The weather forecast for central London indicates that on Sunday we will get four degrees of frost. That's a serious freeze for this part of the word and will be a record low for my rooftop this year. What to do?
The cheapest and simplest thing to do is to go out and buy some horticultural fleece. At Homebase this fleece comes in a 4.5x2.2m sheet and costs £7.95. I use it for forming winter fleece tunnels and for simply laying over, or around, vulnerable plants.
Here you can see one of my raised beds, covered completely by fleece. The hoops are made from plastic water pipes. I hold the whole thing together using large laundry pegs. Alternatively, you can lay a big sheet of fleece right across your crops and secure it with stones round the edges. The fleece is light, so it won't really damage plants by resting on top of them. You can also see two different varieties of cloches that I use. The lightweight plastic bell jars are useful, they have an air vent in the top that helps to dissipate the humidity and provides a useful hole for my watering can. But they are lightweight. In a gale, even with four sturdy ground spikes, they do tend to lift off.
Glass cloches are much more expensive, but don't blow away. You can lift the tops off all together or lay them down at 45 degrees to the base, which allows air and water in, while maintaining a level of protection. But when frost is forecast, make sure that you close them up tight. Cold air and frost falls from high to low areas, so the side walls of a cloche alone would only serve to collect the frost, not protect from it.
The plastic cloches and fleece are good value, but won't last forever. The fleece will get dirty after a season or two, but I've discovered that you can put it in the washing machine and wash it on a cool wash! The glass cloches come from Crocus and cost £59.99 (10 Jan 2013)
The great thing about the fleece is that it lets water in, so you don't have to worry about crops drying out.
Now is the time to go round the plot protecting as much as possible. I shall pay particular attention to my grow-house, which should protect against that level of frost. However, I've noticed that the door does not close very closely and there are gaps at the base of the frame. So I shall earth up right round the edges and put some thin polystyrene quilt across the door opening.
Do not despair if some crops seem to have succumbed. Even if some top growth is damaged the roots and the base of the plant may be fine. Wait a while before grubbing them out. Cabbages, Kale and Broccoli should all survive. In fact they say that Brussel sprouts are less bitter once the frost has got to them.
I also use straw around tender plants, underneath the fleece, for a belt and braces approach. Below is a selection of images taken underneath the fleece.
Chinese broccoli, Pak choi, Spinach bright lights, Land cress
But frost has its benefits too. There is no better way of reducing next year's slug population than letting a bit of frost deep into the soil. And if you have a clay soil, the heave effect of the frost will effectively dig it for you - no sweat!