because growth slows as the days shorten). If today is past that date, the crop won't finish, and the honest move is a shorter-season variety or waiting for next year. This backward-count from the fall frost is the single most useful thing a calendar can't do for you, because it doesn't know your dates.
Days from sowing to first harvest, from the corpus. Each bar is the crop's range across cultivars; where a cultivar group is named, its own range.Source: corpus days_to_maturity — the species field, or a cultivar group's override where one is named.
Warm crops that start before the window opens
One more wrinkle the calendar compresses: some crops are started weeks before they're planted out. A tomato is sown indoors around six weeks before your last frost and only set outside once frost has passed - two different dates for one crop. The starting seeds indoors guide covers which crops need that head start and how to count it.
And remember the date itself is a probability
The whole system rests on your frost dates, and those are averages, not guarantees - a "last frost date" is the day after which frost becomes unlikely, not impossible. The understanding your frost dates guide covers reading them as odds. Put it together and reading a planting calendar is three moves: find your frost dates, count each crop's window from them instead of from the month, and check the fall end to be sure there's time to finish. The first bed walkthrough does exactly this for a first season.
