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.
Promising · suboptimal if ignored → advise
How it works
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.
The evidence
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.
Where this rule stops
Spring-planted crops only. A crop marked sow_season: fall (garlic) is planted in autumn and is not described by a spring-frost offset, so it is excluded; perennials and woody herbs are excluded by applies_to. The offset numbers (21 / 14 days) are the values chosen within the extension range in the pointer below and stay UNVERIFIED until a human reads a planting-date table against them.
Plants this fires on
- Amaranth
- Arugula
- Basil
- Beet
- Borage
- Buckwheat
- Butternut
- Cabbage
- Calendula
- Carrot
- Celery
- Cereal Rye
- Cilantro
- Common Bean
- Corn
- Cosmos
- Cowpea
- Crimson Clover
- Cucumber
- Dill
- Edamame
- Eggplant
- Endive
- Fava Bean
- French Marigold
- Hairy Vetch
- Horseradish
- Lacy Phacelia
- Leek
- Lettuce
- Lima Bean
- Melon
- Mustard Greens
- Nasturtium
- Oats
- Okra
- Onion
- Parsley
- Parsnip
- Pea
- Pepper
- Potato
- Radish
- Rocky Mountain Bee Plant
- Runner Bean
- Rutabaga
- Scallion
- Sorghum
- Spinach
- Summer Savory
- Summer Squash
- Sunflower
- Sweet Alyssum
- Sweet Potato
- Tomatillo
- Tomato
- Turnip
- Watermelon
- Winter Squash
- Zinnia
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- Corpus: another rule here verified - the maintainer read GardenNotes #720
- Corpus: another rule here cites the document that actually states its offsets
- corpus: add another rule here, cool-season sow window (Promising, unverified)
Cite this page
Milpa Gardens. "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." Evidence grade: Promising. https://milpa.garden/rules/cold-hardy-annuals-can-be-sown-transplanted/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.