Heat Zone Mapping Calculator

1) Adjusted heat days

Adjusted heat days = Raw heat days + (Urban + Shade + Wind + Elevation + Irrigation + Trend buffer)

2) Zone mapping

Heat days reflect how often plants face high-temperature stress. Use the mapped zone alongside winter hardiness for better plant selection.

1. Find the average yearly count of days with highs above 86°F (30°C) for your area.
2. Enter that value as Heat days per year (raw) and choose your data period.
3. Apply microclimate adjustments for your exact planting spot.
4. Press Calculate Heat Zone to see the zone above the form.
5. Download CSV or PDF to save results for your garden records.

What a heat zone represents

A heat zone estimates how often plants face intense heat. It uses the average yearly count of days above 86°F. Higher zones mean longer periods of heat stress, faster soil drying, and greater risk of bloom drop. It also correlates with reduced fruit set, sunscald, and higher irrigation demand. Use it to compare sites, not to predict a single day.

Collecting reliable heat-day inputs

Start with multi‑year records from a nearby station. Use at least ten years when possible. If you only have one year, expect large variation. Count each day whose maximum temperature exceeds 86°F, then average the yearly totals. If records are in Celsius, use 30°C as the threshold. Keep your dataset consistent by using the same station and method each year.

Microclimate adjustments that matter

Gardens rarely match the station exactly. Pavement, walls, and reflected sun can add heat days. Tree canopy and shade cloth can reduce them. Wind lowers leaf temperatures but can increase water demand. Elevation shifts temperature patterns; higher sites usually record fewer hot days. Regular irrigation also reduces plant stress during peaks. Courtyards, raised planters, and mulch often warm faster. North‑facing slopes, open lawns, and light mulches typically moderate heat.

Interpreting results for plant selection

Use the mapped zone to screen plants for summer tolerance. Compare it with plant labels and trial data. In higher zones, favor heat‑tolerant cultivars, deeper mulch, and afternoon shade. Protect roots from hot hardscape, and maintain even soil moisture. Schedule planting so roots establish before peak heat. For containers, select larger pots.

Limitations and good practice

Heat zones are not the same as cold‑hardiness zones. They do not describe winter minimums, humidity, or night temperatures. Use this estimate with local observation, soil type, and irrigation capacity. Recheck your numbers after major landscape changes, such as new paving or shade trees, and update using recent climate trends. When in doubt, validate with a season of notes on wilting, scorch, and recovery after heat waves.

1) Where do I find heat-day counts?

Use local meteorological summaries or a nearby airport station dataset. Count days with highs above 86°F, then average across years for a stable input.

2) Should I use one hot summer as my input?

Avoid single-year inputs when possible. One season can be unusually hot or cool. A ten-year average gives a better planning value for plant selection.

3) Why does shade reduce my heat zone estimate?

Shade lowers leaf temperatures and reduces afternoon heat load. It can effectively cut the number of high-stress days experienced by plants in that spot.

4) Does irrigation change the actual zone?

Irrigation does not change air temperature, but it reduces plant stress through cooler soils and improved transpiration. That is why the calculator includes a cooling adjustment.

5) Can I use this for container gardens?

Yes, but containers often run hotter and drier than ground soil. Treat your result as conservative and consider extra shading, larger pots, and frequent watering.

6) Is this the same as winter hardiness zoning?

No. Winter hardiness is based on cold minimums, while heat zoning tracks frequent high-temperature stress. Use both to match plants to your climate year-round.