How do I read a weather forecast for frost risk in my garden?

A forecast gives you a number for a region. Frost is a phenomenon of a square metre. Bridging that gap is a specific skill and it takes about a minute a night in the risky weeks.

Step by step

1. Find the overnight minimum, not the daily low.Use an hourly forecast and look at the small hours, typically between three and seven in the morning. The 'overnight low' headline is often for a broad area and rounded.
2. Check the cloud cover for those same hours.This is the field that changes the answer most. Clear or mostly clear through the small hours means radiation cooling is unimpeded and the actual surface temperature will run well below the forecast air figure.
3. Check the wind speed for those hours.Anything above a light breeze mixes the air and suppresses frost. Calm or nearly calm is a frost condition. A forecast that shows the wind dropping away after midnight is the classic setup.
4. Check the dew point if the forecast gives one.A dew point well below freezing means dry air that will cool hard and frost readily. A dew point close to the air temperature means moisture in the air, likely dew or mist, and a slower cooling.
5. Apply your site correction.Take the forecast minimum and adjust it for what you know about your garden — colder if you sit in a hollow, at the bottom of a slope, on high ground or out in open country; warmer if you are urban, coastal or sheltered.
6. Decide on the plant, not the average.The question is not whether it will frost but whether the specific plants at risk can take it. Just-emerged potato shoots, open blossom, hardened-off bedding and established shrubs have completely different thresholds.
7. Act in the evening, not at bedtime.Fleece has to go on before the ground has lost its heat, because a large part of what it does is trap warmth rising out of the soil. Covering at eleven at night saves much less than covering at seven.

Tips

• Two or three different forecast providers disagreeing by a degree is normal and is a signal in itself — a marginal night is exactly the one to protect on.
• Set an alert for your area in the risky weeks either side of your last frost date rather than checking each night. It removes the failure mode of forgetting.
• Local weather station networks and amateur stations often publish live readings from a few streets away, which is far better than a regional forecast for calibrating how your area compares.
• If in doubt, cover. Fleece left on unnecessarily on a mild night costs nothing; a tray of tender plants lost costs a season.

Common mistakes

• Reading the daytime forecast and assuming a warm day means a warm night — The two are only loosely connected. Some of the sharpest frosts follow bright, sunny, cold days, because the same clear skies that made it sunny let the heat straight out after dark.
• Using a forecast for the nearest large town — Towns run warmer overnight than the countryside around them, sometimes by several degrees on frost nights. A rural garden using a city forecast will be caught out repeatedly.

Questions people ask