On the high lime trees individual leaves go yellow first. There are more limes in Kensington Gardens than any other trees, planted in alleys round the royal palace, and more trees in Kensington Gardens than in the adjoining (and bigger) Hyde Park.

The turning leaves, horse chestnuts first, then limes, and a gradual browning of the grass as autumn sets in, change the mood of the park. The squirrels seem busier than ever. Chestnuts plop to the floor, at its driest now, inviting more picnic parties. It seems to be mostly brown people, from the sub-continent (or perhaps just Notting Hill) who love to congregate in their whole large families, tablecloths spread on the grass, to spend the afternoon drinking tea and emptying plastic boxes of food, little children spinning off at tangents to kick their coloured balls. Two boots on the grass represent a goal without a goalie.

Dogs in dizzying variety are the main distractions, certainly for me, sitting on a bench in the wide walk past the sculpture of Physical Energy. The massive bronze horseman, riding bareback on a pawing steed, was the masterwork of Britain’s’ most respected sculptor of Edwardian times, G F Watts, created at his studio on the road to Guildford.

Watts has been considered as Britain’s Michelangelo. I couldn’t possibly comment, but the huge sculpture certainly commands attention. There is usually more controversial art to be seen three hundred yards away at the Serpentine Gallery. Together with the dogs, plenty to fill an idle afternoon.

With the temperature in what we used to call the 80s (even for a day or two the 90s) a north-facing garden has been a boon. The French window from the kitchen is open; the garden  flows into the house and vice versa, a big white parasol shades the central table, our neighbour’s walnut offers grateful shade and the constant splash into the little faux-lead tank with its residents Cae and Gwian, goldfish named after our woods in Wales, reminds me of the soothing lines in The Ancient Mariner: ‘The noise of a hidden brook, in the leafy month of June, which to the sleeping woods all night singeth a quiet tune.’ This is the sound that tells the mariner his ordeal on his stricken ship is coming to an end. It percolates soothingly through the open bathroom window while we sleep.

But where have the birds gone? Have our resident robin and the pair of blackbirds joined the rest of our neighbours in the Cotswolds, or Cornwall, leaving the shuttered street silent, the road unencumbered with parked cars? August in London is a city muted – save for the screeching parakeets; a town three quarters asleep. In the rest of the year a long day spent reading carries a shadow of guilt. Not in August.

Are we making the most of what water we have in our gardens? Here is Celia Fiennes, a gentlewoman garden-touring in the 1680’s, describing Wilton House.

‘…the river runns through the garden that easeily conveys by pipes water to all parts. (A) grottoe at the end of the garden…. its garnished with many fine figures of the Goddesss, and about 2 yards off the door is severall pipes in a line that with a sluce spoutts water up to wett the Strangers; in the middle roome is a round table, a large pipe in the midst, on which they put a crown or gun or a branch, and so it spouts the water through the carvings and poynts all round the roome at the Artists pleasure to wet the company; there are figures at each corner of the roome that can weep water on the beholders, and by a straight pipe on the table they force up the water into the hollow carving of the roof like a crown or coronet to appearance, but is hollow within to retaine the water forced into ti in great quantetyes, that disperses in the hollow cavity over the roome and descends in a shower of raine all about the roome; on each side is two little roomes which by the turning their wires the water runnes in the rockes you see and hear it, and also it is so contrived in one room that it makes the melody of Nightingerlls and all sorts of birds which engaged the curiosity of the Strangers to go in to see, but at the entrance off each room, is a line of pipes that appear not till by a sluce moved it washes the spectators, designed for diversion.’

Have we lost our sense of humour?

There’s a moment at the beginning of August when the big bush or little tree of Solanum rantonettii scatters its crown with purple potato flowers. Only little ones – and I’m never sure whether to say purple or blue. It’s a matter of judgement -or maybe eyesight. But a fifteen-foot dome of this colour pretty much takes over the view for six weeks or so.

No such luck with it in Hampshire; it can be too cold even a mere two miles from the sea. But for a London garden with some wall shelter and a fair amount of space I can’t think of a more satisfactory little tree. I tucked it into the south-facing corner by the greenhouse door. Now we have to cut off six-foot shoots to stop them casting the greenhouse into shade.

My obsession with blue has its moment at this time of year, and the solanum has a companion little tree in a pot on what I call the quarterdeck, four steps higher at the far end of the garden (for its total length, think a cricket pitch). I use blue in what a grammarian would call ‘sensu lato’ – as one must in discussing a colour that ranges from summer skies to deep distant shadow. Little acnistus (or iochroma if you prefer) dangles inch-long flaring bells of a deep violet hue. I keep this little standard clipped lollipop style the better to see its bells.

The most obliging blue, particularly eager to oblige, scrambling through the border, is Geranium ‘Rozanne’, celebrated last year as the ‘Plant of the Century’ or some such. With clematis ‘Perle d’Azur’ the blue mood needs lightening. Phlox ‘White Admiral’ is the answer.

Since I first saw the scarlet fairy bells of Fuchsia magellanica years ago in its native Patagonia I have loved this modest beauty. When years later I met its albino version I loved it even more, and have planted it wherever I have a spot in the garden that needs a quick and charming lightweight. Then I came across ‘Hawkshead’, a lovely purer white version. I have a tall plant of ‘Alba’at the far end of this little garden, doing its best to block the steps up to what I think of as the quarterdeck. I regularly have to cut off some of its spreading branches; it rapidly adds more on top. It’s certainly taller than me now, an airy tower of tiny dangling bells on the blush side of white.

The contrast with the pure white of ‘Hawkshead’ is telling. I don’t plant many duplicates; I’m too keen on trying new things, but there is the pleasure of subtle variety here.

In the last few days, down by the Solent, I’ve seen more samphire than plants with RHS approval, but I always associate seaside summer with the combination of hydrangeas, usually in a washy pink, with the scarlet shock of montbretia. They are far from being AGM plants, but in bright light and salty air they make a strong holiday statement. Fuchsias love it here too. The other day in a garden by the Solent I saw a hedge fully seven feet high of the original red-flowered variety, the red not strident but just lending it a glow. It’s a splendidly biddable plant: cut it tight or let it gesticulate, a midsummer joy that keeps going well into autumn.

We landed in Normandy on D-day, eighty years after the event, to find jeeps clogging the roads, a sea of ancient khaki uniforms and veteran Dakotas roaring over the surf. But it was another British connection that brought us; we came to revisit the most beautiful English garden in France. How many people know that one of the best of the famous collaborations between Gertrude Jekyll and Edwin Lutyens is on the other side of the Channel? Literally ‘on’. The thirty acres of Le Bois des Moutiers tumbles dramatically down to a cliff-edge and the sea. The garden, and one of Lutyens’ most successful country houses, was commissioned by the Mallet family of bankers at the end of the 19th century. If you book a visit there is still a member of the family to show you round, though today it belongs to the Seydoux, owners of most of France’s cinemas. In 130-odd years it has matured into something that would give its planters justified joy.

Lutyens hit mid-season form with the house; serenely simple at first sight but with precise details that sometimes seem to refer to Gaudi. The partnership with Jekyll gives the surrounding more or less formal gardens complete authority; it is hard to imagine them being otherwise. To the south they melt into orchards, then reach the edge of a slope that comes, in what in Devon would be called a combe, down to the cliff edge far below and what the French, with perhaps unintended poetry, call ‘le grand large’.

The garden, and Vasterival, another woodland treasury of plants created in the 1950s by Princess Greta Sturza, a former Wimbledon champion, face each other across the valley above the sea. The little village of Varengeville, south of Dieppe, is known for its cliff-top church with a richly blue stained glass window by Georges Braque. And inland lies the pastoral Pays d’Auge, source of butter, cider and Calvados.

Sod’s Law visits Chelsea. Monday, Press Day, the royal inspection and the only time you can wander in relative ease round a show without a crowd, was a perfect summer’s day: light sun, a faint breeze under a cloudless sky. Tuesday; cool, sky grey and drizzle turning to rain.

There are fewer Show Gardens than in the past, but one or two were memorable. Trad’s Garden of the Show rarely chimes with the official judgement; it’s the same with wine ratings, but then I’m a heedless hedonist. My favourite was the Roman villa, complete with attendants (not, I think, ‘enslaved persons’) in togas. I don’t know whether the water dripping quietly from the eaves into a marble channel in the floor was really a Roman trick to cool the loggia, but I hope so. (By Tuesday of course it was unneeded; the heavens did the job).

The actual gardens, to my taste, were predictably more about ‘sustainability’ than beauty – or even use. I suspect Tom Stuart-Smith will be rated one day as the Repton of our time: his touch is so sure, authoritative, stylish but somehow realistic and comfortable. His palette this year was calmly green and white, with some purple for contrast. His biggest plants were, of all quiet things, three ordinary hazel bushes of significant size. A substantial wooden hut, a stone water tank and a cluster of clay pots set the tone of almost humdrum existence heightened by intelligent, rather than eye-catching, planting.

There are some colours that burst on us once a year, without precedent: unique, exciting and transient. The new leaves of lime trees have the stage this week; a colour that by coincidence we tend to call lime-green – referring, though, to the citrus fruit rather than one of our champion park trees. Each April I amble round Kensington Gardens, pausing at tree after tree to enjoy the glowing tenderness of the unfolding buds, the little shining package of the leaf pushing out, limp for a day before breathing gives it strength to open to a pale green oval.

Limes were the chosen show-trees of the Georgians. They imported them in thousands from Dutch nurseries to plant their avenues. Kensington Palace has a great crinoline of them centred on the Round Pond, planted perhaps by Queen Charlotte, replanted in the past twenty years or so and now brilliantly verdant, the epitome of a stately spring.