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Sources & Data

Every zone, frost date, and plant fact on WhatCanGrow traces back to a primary source. Here's exactly where each piece comes from.

US hardiness zones

  • USDA Plant Hardiness Zone Map (2023) — the authoritative 13-zone US scale, based on 1991–2020 average annual extreme minimum temperature. Produced by the USDA Agricultural Research Service and Oregon State University's PRISM Climate Group.

Canadian hardiness zones

  • Natural Resources Canada Plant Hardiness Zones (2014) — the Canadian 0–9 scale, a multi-variable formula (minimum winter temps, frost-free period, snow cover, rainfall, wind).
  • Agriculture and Agri-Food Canada — supporting climate and plant-hardiness data.

Frost dates & growing season

  • USDA / NOAA 30-year climate normals (1991–2020) — the basis for average first and last frost dates.
  • Environment and Climate Change Canada — Canadian frost and growing-season norms.

Plant data

  • University extension programs (USDA Cooperative Extension) — verified hardiness ranges, days-to-harvest, soil/light/water needs, and growing tips for each plant.
  • Seed catalogs and plant databases — cross-referenced for days-to-maturity and container-suitability.

A note on "averages"

Frost dates and zones are 30-year averages, not guarantees. A late spring frost or an unusually warm winter shifts real-world outcomes, and local microclimates (elevation, proximity to water, urban heat) can shift your effective zone by a half-step or more. Use our numbers as a strong starting point, and check your local extension service for area-specific guidance.

See About & Methodology for how we turn these sources into a verdict.