Data Sources & Methodology
Transparency is a core value at TheYardwise. This page documents where our data comes from, how it's processed, and what confidence level applies to each data point.
Climate Data
USDA Plant Hardiness Zone Map
Our hardiness zone lookup uses the 2023 USDA Plant Hardiness Zone Map, developed by the PRISM Climate Group at Oregon State University. This map is the standard reference for determining which perennial plants are likely to survive winter in a given location, based on 30-year average annual extreme minimum temperatures.
Source: USDA Agricultural Research Service — Plant Hardiness Zone Map
Open-Meteo ERA5 Reanalysis
We retrieve historical weather data from the Open-Meteo Historical Weather API, which uses the ECMWF ERA5 reanalysis dataset. ERA5 combines historical weather observations with advanced atmospheric modeling to produce a consistent global record dating back to 1940. We use the most recent 30-year period as our climate baseline.
Sources: Open-Meteo Historical Weather API · ECMWF ERA5 Reanalysis
Frost Date Estimation
Last spring frost and first fall frost dates are estimated from ERA5 temperature data using a threshold of 0°C (32°F). We analyze 30-year daily minimum temperatures to determine the 10th percentile frost-free window — a conservative estimate providing 90% probability of frost-free conditions.
Calculation Methods
Growing Degree Days (GDD)
GDD is calculated using the standard agronomic formula:
GDD = (Tmax + Tmin) / 2 − Tbase
Where Tbase = 10°C (50°F), the standard for most warm-season crops. GDD accumulation helps predict plant development stages — from emergence to flowering to harvest — more accurately than calendar days alone.
Reference: This method is widely used by agricultural extension services including University of Minnesota Extension and the USDA NRCS.
Crop Suitability Scoring
Our recommendation engine scores crops by matching your location conditions against crop requirements:
- USDA hardiness zone — perennial survival probability
- Growing Degree Days — sufficient heat accumulation for crop maturation
- Frost-free window — adequate growing season length
- Soil type — drainage and nutrient compatibility
- Sunlight exposure — light requirements for optimal growth
Plant Database
Our plant database contains 100 plant entries across vegetables, herbs, fruits, and roots. Each entry includes growing parameters such as GDD thresholds, soil pH range, soil drainage preference, companion plants, and difficulty rating.
Data Quality Tiers
All database fields are classified into one of three quality tiers:
| Tier | Label | Definition | GDD Coverage |
|---|---|---|---|
| Verified | Cross-referenced | Data confirmed by ≥2 authoritative horticultural sources (extension services, peer-reviewed research, USDA references) | 15 plants (core crops) |
| Estimated | Single-source or derived | Data from one reliable source, or derived from related species with known parameters | 38 plants |
| Null | Not yet available | Insufficient data to assign a value. These fields are left empty rather than filled with unverified guesses. | 47 plants |
Core Verified Crops (GDD 100% verified)
The following 15 crops have fully verified GDD ranges sourced from multiple authoritative references:
Tomato · Pepper · Cucumber · Lettuce · Beans · Carrot · Corn · Squash · Basil · Spinach · Peas · Kale · Beet · Radish · Broccoli
Plant Database Fields
| Field | Type | Description |
|---|---|---|
| gdd_min / gdd_max | Number or null | Growing Degree Day range for crop maturity (base 10°C) |
| gdd_source | verified / estimated / null | Quality tier of GDD data |
| soil_ph_min / soil_ph_max | Number or null | Optimal soil pH range for nutrient uptake |
| soil_drainage | String or null | Preferred drainage: well_drained, moist, dry_tolerant, etc. |
| companion_plants | Array or null | Beneficial companion planting partners |
| difficulty | easy / moderate / hard / null | Growing difficulty for home gardeners |
Important: We never fabricate data. Fields marked as null are intentionally left empty where authoritative sources are insufficient. As our database grows, we progressively validate and upgrade estimated entries to verified status.
Data Limitations
Our recommendations are based on historical climate averages, not real-time weather data. Actual growing conditions in your garden may differ due to:
- Microclimate effects — urban heat islands, slope orientation, wind exposure
- Year-to-year variability — individual seasons may be warmer or cooler than the 30-year average
- Soil variability — even within a single garden, soil conditions can vary significantly
- Climate change — historical averages may not perfectly predict future conditions
We recommend using our planting guides as a starting point and adjusting based on your direct observation of local conditions. For the most accurate local advice, consult your local agricultural extension service.
Ready to Put This Data to Work?
Our Garden Planner combines all these data sources into a personalized planting guide tailored to your exact location.
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