The Cool Greenhouse

Compared with an unheated greenhouse, the scope for a cool one – where the temperature never falls below 4 degrees C (40 degrees F) – is very much wider, opening up the possibility of having plants in flower throughout the year. There are one or two particular aspects of management which need to be treated with particular care if the cool greenhouse is to be successful, but if they are, it offers an unparalleled opportunity for any gardener.

Plants for All Seasons

One of the key attractions of the cool greenhouse is the ability to plan for a succession of colourful displays throughout the year – aside of the ability to propagate a wide range of seeds and cuttings either for the greenhouse itself or for planting out in the garden. At the beginning of the year, daffodils, narcissi, tulips and hyacinths, along with Japanese azaleas, cinerarias and primulas can all provide early colour, long before their outdoor relatives have even begun to grow.

By April, calceolarias, fuchsias, godetias, pelargoniums and schizanthus – a beautiful plant, also known as the Poor Man’s Orchid – will be taking over, providing a display which will last to the end of June. From then until September, begonias, campanulas and gloxinias join the still-flowering fuchsias and pelargoniums lending new interest to the greenhouse. In autumn, pre-cooled bulbs, chrysanthemums, cinerarias, primulas and zonal pelargoniums offer a splash of colour to see out the year.

If the minimum temperature in the greenhouse never falls below 7 degrees C (45 degrees F) perpetual-flowering carnations can be relied on to provide a year-round display themselves. A small propagation unit makes a good addition to the whole set-up, allowing the higher temperatures necessary for the early germination of begonias, gloxinias and lobelia in the spring.

All of the fruit and vegetable options available for the unheated greenhouse are, of course, open to the cool greenhouse too. Lettuces can successfully be grown throughout the winter months and tomatoes through the summer – though the temptation to sow too early should be resisted, particularly if the greenhouse has no additional lighting. Many plants do best when the days are longer, so holding off tomato planting until March at the earliest is generally a good idea. Peaches, nectarines, apples, pears, plums, cherries and strawberries are all possible for the owner of the cool greenhouse too.

Winter Management

Ventilation is always of paramount importance, but the cool greenhouse needs more free-flow of air than its unheated counterpart, chiefly because of the types of plants commonly grown in it. Providing good ventilation – using top and side ventilators – whenever possible is one of the key factors to success, though obviously a bit of discretion is called for when the full icy winds of winter are blowing at their worst.

The key to managing the cool greenhouse in winter is to ensure that the environment is kept much drier and the plants watered sparingly. If the air becomes too wet, there is a real risk of botrytis and other fungal diseases, so watering at this time of year should be done early in the day, rather than the afternoon or early evening, to avoid too much moisture in the atmosphere as the temperature falls.

The cool greenhouse opens up wide possibilities to the gardener and offers the chance to cultivate a broad range of decorative plants – particularly if the temperature can be kept around 8 or 10 degrees C. While it does not allow the same sort of scope that growing under warmer glass-house conditions can provide, neither does it require anything like the same level of expense to heat it; the cool greenhouse can add enormous flexibility and potential to any garden.