Fifteen or 20 years ago I offered readers a prize for the funniest roundabout in France – or rather a photo of it. I had lots of entries. It was the time when the rondpoint was a fairly new way of dealing with a crossroads, and local authorities seized on it as a means of self-expression or advertising. Most involved gardening of some kind, some in a flower-show sort of spirit, some so garish that no lights could compete, some ambitiously advertising local industry – a vineyard or a jet fighter – and many intentionally or unintentionally very funny.

Now it’s gone beyond a joke. The old pleasures of motoring in France are threatened by such aggressive road-building that you can only approach most towns along miles of by-passes, through thickets of road-signs, along narrow lanes of tarmac between overbearing kerbs and bollards, over speed-humps steep enough to shake your head off. ‘Toutes Directions’ is the unhelpful universal signpost. If you can find ‘Centre Ville’ in the forest of instructions it leads you into a maze from which you despair of escaping. If you ever come across the historic market place its buildings are obliterated by more bossy signs and chaotically parked cars. There is no escape.

Worst of all, smaller and smaller towns are boosting their self-importance by aping big ones, installing rondpoints and hiding their ‘Ville Fleurie **’ boards with signs to their ‘Zones d’Activités’ (while activité ebbs from their ancient hearts). Hamlets aspiring to be villages are joining the game. My current prize-winner is a one-horse village in the Médoc we now call Arcins-Les-Quatre-Ralentisseurs. There is one speed-hump to greet you at each end of the short street and two outside the tiny mairie. You laugh so that you may not cry.

I must have been dozing to have missed my own fortieth birthday – or rather Trad’s. It happened in June: forty years since the first number of the Journal of the RHS reborn as The Garden.

I was in charge of the mag then, and looking for an editorial leader column. I decided to write it myself, but quasi-anonymously. I used the nom de plume of Tradescant because it seemed to be a name uniquely associated with gardening, without any current claimants. Icouldn’t find a soul answering to it in England or anywhere else.

I’m still at it. It grew into a habit I was (and am) loathe to shake off. There is an odd comfort in slipping into a persona which is oneself, but not quite. I put on Trad’s old tweed jacket, cuffs fraying and elbows patched, and record his current thoughts or preoccupations. Often they coincide with my own.

For many years we worked together in an ambitious garden of twelve country acres. We have gardened together in France, in Hampshire and Wales, and now have a thousand square feet of Kensington as our headquarters. We have the constant stimulus of visiting gardens and nurseries, of conversations and the library; there is never a lack of matter for gossip and reflection. Will Trad make it to fifty? Will he make it to forty one? We shall see.

The best performer: Fuchsia boliviana

An overnight storm had left the grass cool and green underfoot, but the heat of the low sun was oppressive. I followed the shadows of the trees until I got to the Round Pond; in the open, skirting the pond, the double glare of sky and reflection was dazzling, my face burning. Autumn has only begun in the horse chestnuts; the rest of Kensington and its gardens are still enjoying summer.

There is anticipation in our little garden now, but not much action. A second or third crop of roses is half-hearted, however welcome. The Central and South American salvias should be striking up now with their paintbox colours; I fear they miss the sunshine under our walls and trees. S. uliginosa is too tall and floppy; much as I love its tiny Cambridge-blue flowers, they hardly justify such a leggy plant. S. vitifolia, from Mexico, looks fine with its pale furry-soft leaves; its branching flowerspikes are just giving glimpses of its sapphire blue. And as for our tomatoes, whatever the temperature in its greenhouse, they need more sunlight. They are still only reluctantly beginning to ripen. With Gardener’s Delight doing best. I shan’t bother with the yellow varieties again; they turn soft before (if ever) turning sweet. In fact the real bonus is the smell of the beautiful leaves, now scrambling high into the roof.

Curiosity is a curse in a gardener. You can never be tidy if you are curious. I always want to know what will happen next. Will a faded flower set seed? Can I grow the seed? How tall will this climber grow? What is that seedling in the paving? (I’ll have to leave it till I see its flowers). If your whole garden is a mass of mini-experiments it will never merit a photograph.

Successes and failures? At the end of a long summer there must be conclusions to be drawn. Tomatoes and salvias apart, almost everything could do with more light, too. Hydrangeas have done well, geraniums, phlox, Japanese anemones, campanulas, Thalictrum delavayii, the inevitable Verbena bonariensis all earn their keep.

The best performer, though, by far, is the redoubtable Fuchsia bohiviana. We brought it as a standard in a mere 8-inch pot from Saling Hall. It has grown to four times the size, added four feet to its span and recently three feet to its height, flowering vividly with its scarlet tassels of narrow bells all summer. Its fruit ripens, too, red berries turning black and (tolerably) edible. It propagates easily by either seed or cuttings. I shall have to chop off whole branches to squeeze it into the greenhouse for the winter – although I wonder if a plant this lusty wouldn’t survive if I just stuck it under the verandah.

That's Cap Ferrat across the bay

A new garden diversion: spotting a muster of mega-yachts in the harbour below; the bay of Beaulieu-sur-Mer. They must be assembling for the Monaco boat show next week: Madame Gu, a bright blue boat, sharp-pointed like a dart 325 feet long, RM Elegant, the very opposite of her name, Luna (377 feet, built for Roman Abramovitch), more like a huge tug whose massive aft deck apparently conceals a submarine, and the 155 feet Blush (does the owner?)

They lend a new meaning to the word “eye-catcher” as they glide into the bay and deploy their little navies of tenders and water-toys. Meanwhile the tiny flotilla of children’s instruction dinghies is a bright flight of butterflies by the pink villa on Cap Ferrat still known as David Niven’s.

I have written before about our son-in-law’s Riviera garden. It has grown faster than I believed possible when I oversaw its design only six years ago. The vines forming a tunnel on the top terrace are heavy with grapes. The cypresses diligently clipped by Lucien and Pascale, the devoted part-time gardeners, are perfect green rockets, the orange and lemon trees thick with fruit, the cascading rosemary totally covering the stone walls, the oak-leaved hydrangeas sturdy bushes eight feet high and the agapanthus that fills the box-edged beds as thick as weeds standing upright in a stream. A persimmon tree we moved five times has its first tight globes of fruit. The climbing roses cover the pergola round the pool, and the palest blue Salvia uliginosa mixed with perovskia and deep pink Japanese anemones stands higher than my head. Iceberg (there is no better rose for a hot climate) has flowered and been cut back four times this year.

The secret is water. The micro climate here below the cliffs known as la Petite Afrique is absurdly benign. Warmth, sunshine and daily irrigation by driplines produces almost magical growth – as boar and badgers are well aware. Fencing them out is a challenge. Did Adam and Eve have this problem in their garden? We are not told. Is there a serpent? Only the crabgrass, the only grass that will grow here. Its snaking white shoots creep evilly into beds, through the stones of walls, anywhere they can reach. Now, if we could exorcise them…

Where Tradescant snipped and dug: the East Garden at Hatfield House

Horticulturists are now on a par with accountants and surveyors, loss-adjusters and legal executives. It seems an odd ambition for a gardener, but the Institute of Horticulture received its Royal Charter yesterday in the wonderfully apt setting of Hatfield House.

John Tradescant must have known the Marble Hall where his employer’s successor, the Marchioness of Salisbury, unveiled the charter with its red Royal Seal. His own successors, the country’s senior gardeners, packed the hall in their scores. Then they went out to see one of England’s grandest and loveliest gardens in its late-summer glory.

I have known Hatfield well since the sixth Marquis and marchioness started a garden festival there in the 1980s. Things were deliciously informal in those days. The atmosphere was village fête with a bigger marquee. At the first festival, Lady Salisbury invited some friends to judge the exhibits, innocently unaware that the well-established RHS rules apply wherever gardeners compete.

I was among the amateur judges who spent hours over the vases and vegetables, dithering between golds for this and commendations for that. Our announcement of the winners, though, was met with a baffled silence. A delegation came to tell us that we’d got it all wrong. You don’t judge horticultural shows by personal preferences, by taste or by any yardstick except the rulebook. The agonizing was unnecessary: winners can be measured. And I have never been asked to judge anything since. No charter for me, I fear.

How the garden glowed yesterday, though, in the near-autumnal light. I have seen the topiary in the East Garden, the vast elaborate parterre below the terrace of the house, grow from new plants in the ’80s to solid maturity. Each of the eight big central beds is divided in four by a heraldic-looking star of yew, and the central alley leads between towering yew cylinders. There is room in the beds for tall roses, for huge tree peonies just taking on autumn colours, and for every late-season flower in the medley of warm colours that grows more meddled as autumn draws on. Here, I thought, is the quintessential English country house garden in all its nostalgic splendour, only 20 minutes from King’s Cross.

I’ll spare you all the noisy and dusty details, but our next-door neighbours’ house is being demolished, or most of it. Its Victorian elegance (if that’s not too strong word for the plasterwork and joinery of the 1840s), is going on the skip, to be replaced by the stark spaces of today’s fashion. Only the front and side walls are staying – and while they’ve got the roof off, why not dig a basement as well?

Noise and dust we can live with; worse is the worry that we’ll all fall into the hole, or if not actually fall, see scary cracks in our wall. Just over the road another end-of- terrace house like our neighbours’ moved enough to split its neighbour down the middle. Admittedly they had dug a two-storey basement to fit in a 14-metre swimming pool. (Will the whole house smell of chlorine?)

All this is, of course, ‘permitted development’. The government’s Party Wall Act doesn’t take account of the flimsiness of 19th century spec building. Builders use the cheapest materials they can get away with. Our previous London house, in Islington, was dated to October or November 1838; the evidence being that Baltic deal was used for the internal walls, then plastered over. Apparently there was a glut of deal in London docks that autumn.

If it weren’t for the walnut tree that shades our back garden there might be a basement under the next door garden as well. The law, though, is more protective of trees than of neighbours’ sensibilities. Wisteria is not a tree, says the law, so the massive one on the back wall – about to be demolished – has already gone on the skip. Developers have some mad ideas. Having rebuilt a house opposite (my goodness, what a Media Room they built) they hired a crane to lift five 25-foot cypresses over the roof into the back garden. Unfortunately the crane hit three parked cars, and they forgot to water the cypresses. Another crane, maybe, to lift the brown carcasses out again? And what will the Council say about removing big trees?

Is there a National Collection of blue flowers? There should be. It’s hardly a taxonomic distinction, but I can think of few plant characteristics that spring out at one as much as true blueness.

Just now it’s agapanthus. We bought a dozen bulbs at Chelsea last year for our new garden from a Yorkshire grower, half of them blue, half white. The blue have been flowering for six weeks now and are

still going strong, the white not at all, though their leaves are fine. The textbooks all agree that agapanthus need sun – though it seems they flower for longer without it. Most of ours are in our sunniest bed (still half-shaded), but those in almost total shade, three bulbs in a 12-inch pot, generously watered, are flowering best of all. Perhaps next year they’ll be all leaves and I’ll discover they did need sun after all.

Anemones, scillas, crocuses, campanulas, rosemary, geraniums, lithodora, ceanothus, nigella, clematis and hydrangeas have all given us blues of sorts. Recent sorties to Kew have added more salvias, morning glory and a passion flower too tender, I fear, for London to my list. But the really arresting blue of, for example, Salvia patens, is the gardener’s equivalent of the lapis lazuli renaissance painters reserved for the Virgin’s robe. It has long been my holy grail.

A rowdy at Rousham

Of all England’s great historic gardens Rousham is the one that affects me most. It was my inspiration in creating the garden at Saling Hall – more subconsciously than in any sense of imitation. I vainly hoped to recreate the seemingly casual interaction of grove and glade and stonework, of light and shade and glints of water, that apparently appealed to Georgian country families.

We were at Rousham again the other day. Its unique virtue as a garden to visit is to be open without ceremony every day. “Bring good shoes and a picnic”, say the Cottrell-Dormers, “and it’s yours for the day.” Gleaming white argosies of cloud were sailing in a sapphire sky, the sunshine just edged by a cool summer breeze. Rousham House is inscrutable, plain and prim; indifferent, it seems, to its star-struck visitors. Nor is the garden visible as you circumnavigate the building and set off across the unornamented bowling green to the first startling, even shocking, eye-catcher; a lion mauling a horse. Is this a warning that you are about to see nature untamed? If so, it is a false alarm.

When a garden is yours for the day you amble round it in your own fashion, not systematically but darting here and nipping there to inspect a statue or a plant or to see what happens round a corner. This was not the designer’s intention. He had a clear plan for your visit; the order and timing of each revelation of a new prospect.

I wish we knew more about Kent’s time at Rousham. It was Charles Bridgeman who contrived the broad layout of an Elysian garden on the steep slope 100 feet high and a mile long, leading down to the winding river Cherwell, in a process that may have lasted for fifteen years or so. When the late owner’s brother inherited he employed the über-fashionable William Kent to enlarge and embellish the house and to give the garden his magic touch. Did Kent plant the massed trees that now cast so much of it into deep shade? There are yews as tall and straight as any in England that surely must have been planted 300 years ago. Many of the oaks, beeches and limes, and a cedar of Lebanon by the Temple of Echo, look like the original planting, too.

Today the architectural set-pieces that overlook the river are in distinct glades separated and dominated by high trees, underplanted with evergreens, linked by the famous serpentine rill. I am certainly not alone in finding it a haunting, even spiritual, place where melancholy meets the ghost of glamour. Sitting in the Temple of Echo, the meadow sloping down to the Cherwell dappled with beech tree shade (concave slopes were much admired) the silence is pregnant. Three centuries ago it would have echoed to shouts and shots and hunting horns, dogs barking, the splashing of fountains and the giggling of girls. The English gentry didn’t (and still don’t) do solemn. The Praeneste, the seven arched cloister that looks over the river, has benches in Kent’s most elegant style for 28 well-dressed, and no doubt gossiping, guests.

We have a detailed description in a letter from the head gardener of 1760, John Clary, when Kent’s planting was still young. Melancholy was evidently far from the owner’s mind. Today water dribbles in mossy grottoes where Clary tells us it was flung in fountains forty feet high. The 18th century believed in bling. The woods were underplanted not with plain laurel but with every flowering shrub. Colour was introduced everywhere, and sparkling water splashed around.

Our habit of seeing the past in the sobriety of mossy patina makes it hard to take in the real meaning of the designs we so admire. The Parthenon was brightly painted, full of noise and incense; bare sun-bleached marble is a protestant taste the ancients would not have shared. Perhaps it is my own melancholy that I attribute to a landscape adrift in time.