I feel almost shy about writing this column. It contains no revelations, no call to arms. No one gets savaged - well, only mildly. The subject is almost inconsequential. Yet it has become an obsession which, at this time of year, forbids me to concentrate for long on anything else.

Though we still subsist largely on junk, even bilious old gits like me are forced to admit that the quality and variety of most types of food sold in Britain has greatly improved. But one kind has deteriorated. You can buy mangoes, papayas, custard apples, persimmons, pomegranates, mangosteens, lychees, rambutans and god knows what else. But almost all the fruit sold here now seems to taste the same: either rock-hard and dry, or wet and bland. A mango may be ambrosia in India; it tastes like soggy toilet paper in the UK. And the variety of native fruits on sale is smaller than it has been for 200 years.

Why? Most people believe it's because the supermarkets select for appearance, not taste. This might be true for vegetables, but for fruit it's evidently wrong. Green mangoes, Conference pears, unripe Bramley, Granny Smith or Golden Delicious apples look about as appealing as a shrink-wrapped stool. Appearance has nothing to do with it. What counts to the retailer is how well the variety travels.

Take the Egremont Russet, for example. It's a small apple that looks like a conker wrapped in sandpaper. But it has one inestimable quality. It can be dropped from the top of Canary Wharf, smash a kerbstone and come to no harm. This means it can be trucked from an orchard at Land's End to a packing plant in John O'Groats, via Sydney, Washington and Vladivostock, then back to a superstore in Penzance (this is the preferred route for most of the fruit sold in the UK) and remain fit for sale. The supermarkets must have had some trouble shifting it because of its strange appearance, so they promoted it as a connoisseur's apple. Such is our suggestibility that almost everyone believes this, even though a dispassionate tasting would show you that it's as sweet and juicy as a box of Kleenex.

For the same reason, we are assaulted with Conference pears, most of which resemble some kind of heavy ordnance, rather than any one of a hundred exquisite varieties such as the Durondeau, Belle Julie, Urbaniste, Glou Morceau, Ambrosia, Professeur du Breuil or Althorp Crasanne. It is because these pears are so delicious that they cannot be marketed. They melt in the mouth, which means they would also melt in the truck before it left the farm gate. As the best pears, plums, peaches and cherries are those which go soft and juicy when ripe, the grocers ensure that we never eat them.

To compound the problem, the supermarkets demand that fruit is picked long before it ripens: it doesn't soften until it rots. This makes great commercial sense. It also ensures that no one in his right mind would want to eat it. But, happily for the retailers, we have forgotten what fruit should taste like. The only way to find out is either to travel abroad or - the low-carbon option - to grow your own. I find myself becoming a fruit evangelist, a fructivist, whose mission is to show people what they are missing.

When I lived in Oxford, at a time when allotments were underused, I spent a week in the Bodleian library reading Hogg and Bull's Herefordshire Pomona, a massive book of apples and pears, written in the 1870s (you can now buy it on CD from the Marcher Apple Network). Then I cleared two and a half plots and planted the best varieties I could find. I left just as the trees were ready to fruit. But land here in mid-Wales is cheap. I bought half an acre and have started planting a second orchard.

When I first tried to place an order, I caused great excitement among the nurseries I phoned. Where had I seen these apples? Who recommended them? Two of them, I discovered, had been extinct for at least 50 years. So I have had to settle for second best, by which I mean breeds that still exist. I began by planting a Ribston Pippin and an Ashmead's Kernel. These apples, both exquisite when fully ripe, can be stored from October till May. To spread the fruit as far through the year as possible, I have ordered an apple called the Irish Peach, which ripens in early August; a St Edmund's Pippin (September) and a Wyken Pippin (December to April). After a long search I think I have pinned down the apple I once tasted and loved in a friend's garden. I'm pretty confident that it was a Forfar, also know as the Dutch Mignonne, so I've bought one of those too. If I'd had more space, I would also have planted a Catshead, a Boston Russet, a Sturmer Pippin and a Reinette Grise.

I have bought two pears - a Seckle and a Beurré Rance - a green plum (the Cambridge Gage), a fig, a medlar, a peach, currants, gooseberries, raspberries, loganberries and blueberries. But what excites me most are the suggestions made by a man called Ken Fern. Once a London bus driver, Fern has spent most of his life cataloguing and growing the edible species of fruit and vegetable which can survive in this country. His list now extends to 7,000, some of which are featured in his book Plants for a Future. I've decided to buy an Arnold Thorn (Crataegus arnoldiana), which belongs to the same genus as the hawthorn, but grows sweet juicy fruits the size of cherries, and to replace my hedge with Elaeagnus x ebbingei, which produces sweet red berries with edible seeds, in (uniquely) April and May. This means, if it works out, that I can eat fresh fruit all the year round. I can store apples and Beurré Rance pears until the Eleagnus fruits, then my strawberries should be ready more or less when it stops. One day, when I can afford it, I will buy more land and plant a few dozen of the weird species Fern has found.

Most people have less space than I do, but even a tiny garden can support half a dozen apple trees, if you grow them as cordons (single stems with short spurs) 80cm apart against a wall. If you have room for only a couple of pots, you could grow blueberries, strawberries, cranberries or some of the little shrubs Fern recommends, such as Vaccinium praestans and Gaultheria shallon. Or you could become a guerrilla planter or guerrilla grafter, growing fruit on roadsides, on commons and in parks and wasteland. Apple twigs of any kind can be grafted on to crab trees. Medlars and one breed of pear (a delicious variety called Joséphine des Malines) can be grafted on to hawthorn. Kiwi fruit, passion fruit and a vine called Schisandra grandiflora will climb into trees of any kind.

It's not just the produce I love. When you start growing fruit, you enter a world of recondite knowledge, accumulated over centuries of amateur experiments. You must choose the right rootstocks and pollinators and learn about bees, birds and caterpillars. But above all you must learn patience. Growing fruit forces you to think ahead, to imagine a sweeter future and then to wait. Perhaps it is this, as much as the forgotten flavours, that I have been missing.

monbiot.com