According to my weather book, if you are measuring in degrees Celcius, (and this is what they say)
0 to - 3.5 brings a slight frost
-3.5 to - 6.6 brings a moderate frost
-6.6 to - 11.5 brings a severe frost
-11.5 and below brings a hard frost.
In general terms if it is cloudy you’re not going to get frost, but be careful, the night can change and although when you go to bed it appears cloudy and mild, it can change and you wake up to find it white. Similarly a strong wind can slow down the night-time cooling, but if the temperature falls anyway below freezing, the wind can simply help the frost to penetrate. No easy answers – just be on the watch.
Another thing to note, in the Autumn, you can have air frost when the air temperature at about 1.5m above the ground falls below freezing, but the ground temperature still hasn’t cooled to freezing yet. When the air temperature at 5cm above the ground falls below freezing point, a ground frost may occur, even though higher up it is not so cold. The white frost we’ve been describing above is called hoar frost which, as we said, occurs on clear, calm nights when water vapour touches icy-cold surfaces and instantly freezes onto them. For gardeners, obviously, the hoar frost is likely to do the most damage, but the ground frost may also harm some plants. The air frost is just going to make it feel chilly for you.
While we’re at it, we’d better mention rime frosts which occur when it is very cold indeed and as the wind blows damp, freezing air over branches, foliage etc. it freezes and can form amazing shapes. When dew forms on grass and then freezes and more ice crystals form on top, they can form a fern or leaf-like pattern which we call fern frost.
3. Beware Frosts
How to deal with frosts?
Here are some suggestions:
1. If you have bedding plants that are annuals - forget them. When the frost gets them it’s time to clear them out, chop them and compost them (more on composting another page!)
2. Cover up vulnerable plants. Old hands suggest that virtually anything will do – old blankets, old sacks to cover the roots area particularly. If you try covering a whole plant you need to uncover it to get sunlight – it’s easier to pot them and put them in a light potting shed, cold frame or greenhouse. You can buy ‘fleece’ protection in sheets that make life easier.
3. If your plants are hardy, risk it and leave them uncovered. Even after the minus 3 or 4 of last winter lots of the stuff survived.
4. If you are watering plants, do it a day or so before frosts are expected.
5. Mulching with chips, pine needles, leaves etc. can help but some tender plants don’t like heavy mulching.
6. In the long-term creating raised flower beds is said to help as cold air tends to collect in sunken or lower areas and raised beds may escape the worst of the cold.
7. Thinking about the openness or otherwise of your garden may produce ideas of how to provide protection from th blasts of cold air that may come in from nearby fields, say.
Types of Frost
To see all the different sorts of frost that occur and their severity, we quote below from our 2010 October Seasonal reflections:
Dew goes and frost arrives when the temperatures drop. When the temperatures fall sufficiently, surfaces such as the ground or my shed roof drop below freezing and water vapour settles on it and turns into ice crystals that we call frost – beautiful looking but devastating for some of the plants in our gardens. Frost is white because the ice crystals, of which it is made, contain air.