Hygrometer
It is half past eight, the light has gone, and you are standing at the back door deciding whether to spend twenty minutes dragging fleece across the garden.
Do it and it is probably wasted. Skip it and you might lose the lot. Every clear spring evening presents the same coin toss.
It is not actually a coin toss. Three numbers settle it, and all three are in an ordinary forecast.
The three numbers
1. Will the sky be clear?
This is the biggest single factor. Cloud radiates heat back down and largely prevents surfaces cooling below the air temperature. An overcast night at 34°F is usually safer than a clear one at 38°F.
Overcast: do not bother.
2. Will there be wind?
Even a light breeze mixes warmer air down across the surface and stops the chilled layer forming. Frost needs still air.
Breezy: do not bother.
3. Where is the dew point?
The one people miss. Air cannot cool much below its dew point, because condensing water releases heat that halts the fall. So the dew point is a floor under the night.
| Dew point | What happens | Cover? |
|---|---|---|
| Above 36°F | Cooling stops well above freezing | No |
| 32–36°F | Dew or light frost forms, cooling largely stalls | Marginal |
| 25–32°F | Surfaces reach freezing before anything stops them | Yes |
| Below 25°F | Nothing stops the fall. Hard frost likely | Yes, and check what is exposed |
Why covering works at all
Not by insulating — by blocking the view of the sky.
Leaves lose heat by radiating infrared upward to space. A cover intercepts that and radiates back down, so the plant is exchanging heat with fabric a few inches away instead of with the upper atmosphere.
That is why a single layer of thin horticultural fleece, which has almost no insulating value, works so much better than its thickness suggests. It is not keeping warmth in. It is stopping the radiation getting out.
The same reason a bed under a tree or against a wall survives nights that an open bed does not.
Doing it properly
- Keep the cover off the foliage. Hoops, canes, anything. Where fabric touches leaves it conducts heat away and frost damage appears exactly at the contact points.
- Take it to the ground if you can. Trapping the ground's stored heat under the cover is worth several degrees.
- Water the soil in the afternoon before a frost night. Damp soil holds more heat and releases it slowly overnight.
- Take covers off in the morning. Left on through a sunny day they cook what is underneath and hold humidity that encourages fungal problems.
The one that catches people
Cold air is denser and drains downhill after sunset, pooling in low ground.
So a bed at the bottom of a slope can be several degrees colder than one halfway up, and a walled garden with no low outlet can act as a frost trap. If you lose plants in the same corner every year and the forecast never seemed cold enough, that is why.
The hourly forecast shows the temperature and dew point together, which is all three numbers in one place — and turns a twenty-minute gamble into a thirty-second check.