Where our growing data comes from

Northeast

• University of Maine Cooperative ExtensionMaine · cited 6 times here

  Written for a short, cold season, which makes it useful well beyond Maine if you are in zone 4 or colder.

• UNH ExtensionNew Hampshire · cited 3 times here

  Practical northern-New-England guidance, and the source behind our birdhouse gourd timings.

• Cornell UniversityNew York · cited 41 times here

  Vegetable Varieties for Gardeners is the one database we know of where the variety reviews come from gardeners in a named county rather than from the company selling the seed.

• Penn State ExtensionPennsylvania · cited 4 times here

  Strong on vegetable disease identification, and the source we lean on for companion-planting claims that turn out to have research behind them.

Mid-Atlantic

• University of Maryland ExtensionMaryland · cited 2 times here

  Their Home & Garden Information Center is the source of the target soil pH figures on our plant profiles.

• Virginia Cooperative ExtensionVirginia · cited 3 times here

  Good on the transition zone, where northern and southern growing advice both stop working.

• WVU ExtensionWest Virginia · cited 3 times here

  Useful for Appalachian elevation, where your frost dates have less to do with your latitude than your altitude.

Southeast

• University of Arkansas ExtensionArkansas · cited 1 time here

  Mid-South guidance for the awkward band that is too hot for northern timings and too cold for southern ones.

• UF/IFAS Gardening SolutionsFlorida · cited 9 times here

  Florida's growing calendar looks nothing like the rest of the country's, and this is the source that takes that seriously rather than apologising for it.

• University of Georgia ExtensionGeorgia · cited 23 times here

  Their CAES field reports are unusually willing to publish what failed in trials, not only what worked.

• University of KentuckyKentucky · cited 2 times here

  The Center for Crop Diversification covers specialty crops most extension services never get to.

• Mississippi State University ExtensionMississippi · cited 5 times here

  Deep South heat and humidity, with honest guidance on what simply will not grow through a Gulf summer.

• NC State ExtensionNorth Carolina · cited 133 times here

  The Plant Toolbox is the single most-used source on this site by a wide margin. It carries hardiness, size, life cycle and toxicity for an enormous range of plants, ornamentals included.

• Clemson Home & Garden Information CenterSouth Carolina · cited 27 times here

  Fact sheets written for humid southern summers, which is the condition most national growing advice quietly assumes away.

Midwest

• Illinois ExtensionIllinois · cited 22 times here

  Broad, plainly written, and strong on the vegetable problems people run into rather than the rare ones.

• Purdue ExtensionIndiana · cited 1 time here

  Consumer horticulture guidance for the eastern Corn Belt.

• Iowa State University ExtensionIowa · cited 23 times here

  Yard and Garden covers the Midwest's compact season end to end, from soil prep through storage.

• Michigan State University ExtensionMichigan · cited 1 time here

  Particularly good on fruit, which is unsurprising given what Michigan grows.

• University of Minnesota ExtensionMinnesota · cited 33 times here

  Cold-climate growing done properly, plus the guide behind our molokhia entry, which is not a crop most extension services cover at all.

• Ohio State University ExtensionOhio · cited 11 times here

  Ohioline is a large free fact-sheet library, and one of the easier ones to search when you have a specific pest in front of you.

• University of Wisconsin-Madison ExtensionWisconsin · cited 31 times here

  Their horticulture articles are the best short write-ups we have found on individual ornamental species, and we cite them constantly on flowers.

Great Plains

• South Dakota State University ExtensionSouth Dakota · cited 6 times here

  Wind, short seasons and hard winters, which is a different problem set from anywhere further east.

Southwest

• University of Arizona Cooperative ExtensionArizona · cited 1 time here

  Their Maricopa County planting calendar is what we cross-checked our low-desert zone 10 windows against before publishing them.

• Texas A&M AgriLife ExtensionTexas · cited 1 time here

  Covers a state that spans five hardiness zones, so read the regional pages rather than the state-level ones.

Mountain West

• Montana State University ExtensionMontana · cited 3 times here

  Short-season, high-altitude guidance, including crops worth trying where most things will not ripen.

• Utah State University ExtensionUtah · cited 40 times here

  Excellent on high-desert growing, where the difficulty is aridity and swinging day-night temperatures rather than cold.

Pacific Coast

• University of California Agriculture and Natural ResourcesCalifornia · cited 2 times here

  California spans more growing conditions than most countries, and UC ANR is the only source that covers all of them consistently.

• Oregon State University ExtensionOregon · cited 15 times here

  OSU also hosts the PRISM Climate Group, whose 2023 dataset is where every hardiness zone on this site ultimately comes from. We owe them more than a citation.

• Washington State University ExtensionWashington · cited 5 times here

  Their tree-fruit programme is a national reference, and their master gardener material is good on maritime Pacific Northwest conditions.

Alaska

• University of Alaska Fairbanks ExtensionAlaska · cited 4 times here

  Interior Alaska pairs brutal winters with long summer days, which produces growing advice you will not find anywhere else.

Regional specialists

Not extension services, but worth your time for the region they cover.

• Growing In The GardenLow desert (zones 9b and 10a: Phoenix, Tucson)

  Run by a Phoenix-area horticulturist, and the best plain-English source we found on the low desert's inverted dual growing season. Our zone 10 planting windows are built on it.