WHEN my children were small, they used to be amused by my tales of the old days before fridges, washing machines and central heating. I was born four years after the end of the war and there were not many luxuries.
One of the things they found hard to imagine was a bedroom so cold that frost formed on the inside of the windows. (You had piles of blankets so it was cosy in bed, though nasty stepping out on to chilly lino.) The frost formed the most beautiful patterns, and here are a few examples.
It’s got round to the time of year for night frosts, though here in Lancashire we have not had one yet and the geraniums are still blooming. However you need to get in any tender plants you want to last through the winter (I don’t bother with the geraniums – despite Monty Don’s exhortation not to buy them from garden centres every year, that is what I do. Or supermarkets). Frost kills them by freezing the water in their cells, damaging the cell walls. This can be exacerbated when the sun comes out because the rapid thawing ruptures the walls.
Hardy plants can take that kind of damage which kills the top growth, but they too are vulnerable to prolonged sub-zero spells when the soil becomes frozen. The roots are unable to take up water and the plants die from lack of moisture.
However some plants benefit from frost – sprouts and parsnips are supposed to have better flavour after an icy blast. It is hard to tell these days because these crops are available all year round, not just after the frosts start.
There are several types of frost. It is formed when water vapour comes into contact with a solid surface with a temperature which is below freezing or soon dips below freezing. This changes the water vapour from a gas to a solid (ice), but different circumstances result in different forms.
The window frost of my childhood is formed when the outside of the glass is below freezing and the inside is in contact with warmer air containing moisture (for example, from a sleeping person breathing). It develops slowly throughout the night.
Outside you are likely to see ground frost, which forms when the air is still and cold, usually on clear nights. Water vapour in the air condenses on solid surfaces and as the surface temperature drops below freezing, ice crystals form. It looks like this:
Hoar frost requires slightly different conditions. It forms when the water vapour in the air comes into contact with solid surfaces that are already below freezing point. Ice crystals form immediately, and the ice continues to grow as more water vapour is frozen. On a still night, it can grow thick if the surface temperature is stays below zero for several hours.
Here are a couple of great time lapse videos of hoar frost formation. They say they are taken under controlled conditions but I am not sure what that means.
Rime is different again (and apparently does not technically count as frost) as it involves supercooled droplets of water landing on surfaces which are below zero. They freeze almost instantly, creating a mixture of tiny ice particles and trapped air. This type of ice is more often encountered on aircraft, when it is a dangerous pest.
When the frost does come, it will mean the end of the season for the gunneras. They are already looking a bit sorry for themselves and have taken a battering from high water in the stream. This is them at the end of May:
And this is them yesterday:
All being well they will be back bigger and better in the spring.