GrowDate Atlas

When to plant, crop by crop

One page per crop. Each gives the extension rule in plain words, the soil temperature the sources name, and the planting window computed from the NOAA frost normals of 39 U.S. cities. No date on these pages is typed; every one is a frost date plus a published offset.

The windows side by side: Chicago, Illinois

To see how far apart the rules land, here is every crop at one station, CHICAGO OHARE INTERNATIONAL AIRPORT, IL US (last spring 32°F Apr 22, first fall 32°F Oct 19). Each crop page repeats this for all 39 cities.

J F M A M J J A S O N D TomatoesTomatoes: start indoors Mar 18 – Apr 15 Tomatoes: transplant May 6 – May 20 PeppersPeppers: start indoors Mar 11 – Apr 8 Peppers: transplant May 6 – May 20 Garlic Garlic: plant cloves Oct 26 – Nov 2 Potatoes Potatoes: plant seed potatoes Apr 8 – Apr 22 Onions Onions: plant sets Mar 25 – Apr 8 Strawberries Strawberries: from Apr 22 PumpkinsPumpkins: start indoors Mar 25 – Apr 22 Pumpkins: direct sow (or transplant) Apr 22 – Aug 10 Sunflowers Sunflowers: direct sow (or transplant) Apr 22 – Sep 4 Tulips Tulips: plant by Oct 19 CucumbersCucumbers: start indoors Apr 1 – Apr 15 Cucumbers: direct sow (or transplant) Apr 22 – Apr 29 ZucchiniZucchini: start indoors Apr 1 – Apr 15 Zucchini: direct sow (or transplant) Apr 22 – Apr 29
start seeds indoors plant outdoors (bar) or plant-by date (marker) Computed from CHICAGO OHARE INTERNATIONAL AIRPORT, IL US; hover for dates.

Crops

CropRule (source)Chicago, IL windowSoil temperature
When to plant tomatoes 2 to 4 weeks after the last spring frost (Colorado State University Extension) May 6 – May 20 ≥ 50°F
When to plant peppers 2 to 4 weeks after the last spring frost (Colorado State University Extension) May 6 – May 20 ≥ 60°F
When to plant garlic 1 to 2 weeks after the first fall frost (University of Minnesota Extension) Oct 26 – Nov 2 not stated by the source
When to plant potatoes Up to 2 weeks before the last spring frost (Colorado State University Extension) Apr 8 – Apr 22 ≥ 45°F
When to plant onions 2 to 4 weeks before the last spring frost (Colorado State University Extension) Mar 25 – Apr 8 ≥ 35°F
When to plant strawberries Potted after the last frost, bare-root earlier (University of Minnesota Extension) from Apr 22 not stated by the source
When to plant pumpkins After the last spring frost, once the soil has warmed (Clemson Cooperative Extension HGIC) Apr 22 – Aug 10 ≥ 60°F
When to plant sunflowers After the last spring frost, once the soil has warmed (Oregon State University Extension) Apr 22 – Sep 4 ≥ 50°F
When to plant tulips In fall, before the first frost (Penn State Extension) Oct 19 ≤ 55°F (cooling)
When to plant cucumbers From the last spring frost date to 1 week after it (Colorado State University Extension) Apr 22 – Apr 29 ≥ 60°F
When to plant zucchini From the last spring frost date to 1 week after it (Colorado State University Extension) Apr 22 – Apr 29 ≥ 60°F

How these pages differ from a city calendar

A city calendar fixes the place and lists 27 vegetables. A crop page fixes the crop and lists 39 places, so you can see how the same rule moves from a 297-day season to a 140-day one. Both draw on the same two files, stations.json and crops.json, plus the per-crop rules in cropGuides.json, each with its source and the date it was read.

Garlic and tulips are anchored to the first fall frost rather than the last spring one; strawberries and tulips have a start set by soil temperature, which NOAA normals do not report, so those pages compute only the frost-anchored end of the window and say so. Grass seed is not here: the extension sources read on 2026-10-23 give calendar dates and soil temperatures, not a frost offset, so no page could be computed honestly (details in the data file).

Method and sources: methodology · why a frost date beats a zone number · soil temperature by crop.