The last couple of nights have seen a heavy frost here. Down to -4•C last night. Pretty, but will sadly damage the last of the tender plants in the garden unless action has been taken beforehand.
Frost is normally formed on still, clear and cold nights. Clear skies allow the heat from the ground to escape into space. The cool air causes water vapour in the air to condense and form droplets on the ground, objects or trees, where surfaces have a temperature of freezing or below causing water to freeze. However, because the ground cools quicker than the air around a metre above, it is possible for a ground frost to occur without an air frost.
There’s also something called a grass frost, which is where natural surfaces such as grass freeze when man-made surfaces such as tarmac and concrete don’t as they can retain heat for longer. This type of frost is of most interest to gardeners and can quite often happen in late spring catching gardeners unaware.
Hoar frost is another type of frost typically associated with a frosty morning and has a white appearance. This occurs when dew has formed and then frozen when the temperature hits 0•C.
It is important to lift tender perennials such as some shrubby salvias, dahlias, pelargoniums, cannas, and fuschias before the first frosts as they will not recover, however it is possible to leave the previous seasons’ growth on some less tender plants until spring, for example penstemons, as this provides valuable frost protection during the winter. Taking a few cuttings beforehand is good practice just incase these plants don’t survive the winter.
As a gardener it is important to identify areas in the garden that are frost pockets so that the most tender plants are not situated there. Also plants such as camellias when in flower will suffer from being placed on an east facing wall as the frost will thaw out too quickly and damage the flowers in spring.
As a gardener it is important to know your plot and act accordingly.