Example data table
| Forecast min | Critical temp | Area | Typical recommendation |
|---|---|---|---|
| -2 °C | 0 °C | 120 m² | Double cloth or plastic plus cloth |
| 30 °F | 32 °F | 800 ft² | Single cloth, start before dawn |
| -4 °C | -1 °C | 60 m² | Combine cover with heat or irrigation |
Formula used
The calculator estimates the temperature gain required to keep plants above a chosen critical temperature target. It applies simple adjustments for wind and humidity to reflect faster heat loss on drier, windier nights.
- Target temperature: Target = Critical + Safety Margin
- Base deficit: Deficit = Target − Forecast Minimum
- Adjusted required gain: Required Gain = max(0, Deficit × Adjustment)
- Dew point estimate: Magnus approximation using temperature and RH
- Irrigation guidance: Rate ≈ Required Gain × 2.0 × (1 + 0.15×Wind)
- Heater estimate: Power ≈ Area × Required Gain × 60 W/m²/°C
How to use this calculator
- Select a unit system that matches your forecast data.
- Enter the expected overnight minimum temperature near the crop.
- Enter the crop’s critical temperature for its current stage.
- Add a safety margin to cover microclimates and sensor error.
- Enter wind speed and relative humidity for the same period.
- Enter the total area you want to protect.
- Press Calculate to see protection need and targets.
- Use CSV/PDF downloads to save a simple night-by-night log.
Frost protection planning article
1) Why frost risk is more than a forecast number
A weather station may report 2 °C, yet leaf surfaces can cool below 0 °C by radiating heat to a clear night sky. Gardens behave like small valleys: cold air drains and pools, especially in low spots and between hedges. This calculator treats risk as the gap between your crop’s critical temperature and the expected minimum, then adds a safety margin for microclimates. A 1.5 °C margin is often enough for mixed beds; high-value blossoms may justify 2–3 °C.
2) Critical temperatures: crop stage matters
“Critical temperature” changes with plant stage and variety. Tender seedlings and blossoms can be injured near 0 °C, while hardened foliage may tolerate several degrees lower. For example, many soft annuals show damage around 0 to -1 °C, whereas hardy leafy greens can endure -3 °C or colder if acclimated. Record your variety and stage in the notes field so your nightly logs remain traceable.
3) Wind, humidity, and dew point signals
Wind increases convective heat loss and reduces the “still-air” benefit of row covers. The tool boosts required temperature gain when wind rises, because advective cooling can overwhelm lightweight protection. Humidity and dew point indicate how quickly the air may cool: a low dew point often means drier air and faster radiative cooling. When dew point is below -2 °C, be cautious—temperatures can drop sharply near dawn.
4) Comparing methods using practical numbers
Protection works by adding heat, trapping heat, or releasing heat during ice formation. Typical row-cover gains range from about 2 °C (single layer) to 4–5 °C (double layer or combined tunnel and cloth) in calm conditions. Overhead irrigation protects by latent heat release as water freezes, but it requires uniform application and sufficient flow. The calculator converts your required gain into an indicative irrigation rate and a heater power estimate per protected area.
5) Turning results into an action checklist
Use the “Protection needed” flag as a decision gate, then focus on the required temperature gain. If the gain is small, a single cover and wind shielding may be enough. For moderate deficits, combine covers and seal edges to reduce drafts. For large deficits, consider staged protection: cover early, then add heat or irrigation before the canopy reaches freezing. Save each run using CSV or PDF so you can refine margins using real outcomes.
FAQs
1) What safety margin should I use?
Start with 1–2 °C (or 2–4 °F) for mixed gardens. Increase it for frost pockets, sensitive blossoms, or when your thermometer is far from the crop canopy.
2) Why can plants freeze when the air temperature is above 0 °C?
Leaves can radiate heat to the sky and cool below air temperature, especially under clear skies and light wind. This “radiation frost” can create freezing surfaces even when nearby air remains slightly above zero.
3) Does a row cover always add the same temperature gain?
No. Gains depend on wind, cover fit, moisture, and how well edges are sealed. Calm nights with tight sealing give the best results; windy nights may reduce gains and require combined methods.
4) When should I start frost protection?
Start before the canopy reaches the critical temperature. For covers, apply before sunset if possible. For irrigation, begin before surfaces drop to freezing and continue until ice melts or temperatures safely rise.
5) Is overhead irrigation safe for all gardens?
It works best with reliable flow and uniform distribution. Poor coverage can worsen injury. Avoid if water supply is limited, wind is strong, or you cannot keep the system running through the coldest period.
6) How accurate is the heater estimate?
It is a sizing estimate based on area and required gain. Actual needs vary with enclosure type, leaks, wind, and heater efficiency. Use it to compare options and plan fuel, then validate with real performance.
7) What inputs improve accuracy the most?
Measure temperature at canopy height, use a crop-specific critical temperature, and estimate wind near the plants. If possible, log outcomes after each frost event to tune your margin for your site.