The citrus family have long been favourite greenhouse and conservatory subjects. Indeed they were among the first plants to be so grown. Originally from the Middle and far East the citrus were one of the last old world fruits to be discovered and are not mentioned in the Bible. By the end of the Roman period the first lemons and citrons had reached the Mediterranean. By the time of the Norman conquest the rich were used to them as luxury foods and by Tudor times they had become commonplace.
Citrus proved slow to acclimatise to even Southern European conditions and many doomed attempts were made to grow hardier varieties, or rather in fact to slowly accustom plants to colder conditions. Fortunately citrus are amenable to tub culture so could be taken indoors for the harshest weather -but back then this was into a shed not a conservatory. The shortage of light meant daily transportation in and out, and wrapping with straw on the coldest nights.
As gardens were built to supply the great houses with luxuries citrus became cultivated here and there by clever and ingenious methods. Heated walls with chimneys inside meant the plants could be planted in outdoor borders and one eccentric grew them on a commercial scale by constructing a paper windowed building for them. But it was not until cheap glass enabled greenhouses to be built that citrus really flourished.
However it was not for their fruits. It was for their flowers and their heavy strong perfume. Citrus are very prolific flowerers, especially if regularly cut back and all fruits removed before swelling. The flushes of flowers were used for nosegays, posies and bouquets to try and hide the stink of other people, their unwashed clothes and the sewage strewn streets they walked about in. The Palace of Versailles, famous for it's early Orangery, was built with hundreds of rooms for more than a thousand staff and guests but not a single toilet......
Indeed because the fruits were cheap and easily imported so most citrus were really grown for the flowers, or for ornamental purposes right up until modern times. The Orangery is so called as oranges have the sweeter perfume than lemons though the latter are easier to cultivate and would have been the more successful originally. However although the bulk of plants were still grown for flowers the fresh fruits were a novelty and very attractive visually so once heating could be combined with glass many different varieties were grown for the master's pleasure if not so much directly for his table.
Now with frost free conservatories and greenhouses available to almost everyone citrus are becoming one of the most popular of all tender plants. This is despite them being quite expensive to buy, slow growing and prone to many minor problems. However they are also surprisingly tough and enduring despite their problems and so willing to fruit they are amazing. Lemons are in practice quite economic as they crop so well for so many years. I have one lemon tree that has produced dozens of lemons every year for more than a decade and looks set to go on for ever. I calculate I have recouped the cost price with fresh fruits by several years now and expect decades more. Indeed some specimens can get remarkably old and still remain hugely productive- though by that time they need a huge tub only moved by forklift!
Moving is the key to success. Grown in tubs they can move out of harsh weather and if well fed, watered and pruned confinement does not stop them cropping. Citrus are really much happier outdoors than under cover so as long as there are no frosts they can live in the garden freeing up inside space. But when frosts return they must be brought back inside. This can be simply one trip in and out but is better spread out over weeks.
In spring they are soft from being housed all winter so they need hardening off by going out each day and coming in at night. This can go on for weeks, the longer the better, or rather the sooner you start the better. Indeed citrus can go out any day that is not freezing or seeringly windy -providing they are used to it. And remember that bright sun in early spring can burn foliage that has only seen it through a layer of dirty glass so the first trips out should be on calm cloudy days. The first trips should also be to very sheltered protected spots and the plants kept close together to shelter each other a bit. By the end of spring with the last of the frosts over the plants stay out permanently -but it's still a good idea to throw a sheet over them on very chill nights.
In autumn there is no need to rush to bring them in again. The first frosts are usually light and sporadic, and the plants are tough from their life outdoors. So just covering the plants with a sheet or blanket usually gets them through the odd frosty night. But they can not stand hard frosts (where water in a bird bath freezes over) and if these are likely they must go under cover. However if possible move them back out for sunny days as long into autumn as possible.
Of course these are counsels of perfection and in practice my biggest, and most valuable, tree goes out and in once. It's too big to move unaided so it goes out last and is given sheets nightly until hardened off. Likewise it's nearly first to come in - it's tough so no hurry, indeed small plantlets are much more vulnerable. It just needs to go in place so the others can go around it and though they can have daily trips out it can't as it's too big.
Fortunately citrus do not need very large amounts of compost to perform and so they succeed in what appear relatively small containers compared to say such as peaches. This keeps the weight low and as they are usually bushy and respond as well to shearing as pruning they are compact. Thus the moving may be a hassle but it's not difficult when compared to their productivity. (A neat trick with really heavy containers for the biggest plants is to repot with a nylon netting liner. When moving the rootball can then be lifted out with the tree if the compost is dryish and the container moved separately.)
Citrus definitely prefer terracotta to plastic -and wood slat most of all (avoid concrete as the lime in it kills them). Their roots crave air, and as economy, and weight, mean plastic tubs are most often used then drill multiple air and drainage holes in the sides as well as the bottom. Then use plenty of coarse drainage under and in the compost. Citrus are not Ericaceous but almost as sensitive, they soon suffer chlorosis if much lime is present so their compost should be like an Ericaceous type mix with plenty of grit and leaf mould. The old boys used decayed grass turves from a rich loamy soil, cow dung, bone meal, grit and leaf mould. I use my sieved garden compost plus grit and composted bark.
Feeding is definitely required to keep up their performance and although under glass with heat it can go on lightly all year it is usually best confined to the warmer months unless you are very careful. A rich top dressing in early spring, or potting up if possible, are only the first stages. Liquid feeds (tomato type), well diluted, need giving first lightly then with almost every watering when growth is at a maximum. And foliar feeding with seaweed solution weekly adds extra nutrition and reduces many of the pest and disease problems. In winter any feeding should be light and high in potash.
Frequent misting whenever the conditions are warm and bright suits citrus -though they can endure dry air conditions. When it is cold and damp do not mist as this promotes mould. Spraying, as opposed to gentle misting, can be beneficial whenever the plants are in good growing conditions as jets of water will dislodge pests and any honeydew that builds up on the leaves. Seaweed solution in the spraying and misting water is really beneficial but should be lime free as should the water, rain is definitely preferable to tap. Soft soap sprays will help remove honeydew and kill many pests but should not be done in strong sun or wind as they remove the protective waxy layer from the leaves as well.
Pests and diseases are inevitable but most are easily controlled by such spraying and washing, or sponging. And most disappear when the plants go outdoors. Vine weevils can eat their roots and must be watched out for. Mould is a problem in cold damp conditions. But rarely do any of these problems kill a plant. That is usually our fault. Citrus are quickly killed by water logging, really quickly -but don't show it for weeks! So ideally water their tubs by immersion in a tray of warm water with thorough soaking and then a proper draining. In practice try and keep the rootball moist but not wet, and in cold winters keep it barely moist but never let it dry out.
It is best to purchase your initial plants as named varieties, there are many suppliers with a vast range to choose from. Most will be grafted onto suitable rootstocks. But it is possible to root cuttings and grow many sorts on their own roots -though some do not grow so vigorously. As they need a prune, (in practice just a general shearing back) in early spring there is usually an abundance of cuttings to root and with the aid of bottom heat they will giving you extra plants that will reach fruiting size in a couple of years.
Alternatively you can grow seedlings and if you are practised graft your named varieties onto the seedling roots which will crop quickly and often give stronger plants. However do not try to grow seedlings on to fruiting size unless you are very patient. It can be done, my Ortanique took twenty three years to fruit! And citrus are usefully unusual in that occasionally one seed may give more than one seedling. In these cases the strong one is usually the true genetic mixed offspring you expect but the other(s) is/are clones of the original and will give the same fruits as the parent. (given the same soil and climate of course!)
Lemons are the easiest, toughest, most productive and without doubt the best choice. Meyer's is a compact and reliable form, Lisbon my next favourite but there are many to try. Lemons hold on well and with luck and skill are available year round though most come in winter. Oranges are not as good in the sense that they crop fewer and have less general utility. A lemon available any day of the year is more value to most of us than a crop of oranges in the new year. Your own oranges are wonderful though, I recommend Valencia. However make more sense as they produce more fruits and they are delightful when fresh. Kumquats (not a true citrus but so similar who cares) have those sweet tiny fruits you eat whole and are excellent value. Grapefruits and other hybrids are more tender and demanding but still possible. There is even a grapefruit surviving outdoors in the Chelsea Physic Garden in London, where in a sheltered spot it even crops!