There is no doubt that the ideal site for a greenhouse is one in full sun. You can shade easily enough, but it is not so simple to provide light of solar intensity. The sun is also an important source of free heat in winter.
Nevertheless, you can still put a shady greenhouse site to excellent use.
Town dwellers, in particular, often have little choice but to put their greenhouses in the shade. Small town gardens are frequently overshadowed by walls or other buildings. But don’t let this discourage you; it merely means you will have to choose your plants with care and common sense.
There is a wide range of greenhouse plants for which shade can be a great advantage (if not vital), at least during their flowering period. And if you already have a sunny-sited greenhouse and are contemplating erecting an extra one, you may consider deliberately putting it in a shady place. For example, a north-facing conservatory built as a lean-to against your house may not be ideal as a suntrap but it will be excellent for displaying most of your favourite pot plants for a large part of the year.
If you wish to put a shady greenhouse to use in winter, remember that it will cost more to heat than one that traps the sun’s warmth. If you cannot provide artificial heat, choose hardy (or almost hardy) shade-tolerant plants.
Enough warmth to keep out frost will greatly extend your range. Although a little extra warmth does not necessarily mean more scope in terms of colourful and decorative plants, it does mean that you can grow some less common and more exotic species.
Exotic hardy plants
Where there is no heat at all, create an impression of tropical warmth with foliage plants like Fatsia japonica and the various hardy palms. These will thrive in semi-shade, but not in extreme gloom. Most of the hardy ferns, however, positively enjoy having very little light.
For both attractive evergreen foliage and glorious flowers, the camellias are very important where light is limited.
They will usually flower well as quite young plants in pots as small as 13mm (5 in). When they eventually become too large for the cold greenhouse you can put them out in the garden. Another evergreen (and an indispensable climber for the rear wall of a cold, shady lean-to) is Lapageria rosea (Chilean bell-flower). This has shiny foliage and large, tubular, waxy flowers that are usually carmine in colour — less commonly pink. They are borne in early autumn to early winter (August to November).
Most of the hedera (ivies) will tolerate a good deal of shade and there are now many decorative variegated forms that can be used as small pot plants or to cover an expanse of wall. Hedera canariensis is particularly pleasing and in cool conditions develops a reddish tint to the green and cream variegated foliage. This ivy is almost hardy, if not entirely so in sheltered places, and so it will grow especially vigorously in a warm greenhouse or room.
Hardy border plants
A number of hardy border plants make good pot subjects for an unheated shady house. Especially suitable are the hardy cyclamen, bergenia (the handsome flowers tend to get damaged outdoors in winter), primulas of various kinds, a number of lilies, trillium (which should be given a leafy compost and will even enjoy deep shade), the long-spurred aquilegias (columbine), Excelsior hybrid digitalis (foxglove) — if there’s height for them to grow to about 1-5m (5 ft) — and hostas (plantain lily) that bear delightful flowers as well as having beautiful foliage. All the hardy spring-flowering bulbs should ideally be given a cold, shady greenhouse for them to display their flowers which will then last much longer, but good light is essential in mid and late winter (December and January) to keep their foliage short and green.
Pot plant displays
A shady, frost-free greenhouse makes an ideal conservatory for the display of nearly all the popular pot plants that are grown to give a mass of colour from mid winter to early summer (December to May). These include cineraria, calceolaria, primulas such as Primulus malacoides, Primulus obconica, Primulus sinensis and Primulus x kewensis, autumn-sown schizanthus (butterfly flower), salpiglossis and other annuals, and cyclamen.
If the house is not too shady most of these can also be grown in it from the earliest stages — from seed or storage organs. Where the amount of shade is considerable, however, it is better to use frames warmed with soil-warming cables. Place them where there is good light for the earlier stages of growing, so that strong, compact plants will develop. Ideally, of course, this is where another greenhouse with an open, sunny position can help enormously.
Flowers for summer and autumn
For summer- and autumn-flowering plants shade is usually vital, and as most of those grown in the home greenhouse will need no extra heat during the summer, a shady greenhouse is the ideal home for them. The ‘top’ group of shade lovers is the family GESNERIACEAE. This includes saintpaulia (African violet), streptocarpus (Cape primrose), gloxinia, Rechsteineria leucotricha (silver song), now commonly grown from seed, the gesneria varieties (that are now classed with the genus rechsteineria), achimenes and smithianthas.
Nearly all the popular foliage plants, with the exception of coleus, also like a fair degree of shade. Shade in summer is particularly essential to most of those now known as house plants, such as foliage begonias, peperomia, pilea, maranta (prayer plant) and calathea. Most of these will be happy under the staging.
Fuchsias will generally do better with some summer shade, more especially to prevent high temperatures that cause the flowers to fall or be short-lived, but the shade must not be too heavy. Regal pelargoniums also benefit from slight shade; it makes the flowers last longer.
Bulbs and vegetables
Many of the summer-to-autumn flowering bulbs or other storage organs may have to be fairly heavily shaded, both to keep the temperature down and to prevent sun scorching flowers or foliage. The giant-flowered begonias can be severely damaged by excessive light, the flower petals becoming bleached or scorched at the edges. Hippeastrums and lilies may suffer similarly.
Few vegetables will enjoy a shady greenhouse, but if there is warmth it can be used for blanching plants like endive, and for forcing and blanching seakale, chicory and rhubarb. One exception, however, is the mushroom. This does not need light — but darkness is not essential either, as many people suppose.