Fields of joy: Thanks to a morphine shortage, poppy growing in the UK is now all the rage

Opium poppies (Papaver somniferum) are becoming a lucrative crop for British farmers. I first saw them last year, shimmering in a field alongside the A35. It was the colour that caught my eye – too mauve for flax, too delicate for field beans. I swerved into a lay-by and trekked back to the field gate. Yes. Definitely poppies. This year there have been great spreads of them, particularly around Oxfordshire, Northamptonshire and Lincolnshire, where the good, light arable land suits their needs.

Poppy growing on a commercial scale in this country started about six years ago, prompted by a world-wide shortage of morphine. The seedpods of opium poppies are full of a milky latex which dries to an amber-coloured resin, rich in codeine, morphine, narceine, narcotine, papaverine and thebaine, all of significant use to medicine makers.

All the crops in Britain are grown under licence for an Edinburgh-based pharmaceuticals company, Macfarlan Smith. The company tells the Home Office where the crops are and how many acres have been licenced. The Home Office writes to the various farmers, confirming the arrangement and inviting them to let the local police know what they are doing. I haven't yet been joined by PC Plod on any of the occasions I've stopped recently to admire the poppy fields. Just as well perhaps. It might not be easy to explain why I'm taking such an interest in them.

The advantage for the grower is that he doesn't have the bother of harvesting the crop. As Oxfordshire farmer Tom Allan points out, he has simply to prepare the land, drill the seed and watch the poppies grow. Macfarlan Smith take care of the gathering and the processing. So it's not surprising that opium poppies are now cultivated on about 8,000 acres of arable land in Britain.

That's nothing, though, compared to Tasmania, which with 20,000 hectares under production is the world's largest provider of opium poppies for the pharmaceutical industry. A nice story was doing the rounds a couple of weeks ago about Tasmanian wallabies getting high on the crop. They'd have to be hard-pushed to try it – it's very bitter – but before long there are bound to be equivalent myths over here: stoned rabbits, deer high on hallucinogens, glassy-eyed sheep.

From a gardener's point of view, the form the farmers are growing is unexciting, a single flower of dull, dirty mauve. But the dullest flowers sometimes produce the best seedpods. Specially for flower arrangers, Chiltern Seeds sell a type (Papaver somniferum 'Giganteum' £1.62) with larger-than-usual seed capsules. Since these are also what the pharmaceutical company is interested in, the field crops must be a similar strain. The opium poppies which crammed our garden in late June were altogether madder: singles and doubles in deep, silky purple, brilliant pillar-box red, a rich crimson and some strange tricolours, which have never appeared before.

If you start by chucking a few different kinds of opium poppies about in your garden, you will have them forever after. They seed themselves liberally and it is up to you to pull them out of places where they are not wanted and to rogue the flowers as they open, to get rid of the colours you don't like and increase the chance of getting more of those you want.

Gradually, doubles drift back to becoming singles, but you can easily add a few new doubles to the mix each year: perhaps 'Double Raspberry Blush' (Thompson & Morgan £1.49) or 'Black Peony' (Chiltern Seeds £2.23). Proper doubles last longer in flower than singles because they don't get pollinated (stamens have turned to petals, so they can't). The singles are more elegant, though, and great favourites with bumblebees which (despite the badgers which dig up their nests) seem to be around in great numbers this year. A fantastic noise comes from the inside of an opium poppy as a bumblebee rolls around the bowl of the petals, bouncing on the forest of stamens.

The petals – huge, silky things – break from a bud that seems impossibly small to contain them. At night, they close up like a clam shell to protect the stamens and the pollen inside. If you want the flowers to self-seed, you have to leave the seed pods to ripen. They are handsome things with their mortar-board tops, but unfortunately by this stage the glaucous leaves, lovely when young, are ragged and decrepit. That is why it is useful to grow opium poppies through something that disguises their lower half. In our garden, they do a good job poking through low arching bushes of 'Buff Beauty', a creamy apricot rose that is just at the end of its first flush of flower when the poppies are at their best. The one gives way to the other. They are also filling in gaps on a dry, south-facing slope that eventually will be filled with rosemary, myrtle and box, clipped in the French fashion.

Self-seeded poppies stick to nature's own timetable, sowing themselves in late summer and early autumn. By that time, little holes will have opened up in the pepper pot seed heads to let out the seed. You can encourage the self-seeding by pulling up the stems and shaking them over the areas where you want the poppies to grow. This autumn seed will give you bigger, stronger plants with more flowers on a stem than spring-sown seed. You can extend the poppy season by doing both, for spring-sown seed will usually produce later flowers than autumn-sown ones.

Few of us think of buying flower seed at this time of year, but with opium poppies it is definitely worth it. If you want to introduce some unusual varieties to your mix, try 'Blackcurrant Fizz', a new variety from Thompson & Morgan (69p) with thread-like petals of deep red and purple, or 'Flemish Antique' (Chiltern Seeds £2.43), the kind of outrageous double poppy you see in Dutch paintings of the Golden Age, with creamy flowers spattered in coral red. While you are at it, you might think of picking up a few other flowers that, sown in September, will have a headstart in the garden next year: aquilegias, pot marigolds (calendula), cornflowers, Californian poppy (eschscholzia), godetia, gypsophila, larkspur, lavatera, night-scented stock (matthiola) and Virginia stock.

Chiltern Seeds, Bortree Stile, Ulverston, Cumbria LA12 7PB, 01229 581137, chilternseeds.co.uk; Thompson & Morgan, Poplar Lane, Ipswich, Suffolk IP8 3BU, 01473 695225, thompson-morgan.com. For a masterly discussion of the whole poppy family, get 'Poppies' by Christopher Grey-Wilson (Batsford, £14.99)