Orchids in the Greenhouse: Cool House Orchids

ALL types of orchids can be grown in heated greenhouses, provided their natural living conditions are imitated.

The jungle, with its high temperatures, heavy shading and excessive and continuous humidity, is the home of the ‘tropical’ orchids. In the savannah, at about 2,000 ft. above sea level, where the jungle is not so thick and where the temperatures are more moderate by night, can be found the ‘intermediate house’ orchids, clinging to the drooping branches of the trees that line the streams and gullies. In the Himalayas and in the Andes, at heights between 5,000 and 10,000 ft., where conditions are cooler and the atmosphere more rarefied, grow the so-called ‘cool house’ orchids.

The latter are the orchids whose conditions of growth can be most easily imitated in the greenhouse, and they and the genus Cymbidium—the easiest orchids to grow—are discussed here.


It is possible to buy seedlings of orchids such as cymbidiums by the dozen relatively cheaply, although seedlings from first-class parents are more expensive and are sometimes more difficult to grow successfully. It is therefore advisable to buy a number of larger, robust plants which are of flowering size.

Very cheap plants should be avoided, as they are probably rootless or one of the poorest types of orchid.

Cymbidiums come into flower during the winter and will remain in flower for many weeks. The flowers should be cut a few weeks after they open, because they are waxy and heavy, and if the plant is strained, the following year’s growth will be delayed.


Normal cool house orchids include approximately 100 genera, 4,000 or 5,000 species and many thousands of hybrids. Among these there are even some of the more exotic and magnificent orchids, such as Vanda coerulea, the blue Indian orchid; the rosy-white Cymbidium insigne (syn. C. sanderi) from Cochin-China; the brilliant orange Dendrobium fimbriatum from India, and its variety oculatum with two rich crimson eyes, also from India.

These orchids need a constantly moist atmosphere, though humidity will reach about 100 per cent in the spring and summer months during the day, and drop to as low as 60 per cent during the winter nights.

In the winter the temperature in the greenhouse can be allowed to drop to as low as 45 to 48° F. (7 to 9° C.) and should rarely be raised artificially above 50 to 55° F. (10 to 13° C).

Give the orchids a little extra artificial heat during the spring months when it is often cold and dank in the early morning, but avoid high temperatures towards the end of the year when the bulbs are maturing.

If there is adequate moisture in the atmosphere, sun heat can increase the temperature to as high as 80 to 85° F. (27 to 29° C.) without damaging the plants.

The exception to these orchids which tolerate extremes of temperature are the pseudo-bulbless orchids. These do not have bulbs above ground to contain moisture and nourishment, and so require a constantly even temperature, preferring a range of not more than 20° F. (11° C), approximately between 48 and 68° F. (9 and 20° C).

A typical pseudo-bulbless orchid is Disa grandiflora, from The Cape, which grows at a high altitude in a cool, moist atmosphere; Cypripedium insigne is another popular species.

Odontoglossum crispum, from Colombia, which grows on trees clinging to the almost vertical sides of north-south valleys and gorges, through which run fast-moving, moisture-giving streams, is representative of another cool house group now widely grown.


The shading of orchids is essential, but should be carried out judiciously as it cuts out the sun’s heat as well as the sun’s light. Few plants suffer from too much sunlight, whereas many suffer from too much shade, and it is far easier to over-water a plant in a heavily shaded greenhouse than in a house that receives a lot of light.

From November to February the green-house can be left fully exposed to direct light. In March and April such exposure would be dangerous, because the plants are often rather soft after the winter, and at midday the greenhouse should be shaded on the side exposed to the sun. An easy method is to stipple about 50 per cent of each pane of glass, usually the central part, with whitewash. During March, April, September and October the blinds should be in position ready to be let down in case there are some exceptionally sunny days, and in the four summer months, from about mid-April to mid-August, full use should be made of the blinds.


The heating of the greenhouse and the shading of the glass have a direct bearing on the amount of water needed to keep the plants in perfect condition.

Do not water the plants very often until new growth appears after the turn of the year. The majority will start to grow as late as February or March, or even April, but some tend to grow rather quickly after their short winter rest. They should be examined at least once a week in the first months of the year, and twice a week by the middle of March, when the days are lengthening.

Never water a wet plant, but never allow a plant to dry out to such an extent that it is ‘paper’ dry. The leaves and the surface of the compost will indicate if the plant is too dry, and if it is, dip the pot into a bucket of rain-water to expel all the air, before watering again with a watering-can.

During the winter months orchids with pseudo-bulbs (the moisture containing, bulbous parts of the plants) require relatively little water. If they are watered excessively at this time the plants will tend to grow again too quickly and will fail to produce their flower spikes.


Fresh air is almost as essential to orchids as a moist atmosphere. It is difficult to use the roof ventilators freely in the winter, and often also in the spring and autumn when the wind frequently veers to the east; therefore it would be wise to introduce an electric ventilator into the gable end of the greenhouse with a corresponding small vent in the entrance door.

But an electric fan will not be necessary if the ventilators on the leeward side are used judiciously —an opening of even 1/2 in. is often enough to change the air in a short time.

The amount of fresh air introduced into the house should be related to the prevailing atmospheric moisture content and temperature. A large proportion of the air in a small greenhouse of 10 by 12 ft. is changed by merely entering the house twice a day.


The correct time to repot orchid plants — they are bought from the nursery in pots — is when the new growth starts, that is when the base of the bulb swells and new roots appear.

Never use dirty materials in the culture of orchids. Wash the pots and tureens carefully before use. The pots should be porous, and filled with crocks to a quarter of their depth.

Cover the crocks with a handful of compost. Trim the roots and spread them over the compost and pack them in with pinches of more compost, which should be worked in with a pointed piece of hardwood (a wooden label is suitable) and with a downward and inward motion. Each plant should be positioned so that the base of the leading growth is level with the pot rim and l to 2 in. away from it. Continue to pack in the compost and surface it evenly by trimming it to within a ¼ in. of the top of the pot.

The compost should be made up of three parts Osmunda fibre (Osmunda regalis) to one part sphagnum moss, half a part crushed crocks and charcoal and a handful or two of crushed bone— one part beech or oak leaves, well and evenly chopped, may also be added as desired.

16. February 2012 by Dave Pinkney
Categories: Featured Articles, Garden Management, Gardening Calendar | Tags: , , , | Comments Off on Orchids in the Greenhouse: Cool House Orchids


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