While the slightly heated surroundings of the cool greenhouse expand the scope of plants which can be kept enormously, the warm greenhouse – where the temperature never falls below 10 degrees C (50 degrees F) – takes this to new heights. However, allowing gardeners to realise their ambitions for growing exotics and true “hot-house” plants comes at a cost, although the results should make it all worthwhile.
For the ambitious gardener, the warm greenhouse opens up the opportunity to grow many more varieties of plants than would otherwise have been possible and the additional warmth offers particular benefits for propagation during winter and early spring. With a winter temperature of around 10 -16 degrees C (50 – 60 degrees F), plants such as winter-flowering begonias, cyclamens, dracaenas and poinsettias can all been grown with relative ease, along with a number of ferns such as Nephrolepsis, the Ladder Fern.
A wide range of other plants are also ideally suited to these conditions, including allamanda – a climber with spectacular flowers – streptocarpus, trailing columnea, various exotic orchids and the highly decorative crotons. This type of glasshouse allows many plants to be propagated from cuttings – and a misting system can be very useful, making the whole job much easier. There are several types available, ranging from small and simple misting sticks for around £20, to highly sophisticated dedicated units costing twenty times as much.
The warm greenhouse is not just the place for exotics however and many of the usual plants can also be grown here very successfully. In the warmth of this environment, bulbs including the likes of hyacinths and pre-cooled daffodils and narcissi can be urged into flowering long ahead of their outdoor relatives. Pot-grown strawberries can be ready for picking as early as the beginning of April – three weeks or a month ahead of the cool greenhouse and perhaps two months earlier than those grown outdoors. Other fruits – peaches and nectarines, for instance – also ripen much sooner. Seeds and cuttings for planting in the garden can be raised very easily in the warm greenhouse, but most will need an extra period of acclimatisation to make sure the shock of being transplanted is not too great. The garden frame is an essential extra for the owner of the warm greenhouse, enabling early bedding plants and the like to be hardened off before they have to face the full rigours of the British outdoors.
Managing the Warm Greenhouse
Generally speaking, throughout spring and summer, the management of this type of greenhouse is more or less the same as for cool greenhouses – chiefly ensuring that the plants are well ventilated, shaded from the worst effects of direct sunlight and properly watered. However, it is during late autumn and winter that the differences between the two really make their presence felt, particularly where winter watering is concerned. While damp, cool greenhouses provide ideal conditions for grey mould (botrytis) making it essential to water carefully in cold weather, the problem is seldom encountered in warmer, well ventilated greenhouses. As a result, careful watering need not be the big issue it is for cool or unheated greenhouses – though this advantage comes at a price. Heating the warm greenhouse inevitably costs more than maintaining the lower temperature of its cool counterpart – the bill perhaps being double or more, depending on the types of plants being grown. The cost of fuel may well have a bearing on the type of heating system chosen – since the larger and hotter the greenhouse, the more heating it will require. While the judicious use of insulation can be a big help in reducing the bills, the warm greenhouse will always remain a more costly affair and it is obviously important to consider this aspect very carefully before opting to grow at higher temperatures.
Although the warm greenhouse is bound to cost more to heat, it does bring significant advantages, not least by making optimum growing conditions for many types of plants much easier to provide and so greatly expanding the range of what it is possible to grow. Provided the running costs are taken into account at the outset, growing in a warm environment is a great way to widen any gardener’s horizons.