My tropical paradise double tunnel evolved from a simple coldframe used in the greenhouse. Many years ago in my greenhouse I constructed a coldframe made from old windows. This was initially to hold plants moving from the propagator until they were big enough, and it was warm enough, to plant them in the greenhouse border. Once the tomatoes and peppers had moved on I used the coldframe to bring on the marrows and gherkins before they went outside in May. Each summer I noticed melons and cucumbers were much happier in this coldframe than they were in the open greenhouse; the warmer moister air inside more than compensating for the diminished light. The coldframe was at first unused for the winter as at that time I grew few tender over-wintering plants.

However, as I'm sure you all will appreciate, plants were soon filling the empty space. Of course at first it was just a few Fuchsias and Pelargoniums, then surplus house plants and a few oddities grown from pips and seeds such as lemon seedlings, date palms and avocado plants. What surprised me was that following winter I didn't lose as many as I had expected even though neither greenhouse or frame were heated in any way. Indeed most of the plants looked happier there than did their compatriots on my windowsills.

I realised this doubled glazing had made a really good if small Orangery and invested in a some young grafted citrus bushes; a mandarin, orange and lemon. These were already yielding when I got them and have been producing ever since. Each year they get bigger, hold larger crops and their flavour is superb, and the scent of the flowers is divine. Citrus plants sit on the patio all summer and need only to be frost free in winter so they are cheap to run.

Sadly citrus from seed take years to fruit and grafted plants are expensive. So I started trying other more exotic plants from seeds and cuttings. These cost almost nothing; nearly every fruit from the supermarket comes with a free packet of seeds inside: such as Pawpaws, prickly pears, passion fruits. And many others can be grown vegetatively such as, ginger, lemon grass and pineapples.

Soon I made a plastic tent inside my polythene tunnel. Walk-in polythene tunnels are very different to greenhouses, not only are most of them much larger but they are less draughty and more humid. The plastic is not as clear as glass, and using two layers cut out much more light. I was amazed how well some plants did in this tent despite the dimmer light. Melons and cucumbers grew as they never had before though tomatoes resented the moist dimness.

As many of my tender plants got bigger I wanted to make this tent their winter home. Unfortunately plastic tunnels may be warmer than greenhouses during the day but they cool much more at night as they have almost no mass. However I reckoned that with some heat I could keep the inner tent frost free. That winter almost all my plants made it with just a bathroom fan heater to keep them warm.

But of course very soon the plants were too big again. So I bought an even larger tunnel, so big I could put up my first small tunnel inside. I could grow even more plants and was now trying some that I had thought needed much more exotic conditions. The bananas were staggering. Sure I had expected to be able to grow the plant awhile, even to get it to flower but I'd never really dreamed it would ripen a crop, and what a crop. They were as good as if not better than any I'd had abroad.

To crop well bananas must be a vegetative clone and not grown from seed. I have a good variety; the dwarf Chinese or Cavendish sort grown in the Canaries. This crops at only eight or nine feet tall and produces delicious stubby fruits in abundance. You can import a 'bulb' quite legally when you come back from holiday as long as no soil is attached. Most banana plantations will sell them so just ask. Ideally you want a big fat 'sword' 'bulb' that has not opened it's leaves much yet, avoid squat open leaved ones. Planted up, potted on regularly and kept warm and moist this will grow to full height within the first summer, the second year it flowers and fruits, then you cut it down. Meanwhile you remove all other shoots bar one replacement. Hey presto you get a crop each year from then on.

So I grow bananas, in Norfolk, and not with some mega-expensive Kew garden palace but with really cheap and easy resources. All that is required, after all, is to keep the plant warm and snug, a couple of layers of plastic and a fan heater is simple enough to arrange. So I carried on trying more, and more tender tropical crops. Very soon I realised even the double tunnel just was not big enough for all the plants I was growing. Indeed I was not just trying out a few plants but was now expecting to get serious amounts of ginger, lemon grass and sweet potatoes, to say nothing of several pineapple and banana groves in big pots and dustbins.

So I bought the biggest tunnel I could fit into my garden. Several full grown plum, peach and pear trees had to be sacrificed to make room. At twenty four feet by sixty, and ten foot tall, the new tunnel had room for me to erect a fourteen by forty tunnel inside and leave plenty of room to grow tomatoes etc. alongside. As the inner tunnel has no weather to contend with I made it extra tall so it nearly touches the roof of the outer one. Hanging plastic sheet curtains not only close both ends but another pair seal off a central bay where my tropical plants live year round. The two end foyers are excellent for over-wintering the semi-tender plants such as citrus and for growing cucumbers and melons during the summer.

Apart from a soil warming cable for the most tender plants the only heat is two bathroom fan heaters. During the coldest months I throw a third plastic sheet over the inner tunnel supported on clear plastic bottles strung on wire. This triple glazing makes it warmer still though very dim. Cold is a bigger danger than low light to these plants, they just sulk and wait for spring. I've tried artificial light and it does help but it's not actually necessary. With plenty of space I find bananas thrive, passion fruit vines become huge, beautiful and productive, and lemon grass grows like a weed. I continue to try with as yet unsuccessful crops (for example I've failed to crop coconuts, tamarinds, custard apples and Pawpaws so far) but am staggered by the yield and quality of the sucesses.

Passion fruits and guavas come fairly true from seed from supermarket fruits and both crop within their first years. My guavas have grown bigger and more perfumed fruits than I have ever seen in their native tropics. They make very nice conservatory plants anyway and their fruit is a real bonus. Guavas quickly make shoulder height fruiting bushes yet need only a pot the size of a large pail though then they do require copious watering. The Strawberry guava is also lovely, it has smaller fruit, dark red and deliciously strawberry, the foliage is neater than it's coarser sister.

Eddoes are Colocasias and close relatives are sold as house plants. The edible 'tubers' are sold by supermarkets and once started in the warm soon produce large arrow shaped leaves. Potted up regularly and grown on these return excellent crops by the autumn or can even be planted outdoors as ornamentals for the summer.

Sweet potatoes are prolific and give bigger yields per plant than Irish. The secret of their culture is to propagate tip cuttings in the autumn so the plants grow from early on the following year. Shoots only grow reluctantly from the 'tuber' which can be made to sprout with bottom heat but if the shoots are left attached they tend to all rot away together. As soon as the shoots are a few inches long they need be detached to grow on separately. Once growing strongly in large containers (1/2cwt black plastic coal sacks work well) the stems should be tied up and not allowed to touch the soil as supplementary rooting diverts growth away from the main crop. The plants never go dormant as do Irish potatoes so the 'tubers' need heat treatment if they are to be stored for any time, however, if left just to grow on the roots can be had fresh almost anytime. Left to the second year they do become more fibrous and woody though! Some varieties of sweet potatoes also flower and are then as ornamental as any house plant. They can be grown in an ordinary plastic tunnel in summer but need the warmth of a double to stay alive in winter.

Pineapples have proved the most rewarding fruit. The fresh top can be pulled out of a fruit and dried for a day or three. Then the dead and lowest leaves are pulled off singly revealing little fledgling roots. The top can then be planted in an open gritty compost and grown on. It takes up to three years to reach fruiting size then up pops the flower which is very pretty like a red teasel with purple petals sticking out and then some months later the fruits ripen. Each plant eventually gives several fruits before needing renewal for which the sideshoots can be split off and rooted separately. I grew my first fruit just keeping the plant frost free, but with some extra bottom heat from a soil warming cable I now grow bigger fruits (averaging six pounds apiece) by the handful. They really are worth the effort!

Sure there is some skill required with watering diverse plants and I have to open up early on hot days or everything cooks. There does tend to be too much humidity and condensation for some plants so good ventilation is needed to keep moulds at bay. But apart from some hefty electric bills there have been no real problems. In fact the only drawback is that it's so nice in my tropical paradise that I lurk in there and am reluctant to get on with the work outdoors.