The flood of tropical plants that swept into Britain in the 19th century changed gardeners' perceptions of the glasshouse. Instead of being a store to overwinter tender evergreens, it became a place of permanent display. Glass roofs replaced the solid covering of the typical 18th-century orangery, and more and more glass was set into the walls to provide the extra light that the new plants demanded.
This century there has been another shift, as the conservatory has become a place for living as well as a space to grow plants. But how do you balance the two conflicting requirements of people and plants?
"Invest in a dehumidifier," say Andrew and Penny Bullivant, who built a conservatory 30ft long by 15ft wide against the east-facing wall of their Somerset farmhouse. Their dehumidifier collects five litres of water every 24 hours.
The Bullivants' conservatory smelt of `Paperwhite' narcissus and tender white jasmine on the day I visited. Raspberry-red abutilons flowered along one beam; deep purplish-blue tibouchinas waved enticingly from the roof. Most of the vines had already been pruned, though butter-yellow leaves still hung on one forgotten branch of the black grape `Madresfield Court'. Tender ferns, clivias and the soft leaves of a peppermint-scented geranium spilled over the fronts of the planting bays.
This custom-made conservatory started as a sketch on the back of an envelope; the building has two solid walls, two walls mostly of windows and a double- glazed, pitched roof. One of the advantages of building it lengthways on to the house was that the Bullivants could borrow one long wall (and its heat) from the house itself. The east-facing position is ideal. The morning sun warms up the conservatory, but plants do not get frazzled, as they tend to in a south-facing one.
The other solid wall is on the adjoining, short, north side, with the windowed walls to the west and south. French oak was used for the window frames and for the roof struts. Penny Bullivant washed all the wood with a white wood stain and then wire-brushed it so that it looks like pale limed oak. The walls were rendered with cement and faintly coloured with pale ochre paint, the floor and window ledges covered with buff-coloured terracotta tiles from Italy. The two solid walls provide plenty of growing- space for plants. The two windowed walls and the glass roof keep the place light, even in bleak midwinter.
A door in the east wall connects the conservatory with the sitting-room beyond, but there's another escape route, a glass door, which leads on to a terrace on the south side. Heat, when it is wanted, comes from three radiators set under the windows. They weren't switched on the day I visited, but the temperature in the conservatory was still 54F.
Heat will dictate the terms on which you use your conservatory. The earliest glasshouses were warmed with pans of charcoal, carried inside by gardeners to keep the temperature above freezing point. Steam heat was popular for a while, but boilers had a distressing habit of exploding. A Scottish enthusiast once floated the idea of using the hot breath of cattle to warm the conservatory air. I can't see that catching on, even with the Bullivants who have plenty of the beasts around them.
The farm, though, did provide the planting-containers sunk against the east and west walls, 40-gallon drums which once contained Sylade. That is the stuff that farmers tip on their silage heaps to turn green grass into fruity winter cattle feed. When the plastic drums were in position, they were half-filled first with biggish stones, then with gravel. On top of this went the compost, into which Penny Bullivant planted Salvia guaranitica, passion flower and jasmine. When she waters the containers, excess water drains through the gravel to the bottom of the drums where it provides a reservoir for the plants if they need it.
The salvia, she said, had been outstanding, flowering with rich blue spikes over a long period from autumn into winter. The best one to use in conservatories is the variety `Black and Blue', which is more tender than the ordinary S guaranitica - but with protection, that does not matter. Nor does the fact that it grows so tall - up to 8ft if it is happy.
The biggest planting bed, 8ft by 3ft, is made against the short, windowless north wall, with tibouchina, a huge-leaved honeysuckle, Lonicera hildebrandiana, plumbago and cobaea fighting for supremacy. All the climbers are tied into wires stretched across between screws plugged into the wall. The honeysuckle has big, leathery, evergreen leaves and enormous trumpet flowers, about 6in long and richly scented. When they first open they are white, but as they age they gradually darken, until they finish a yellowy-orange colour.
Of all the plants in the conservatory, Penny Bullivant likes the vines best. They provide the feeling of luxuriance that so many conservatories lack. They shade the roof in summer, and they fruit with mad abandon. For the first couple of years she let them rampage, training out the growths over wires stretched under the roof. When they had covered as much ground as she could spare, she began pruning them. They get severely cut back in late autumn each year, but she finds that they also need controlling during the growing season, when she gets rid of a great deal of the new, sappy growth.
The Bullivants get an equally good harvest from their passion flower, the edible kind (Passiflora edulis), rather than the showy one (P caerulea) generally used in gardens. Penny Bullivant grew it from a seed and for 10 years, before the conservatory was built, she kept it in a pot on a window ledge. As soon as it was set free in the conservatory bed, it grew at a prodigious rate and started fruiting. Its foliage is much glossier than P caerulea's.
How much work does a conservatory like this need, to keep it in order? The worst job, say the Bullivants, is cleaning the outside of the glass roof. There's a wide, lead-lined gutter between this roof and the wall of the house, strong enough to take a ladder. It's the only way they can reach the glass of the inner pitch. Inside, the glass is washed over with vinegar water to get rid of any mould.
The relative dampness of the building keeps away bugs such as red spider, the conservatory gardener's worst enemy. Occasionally greenfly home in on a juicy shoot of jasmine, but Penny Bullivant just nips off the affected tips and throws them away. Rarer still are clouds of whitefly round the geraniums, but a smoke cone soon deals with those. She hoses all the plants down frequently, and thinks that this is the best defence against bugs. Most conservatories are much too dry. Even with a dehumidifier, theirs is not.
Once a year she tops up the compost in the beds, and, in spring, sprinkles Phostrogen round all the plants, including the ones in pots (lemon trees, clivias, camellias). All through the summer the more rampant climbers, such as the passion flower, need cutting back.
"If you go away for too long, the conservatory reverts to Amazonian jungle" she warns. That's a small price to pay for an place that will give you in January a flower as searingly brilliant as the tibouchina. Frankly, I'm jealous.
For an exhaustive catalogue of plants suitable for conservatories, get hold of `Conservatory and Indoor Plants', by Roger Phillips and Martyn Rix, published in two volumes (Macmillan, pounds 19.99 each).Reuse content