Frost Protection & Pick the Method That Saves Tonight

It depends on three things the tool checks together: how far below your crop's critical temperature the forecast low is, whether there is a radiation inversion (warm air aloft), and whether you have continuous water with a safe dewpoint. With a strong inversion a wind machine raises the canopy about 1.5–3°C cheaply; with no inversion you need heaters or an over-tree sprinkler. The selector picks the cheapest single method that lifts the canopy above the 10% kill point, and tells you to stack methods when none does alone.

T10 is the temperature at which about 10% of buds, blossoms or fruitlets are killed in a 30-minute exposure — the point where damage begins. T90 is where roughly 90% are killed — a near-total loss. For apple at full bloom T10 is about −2.2°C and T90 about −3.9°C, so even a 1–2°C lift can be the difference between a light hit and a wipeout. The values get less cold-hardy as the bud opens from tight cluster to bloom to fruit set.

Yes. Apple at full bloom has a T10 of about −2.2°C and a T90 of −3.9°C, so −3°C sits in the light-to-moderate damage band and is heading toward severe. You would need roughly +1.3°C of active protection to get the canopy comfortably above the 10% kill point. An over-tree sprinkler (with water and an adequate dewpoint) or heaters would clear it; a wind machine would only do so with a strong inversion.

A wind machine (frost fan) typically raises canopy temperature by about 1.5–3°C — but only by mixing warmer inversion air down onto the crop. If there is no inversion (an advective, windy freeze) there is no warm air to pull down and the fan does almost nothing. Fans also lose effect once natural wind exceeds roughly 8 km/h, because the air is already mixed.

Over-tree sprinkling protects by releasing latent heat as water freezes on the buds, holding them near 0°C — but only if water is applied continuously and the air is humid enough. If the dewpoint is too low (very dry air), evaporation cools the bud faster than freezing warms it, and the crop ends up colder than with no protection. That is why the tool flags a large air-minus-dewpoint gap as an evaporative-freeze danger.

A radiation frost happens on calm, clear nights when the ground radiates heat to the sky and a temperature inversion forms (cold at the surface, warmer aloft). An advective freeze is driven by a cold, windy air mass with no inversion. Wind machines, fog and row covers only help with radiation frosts; heaters and over-tree sprinklers can also help in an advective freeze because they add or release real heat rather than relying on mixing.

Start before the canopy reaches the danger zone — typically when the temperature falls to about 0°C for sprinklers (so ice forms continuously) and a degree or two above the critical temperature for fans and heaters. Run continuously through the coldest hours and, for sprinklers, keep going past sunrise until the ice has melted, because turning off too early during melt can re-cool the buds. Monitor with a wet-bulb or canopy-level thermometer, not the official forecast.

Yes, for low crops. A floating row cover or frost cloth traps soil and radiative heat against berries, vegetables and nursery beds, giving roughly 1–4°C of protection depending on fabric weight. They are the cheapest method and need no inversion, water or fuel, but they only suit crops low enough to cover and give less lift than heaters or sprinklers on a hard freeze.

The figures are rough running-and-owning guides per hectare per night — for example a wind machine around $30/ha, an over-tree sprinkler around $18/ha, heaters around $60/ha and a row cover around $6/ha. They cover fuel, water and the amortised equipment, not the capital purchase. Use them to compare methods on the night, not as a precise budget; local energy and labour costs shift them significantly.

On a deep freeze the deficit can be larger than any one method's rise. The selector then tells you no single method is sufficient and to stack them — for example heaters plus a wind machine recirculate heated air for a combined 3–6°C lift, or an over-tree sprinkler plus a cover. Combining methods is also more fuel-efficient than running heaters alone, and accepting some loss may be the realistic outcome below the T90 line.

The tool adds the method's mid-range temperature rise to your forecast low: protected temperature = forecast low + rise, but only when the method is feasible tonight. It then compares that protected temperature against the crop stage's T10. For example a −3°C low plus an over-tree sprinkler's +4.5°C gives a 1.5°C canopy, comfortably above apple full bloom's −2.2°C critical line.

Yes — that is the most important point. Dormant and tight-cluster buds are very cold-hardy (apple silver tip survives below −7°C), but hardiness drops fast as the bud swells, opens and sets fruit. By full bloom and especially at fruit set the same crop is killed only a degree or two below freezing. Always select the current growth stage, because using a dormant critical temperature on an open blossom badly underestimates the risk.