How We Research

PlantingCalc exists because home gardeners deserve the same data-driven planning tools that commercial growers and university extension offices rely on. Without the $200/year software subscriptions, the paywalled journals, or the half-day of digging through academic PDFs to answer a one-line question. This page explains exactly where our data comes from, how we turn it into each calculator, what we add on top, and what we deliberately don't do.

1. Where the data comes from

Every calculator and every zone guide on PlantingCalc traces back to one of four underlying data sources:

• USDA Plant Hardiness Zone Map. The canonical U.S. growing-zone dataset, accessed via the public phzmapi.org API. When you submit a ZIP code to our planting-dates calculator, your ZIP is resolved to a hardiness zone by this API and then used to select the right frost-date window and crop list for you. We do not store the ZIP.
• Typical frost dates. A table of typical last-frost and first-frost dates for each USDA hardiness subzone, with state averages where a subzone has no entry. Your ZIP gives your subzone, and the subzone gives the dates. These are midpoints, not safe dates. The frost date and countdown shown beside your ZIP (in the ZIP boxes and the location bar) are instead the median of the last 30 years of NOAA daily lows at your location, once those load, with the subzone date until then.
• NOAA climate records. The frost probability and seed start tools fetch daily low temperatures for your ZIP from NOAA's Regional Climate Centers (ACIS, PRISM grid) and compute frost dates from those years directly.
• University agricultural extension publications, peer-reviewed planting guides, spacing tables, fertilizer recommendations, and companion-planting research from land-grant university extension offices (Cornell, UMass, UF/IFAS, Oregon State, and others). These are publicly available and are treated across the industry as the authoritative, research-backed source for "what to plant, when, how far apart, and how much."
• Open-source plant databases. Vetted community-maintained plant datasets for vegetable metadata (days to harvest, category, basic care notes) that we cross-reference against extension publications before relying on.

None of these sources are paywalled. They're the same datasets used by commercial growers, licensed nursery professionals, and government-facing planning tools.

2. How we turn data into calculators

Each calculator is a specific formula or lookup, documented in the code:

• Planting dates use the standard extension office formula: your typical last frost date (from your ZIP → USDA subzone) plus or minus an offset per crop (e.g., "tomatoes: last frost + 14 days") from published extension schedules.
• Soil volume is a straight cubic-foot calculation from your bed dimensions. The soil calculator lets you add an extra allowance for settling if you want one; it adds nothing by default.
• Seed spacing pulls the recommended in-row and between-row spacing for each vegetable from extension publications and divides your bed area by those values to estimate plants per bed.
• Companion planting uses a compatibility matrix built from extension research and widely cited companion planting publications. Relationships are tagged as compatible, antagonistic, or neutral.
• Fertilizer uses crop-specific NPK demand values from published soil science research, converted into pounds-per-square-foot estimates for common fertilizer blends.
• Watering schedule uses base rates from agricultural research, adjusted by multipliers for climate zone, soil type, container vs. in-ground, and growth stage.

When our calculators disagree with your own experience, trust your garden. Local microclimate, soil, and weather variance will routinely beat any averaged national dataset.

3. Refresh cadence

• USDA zone data is looked up live from phzmapi.org when you submit a ZIP. No staleness.
• Typical frost dates come from a table of dates per USDA subzone. The frost probability and seed start tools instead compute from NOAA climate records for your ZIP, fetched live.
• Extension spacing and fertilizer data lives as structured data in our codebase and is reviewed at least once per year against the latest extension publications.
• Zone guides are refreshed whenever any upstream source updates, with a version note in the repo.

4. What we add on top

Raw data alone is not a garden plan. Where useful, we add a plain-language editorial layer on top of the calculator output. For example, an explanation of what a specific output means for your bed, or a practical note about common failure modes for a given vegetable in a given zone. These editorial sections are clearly separated from the calculator outputs and from the raw underlying data. They are our own interpretation at the time of writing.

5. What we deliberately don't do

• We don't store your ZIP, bed dimensions, or any other calculator inputs. The only ZIP we keep is one you give with your email for frost reminders.
• We don't sell, rent, or share email addresses collected through our subscribe forms.
• We don't accept payment from seed companies, fertilizer brands, or nursery chains to influence our recommendations.
• We don't invent spacing, timing, or fertilizer values. If we state a number, it comes from a named source.
• We don't give individualized professional advice. See our disclaimer.

6. Corrections policy

If you believe a page on PlantingCalc is wrong. An incorrect zone assignment, a spacing value that doesn't match a published extension guide, an outdated frost date, or a factual error in an editorial section. Please contact us with the page URL and what you believe is wrong. For calculator-level data we verify against the live source before making changes; for editorial content we fix it directly.

7. Who runs this site

PlantingCalc is built and maintained by a small, independent team of home gardeners who wanted the planning tools commercial growers have. Without the subscription. We are not university extension agents or licensed horticulturists. We are not funded by any seed company, fertilizer brand, or nursery network. Some pages link to related garden supplies on Amazon, and PlantingCalc earns from qualifying purchases through those links; that never influences calculator outputs or zone recommendations.