OUR neck of the woods must of course shoulder its share of the blame for the national predicament but it is a bit of a shock when the region stands accused of propagating, literally, the end of civilisation as we know it.

For, if the emotions raised by a Triffidian phenomenon we have spawned are anything to go by, no less than that is threatened. Already this invader from the North-East has caused murder and mayhem, neighbour has turned shotgun on neighbour, peaceful communities have been been split.

Class warfare looms. Broadly it will be between the horticultural elite, the toffs whose give-away is to talk about plants grown from seed, and the suburban proletariat who sow seeds, but there are fifth-columnists among both factions.

I say "our neck of the woods" advisedly. The alleged catastrophe is a tree. Incredibly, it is already one in five of all the trees in Britain.

Parliament is split on the issue, which transcends party political loyalties. For good measure, it is about to be wheeled on as a big gun to be fired by both sides in the until-now separate battle over GM crops; that, at any rate, is my confident prediction.

And, depending on your allegiance in this arborial civil war, the tree is at this moment quite likely to be either affording you enviable privacy and shade ... or depriving your loved ones of sunlight, aggravating Seasonal Affective Disorder and starving your perennials of nutrients.

It is X Cupressdcyparis leylandii, the Leyland cypress. One Christopher John Leyland was the first seriously to encourage the incipient troublemaker, just over a century ago on his vast Haggerston estate on the Northumberland coast.

In fact, it was a hybrid born in 1888 at Leighton Hall, which had been Mr Leyland's home in Wales before he inherited Haggerston from his uncle, Capt Thomas Leyland, and changed his surname. There was accidental cross-breeding between two quite separate species of cypress native to the west coast of North America: pollen from one spontaneously impregnated the other.

Leyland's brother-in-law, a Naylor who apparently remained at Leighton Hall, gave him six of the little bastards. They took to the Haggerston soil like ducks to water. For which many thanks, thousands of of caravanners say during their holidays in the 350-acre leisure park was now surrounds the ruins of the 14th-century Haggerston Castle near Berwick: leylandii make a splendid windbreak.

Shock, horror and jubilation, then, from the anti-GM fanatics at the paragraph about unintended cross-pollination. It just proves, they'll say, that these experimental fields of GM corn are the work of the devil to make human Pompidou Centres of us, with vital organs displayed in throbbing detail on our outsides.

Rather, I'd say, it indicates even more persuasively than do those delicious hybrid cherry tomatoes that vegetable miscegenation is the best thing since wafer-thin sliced cucumber. It has been going on since the Garden of Eden and it's as likely as not that Adam's fig-leaves were first-generation mutants more easily to be sown into an apron.

If anything poisonous ever results, as it occasionally must, then don't eat it, just as we make a point of not serving deadly-nightshade jelly or handing around yew-tree sandwiches. If anything turns up that, for instance, causes skin blotches, then cut it down, as we do stinging nettles, or avoid it, rather as we do not seek the company of adders.

Still more furiously whipping my hobby horse, I do not doubt that GM agriculture is one of the two most urgent research programmes needed if our planet is to see out the third millennium: GM, as China has already accepted without reservation, vastly to increase food production as the world population soars; and, never mind taxing ordinary people out of their cars and into non-existent buses, technology to tackle the most ubiquitous cause of global warning - pollution caused by the burning of billions of pounds worth of (untaxed) aviation fuel.

Unlike most ground-level nasties, the nitrous oxides, noxious particles and carbon dioxide ceaselessly pumped out by high-flying aircraft do not wash out of the atmosphere, because they are too far above the rain clouds. Much of it is released into the lower stratosphere and there has maximum effect on the ozone layer.

Oh, leylandii ... it was not until 1925 that a firm of nurserymen specialising in conifers had their attention drawn to the Haggerston hybrid and, naming it in Mr Leyland's honour, began to clone it in commercial quantities.

One version, called Naylor's Blue, a smoky grey variety now reckoned to be one of the best Leylands, was not propagated until 1954 when the original tree was brought down by a freak tornado which spared its five siblings, still to be seen at Haggerston.

IT was a clever headline in The (London) Times that led me into a report of this month's Hampton Court Palace flower show which listed some of the ramifications of the tree's astonishing vigour: "The unstoppable growth of British Leyland".

Unstoppable in the sense of sales which began to take off in the 50s when newly introduced mist propagation boosted "production" and which, by the late 90s, had reached 300,000 a year.

Unstoppable too in that, if gardeners do not deal regularly with its average 2ft of growth each year, a leylandii hedge soon gets beyond control. The free-standing example prominent behind the opening credits of TV's Brookside looks set to reach the 80ft possible for a mature cypress.

It is a situation which could lead soon to the Sun dusting down one of its own most notorious headlines: "Gotcha!" would be appropriate if a government working party, which has taken submissions this year, urges a ban.

The expectation in the trade is that the recommendations of this "High Hedges" consultation group will not be quite so draconian as that. But the Association of British Conifer Growers (which is probably seen by andi-leylandii campaigners as a National Rifle Club-equivalent representing the redneck tendency among gardeners) is bracing itself against the likelihood of measures to protect the civil liberties of those on the wrong side of these jolly green giants.

Some developers already require buyers of houses on new estates to sign covenants forbidding the planting of the offending hedges.

Germany already imposes height limits on hedges and 6ft is being mooted here. Another possibility is that advertisements for leylandii must include advice on maintenance; indeed, why not also a kind of government health warning along the lines that one afternoon your neighbour may stride out of his shade into your sunshine and throttle you.

In January, the aptly named Baroness Gardner of Parkes, after being lobbied by a pressure group, Hedgeline, introduced the second reading of the Residential Hedgerows Bill. But whether, or what, statutory action is eventually taken really depends on how much consensus emerges after the working party presents its findings this autumn.

The Association of British Conifer Growers is not taking all this lying down. Besides pointing to Southampton university research suggesting that the leyland filters atmospheric pollution, it is stressing the effectiveness of a disciplined hedge as a windbreak and a cover for wildlife.

It threw down the gauntlet at the Hampton Court show by choosing its controversial bestseller as the backdrop against which to display more than 80 other varieties of conifers.

Launched at Hampton Court (where leylandii have not been used to bolster the famous 17th-century maze, originally of hornbeam but since the 50s much reinforced by yews) were two enterprising attempts to cash in on the tree's notoriety: the telescopic leylandii shear, for use when the summit is only just out of reach, and the leylandii pruning platform, for when it is nearly out of sight.

Next year, the leylandii helicopter with rotary cutting blades?