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About PlantingCalc

PlantingCalc provides free gardening calculators powered by USDA hardiness zone data, typical frost dates for each zone, NOAA weather records, and agricultural extension guidance. Every tool on this site is designed to help home gardeners make data-driven decisions, from soil preparation to harvest timing.

Our Data Sources

  • USDA Plant Hardiness Zone Map (via phzmapi.org): determines your growing zone by ZIP code.
  • Typical frost dates by hardiness zone: the usual last spring and first fall frost for each USDA subzone, used for planting windows. They are zone averages, so your own garden can differ.
  • NOAA and National Weather Service data: 30 years of temperature records for your location on the frost probability tool, and the current forecast on Frost Alert.
  • University Agricultural Extension Publications: planting timing, spacing and soil amendment guidelines. Where a page uses a specific rate, it names the source.
  • Companion planting references: traditional good and bad pairings. Few of these have been tested in field trials, so treat them as guidance.

Why Trust Our Calculators

  • Real government data: USDA zones, NOAA weather records and extension guidance, not guesswork.
  • Sources named: where a page gives a specific rate, it says where the number comes from.
  • Transparent methodology: every calculation explains how it works.
  • Corrected when wrong: when a number does not match its source, we fix it and update the page date.

Who We Are

PlantingCalc is a small independent publisher of free tools for home gardeners who want to plan from data instead of guessing: the kind of planning help commercial growers get, without paying for planning software.

Every calculator on this site is backed by published agricultural data, not guesswork. When we say “plant tomatoes 2 weeks after your last frost,” that comes from the same extension office guidelines that professional farmers follow.

How We Calculate

  • Frost dates are the typical dates for your USDA hardiness subzone, which we look up from your ZIP through the phzmapi.org API. They are zone averages, not readings from a station near you; the frost probability tool, and the frost date shown beside your ZIP, use 30 years of NOAA records for your location instead.
  • Planting windows use the standard extension office formula: weeks before or after your local last frost date, calibrated per crop.
  • Spacing recommendations come from university agricultural extension publications (Cornell, UMass, UF/IFAS, Oregon State).
  • Fertilizer profiles are based on published NPK requirements per crop from soil science research and extension guides.
  • Watering estimates use base rates from agricultural research, adjusted by multipliers for climate, soil type, container vs. in-ground, and season.

Our Calculators