Methodology & Sources

This page exists so that every dated claim on GardenPairs - a frost window, a zone's temperature range, an evidence rating on a companion pair - can be checked rather than taken on faith. It is reviewed and re-dated on an annual cadence, because USDA hardiness zone maps are periodically revised and frost patterns shift year to year.

USDA Hardiness Zones and Frost Windows

Zone temperature ranges on this site follow the USDA Plant Hardiness Zone Map framework, which divides North America into zones based on the average annual extreme minimum winter temperature for that area, in ten-degree-Fahrenheit bands. A zone number is not a promise about any single winter - it is a long-term average, which is exactly why our zone pages describe typical frost windows as a range rather than a single date, and why the Frost-Date Lookup tool is explicitly framed as typical ranges, never a forecast for the coming season.

How We Decide Evidence-Backed vs. Traditional Lore

Every companion-pairing page carries one of three honesty labels, and the distinction is deliberately narrow and consistent:

• Evidence-backed: the mechanism has documented support beyond casual observation - for example, legumes fixing atmospheric nitrogen via root-nodule bacteria is basic, well-established plant physiology, and certain marigold species suppressing specific root-knot nematode populations has been studied directly.
• Traditional lore: the pairing is widely repeated in gardening tradition, but we are not aware of a controlled study isolating the claimed effect - the classic idea that basil improves a nearby tomato's flavor falls here, since flavor claims of this kind are essentially untested.
• Mixed: part of the claim has real support and part does not, or the evidence is anecdotal but plausible and consistent enough across independent gardener reports that we don't dismiss it outright, while still being honest that it isn't proven the way the evidence-backed category is.

A rating is a judgment call in the mixed and lore categories, and we would rather under-claim than over-claim - if a mechanism sounds impressive but we cannot point to why it would actually work biologically (nitrogen in the soil, a documented scent-masking effect, a physical trellis relationship, competing root depths), we do not upgrade it to evidence-backed just because it is popular.

Crop Data (Spacing, Days to Maturity, Sun Needs)

Quick-facts figures on each crop guide (spacing, typical days to maturity, sun requirement, frost tolerance) reflect commonly cited ranges from mainstream vegetable-gardening references and seed-packet conventions rather than a single cultivar's exact numbers - real varieties differ, which is why every figure on this site is presented as a range rather than a single precise number, and gardeners should treat their own seed packet's specific guidance as the more precise source for the variety they actually bought.

The Shared Relationship Dataset

Every companion-pair page, the Companion Pairing Checker, and the Bed Layout Helper all read from one internal structured dataset of crop relationships (which crop pairs beneficially or antagonistically with which, and why). This is a deliberate architecture decision: a page and a tool disagreeing with each other about the same pairing would be worse than having no tool at all, so both are built to draw from the same source rather than maintaining separate copies that could quietly drift apart.

Corrections

If you find a factual error - a wrong spacing figure, a mischaracterized mechanism, or an evidence rating you think is wrong - the Contact page is the right place to flag it. We would rather fix a mistake than leave it standing for the sake of consistency.