Cold? It's freezing

What can you grow in an unheated greenhouse in winter? In her Workshop series, Anna Pavord advises the Smiths from Scotland My vision of them happily cropping vegetables all winter shattered
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The Independent Online
We have made a garden out of what had been a derelict piece of ground with a motorbike shed and some hens on it, and in spite of the fact that much of it is overshadowed by trees, we are managing to grow things reasonably well. This area is, howev er, a frost pocket and we get temperatures of -23C sometimes at night, with -12C at noon being fairly common in late December, January and February.

Our family has now given us a greenhouse (6ft by 8ft) which is not heated, but we have lined it with bubble stuff to help insulation. We were given some tomato plants and they grew remarkably well and ripened in the airing cupboard when we picked them green. Now we are left with an empty greenhouse and no knowledge of what to use it for in the winter. Can we sow seeds to plant out in the spring, or will they come too soon, as we are likely to have frost at least until the beginning of May? We looked forbooks in the library, which is a van that comes once a fortnight, but there was nothing about greenhouses except heated ones and exotic flowering plants. We would much rather be able to supplement our pension by growing our own vegetables.

Can you advise us what to put in and when, and also recommend any simple book about cold greenhouses which we could ask the library van to get for us?

For three days before Christmas, the Smiths, who live near Dunkeld in Perthshire, could not get into their greenhouse at all as the door was frozen shut.

"Cold" is a mild way of describing their particular situation. It is icy. I asked Mr Smith to take a couple of readings inside and outside the greenhouse this week, to work out what sort of growing conditions the greenhouse might provide. It is lined with polythene bubble-pack.

The news was grim. The thermometer registered no more than -0.5C both outside and inside the building. He got the same message with a second reading the following day: one degree centigrade outside and in. During the night, he said cheerfully, the temperature had gone down to -11C.

This was surprising. You would generally expect a greenhouse, especially an insulated one, to improve on the outside temperature by at least five degrees. But in front and to the south of the Smiths' house is a hill, Bell Mount. In the winter, the sun isso low in the sky, it never reaches over the top of the hill.

A greenhouse can only trap what heat it receives from other sources. As the Smiths' house was not warming up in the sun, there was nothing for the bubble polythene to hang on to. My vision of them happily cropping lettuce, radishes and carrots all through the winter months splintered with the sound of crashing icicles.

There are several different regimes that you can impose on a greenhouse. If you run it as a stove house, with a minimum temperature of 18C (65F), you can grow tropical plants, but will bankrupt yourself in the process. If you maintain it as a warm greenhouse (13C/55F) you will still spend three times as much on fuel as you would on a cool greenhouse which is kept around 7C (45F).

The Smiths plan to use their new greenhouse as a cold house, with no supplementary heating. Because of this they will not be able to use it to overwinter frost-sensitive plants such as geraniums, but the greenhouse will provide protection from wind, rainand snow and should enable them to cheat the seasons by three or four weeks, when the sun finally comes back up over the hill.

It is a standard span-roof greenhouse from B&Q, the aluminium frame glazed to the ground. A wooden-framed house might have provided marginally higher temperatures, but would have been trickier to maintain. Solid walls up as far as the level of the stageing would also have made the greenhouse a bit cosier, but more expensive.

The Smiths have laid concrete slabs to make a path down the centre of the greenhouse, leaving beds of earth roughly two feet wide on either side. Another earth bed stretches along the back of the house. A trestle worktop takes up some of the space along one side.

They want to use the greenhouse to grow food rather than flowers. Tomatoes and cucumbers are the first things that spring to mind. Aubergine, early beetroot and carrots, courgettes, French beans, lettuces, peppers, early potatoes, radishes and melons also do well in greenhouses, but the Smiths will have to gamble with their earliest and latest crops and make a note in their diaries of the dates when the sun disappears from view. That will bring the curtain down on their growing activities.

If I were the Smiths (and I have to say I'm glad I'm not: much as I love Scotland, in temperatures of -23C I would shrivel and die like the lettuce) I would raise plants of tomato, cucumber, courgette, aubergine, peppers and melons in 3in pots on the windowsill of the kitchen or living-room and introduce them gradually into the greenhouse borders as temperatures crept up in spring. The biggest danger is likely to be the gulf in day and night temperatures. This is where trial and error prevails - at least for the first year.

Beetroot, carrot, lettuce and radish you could sow direct in short drills in the borders, a row a week, using radish as the indicator (or sacrificial victim). It is quick to germinate and if it survives, the others are likely to as well. I would try a sowing in late February, if temperatures seemed to be shifting from their winter nadir. It would also be worth experimenting with late sowings in September and October.

It is at this end of the season that the greenhouse is likely to be most useful. The soil inside will have warmed up through the summer and will cool down more slowly than that outside. Heat, without some artificial boost, will be much slower to build upin spring.

Some varieties of lettuce are specially bred to grow in winter under glass, but they need a cool rather than a cold greenhouse. `Kwiek', `Winter Density' and `Valdor' are the ones to look out for.

The Smiths have already had success with tomatoes, and if they grow cordon rather than bush types, they will still have enough space in their borders to intercrop with carrots, lettuces or other vegetables. They should perhaps switch the tomatoes to a different border each season. If you grow tomatoes in the same ground all the time, pests and diseases build up in the soil. But tomato plants grown in borders where roots can run extensively usually have heavier crops than those grown in pots or Gro-bags.Watering is less critical, too.

Whether or not the Smiths will be able to bring in a crop of early potatoes from the greenhouse for Christmas dinner will depend on there being another long mild autumn such as the one we have just had. Even in chilly Perthshire there were only two frosts before mid-December.

You need some 10in pots and a handful of early potatoes such as `Maris Bard' or `Epicure'. Plant one potato to a pot in August and keep them well fed and watered. The crops are not heavy but they have the authentic new potato taste.

The Smiths might even be able to force a few sprigs of mint to go with the potatoes. Chives, marjoram, mint, parsley, rosemary, and thyme all flourish in the shelter of a greenhouse. Pot them up in autumn and bring them under cover.

As for the book the Smiths hope to get via their library van, I would recommend Dr D.G. Hessayon's The Greenhouse Expert. It is the latest in his long line of "Expert'' books and covers conservatories as well as greenhouses.

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