The Cool Greenhouse

The cool greenhouse is one which is kept absolutely frost free and in which a minimum temperature of 4°C. (40°F.) is maintained. If the temperature is nearer to 10°C. (50° F.) so much the better for many plants, excluding annuals which much prefer a temperature nearer 4°C. This kind of greenhouse is sheer delight for the adventurous gardener, for compared with the unheated greenhouse the scope is wide indeed. There is no difficulty in having plants in flower during every month of the year. If you wish to make more practical use of such a house it is possible also to grow tomatoes during the summer months and lettuces during the winter.

It is desirable to have a small propagating frame in a cool greenhouse in which a high temperature can be maintained in the spring – say 16 to 18°C. (60 to 65°F.). This is used for germinating seeds of begonias, gloxinias, antirrhinums, lobelias and a host of other plants which are going to be grown on throughout the summer.

I have said that with this kind of greenhouse it is possible to have colour throughout the year, so let us now consider some of the plants which can be grown to provide a succession of colour and interest. If we divide the year into quarters, then in January. February and March we can have Azalea indica, cinerarias, Primula obconica and the most popular of all flowering bulbs such as daffodils, narcissi, Tulips and hyacinths in bloom. The joy these flowers can bring in these bleak, often grey months will be readily appreciated.

In the second quarter, April. May and June, the pleasures are different for plants like the pelargoniums (geraniums) fuchsias, the large-flowered calceolarias and that most lovely of half-hardy annuals. Schizanthus, (the Poor Man’s Orchid or Butterfly Flower) are coming into flower. Also coming into bloom now are such autumn-sown annuals as clarkias, godetias, stocks and salpiglossis.

In July, August and September even the cool greenhouse can begin to look exotic with begonias, gloxinias and fuchsias continuing to bloom, and achimenes, Lilium auratum and L. speciosum rubrum, pelargoniums and Campanula isophylla all making their contribution.

In the last three months of the year there are chrysanthemums, pre-cooled narcissi, daffodils and hyacinths, abutilons, Zonal pelargoniums, Lorraine begonias, primulas and cinerarias in flower, and Solarium capsicastrum, the popular Winter Cherry, will be bearing its orange-red berries.

The perpetual-flowering carnations, or tree carnations as they are sometimes called, are ideal for growing in a greenhouse with a minimum temperature of 7° C. (45°F.) and these will provide a continual display of flowers throughout the year.

The cool greenhouse needs even more ventilation than the unheated greenhouse because of the type of plants grown in it. Most of these need a free circulation of air and can be harmed by too high a temperature. It is, therefore, necessary to use the top and bottom ventilators during the spring, summer and into early autumn, and the top ventilators should also be used throughout the year whenever the weather is what gardeners call ‘open’, that is to say whenever the air is not cold or damp.

Greenhouse Plants

Greenhouse Plants (Photo credit: azmichelle)

Another point to keep in mind with this kind of greenhouse is that it is inadvisable to sow seed too early. To use tomatoes as an example, I would not recommend sowing seed of this fruit until the middle of March and then only in a warm propagating frame and they should certainly not be planted out in a bed or in pots or boxes until towards the end of April. Tomatoes give a better return if they are planted out later when the days are longer and the sun warmer. Generally speaking, most propagating carried out in a cool greenhouse should be done three weeks to a month later than that in a warm greenhouse.

An important environmental factor in managing a cool greenhouse successfully is that the atmosphere in winter must be kept much drier than that in warmer greenhouses, and the plants must be watered with more care. During winter, watering should be done during the early part of the day rather than in the afternoon or evening. If the atmosphere in this type of greenhouse becomes too moisture laden or stagnant due to faulty ventilation then it is only too easy for botrytis to gain a hold on plants such as pelargoniums and fuchsias and be the cause of many losses.

If these cautionary words are heeded and your cool greenhouse is fully exploited you can be sure that it will give you enjoyment in full measure.

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01. March 2012 by admin
Categories: Equipment, Greenhouse Gardening | Tags: , , | Comments Off on The Cool Greenhouse


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