After St Paul’s, Exbury the next weekend came as a complete contrast: a garden that shows off its astonishing collection of plants like a museum.

The Rothschild style (Exbury was largely the creation of Lionel de Rothschild in the 1920’s) is full-on; what J C London would have called “gardenesque”. If nature never made sylvan glades with smooth straight hedges, still less did it fill woods to the brim with a kaleidoscope of flowers of different species and varieties in the full spectrum of colours.

Both gardens, Exbury and St Paul’s, gain their dignity from their high vaults of towering trees – above all oaks – that moderate light and

temperature in their shade. St Paul’s is decorated with Augustan restraint; the confident simplicity of the Georgian  age. Exbury revels in the excitable showmanship of the Victorians. In fact the Rothschild style, indoors or out. You see it at Waddesdon Manor in the rich recipe of Gainsboroughs and Versailles furniture and the extravagant full-dress bedding of the parterres. In the New Forest it takes the form of a dazzling display of rhododendrons, azaleas, camellias and every flowering plant and tree that can be fitted in 220 acres of immaculate woodland – along with the smartest of miniature steam railways.

Perhaps its high point, for this romantic gardener, is where after a sustained passage of ravishing camellias the flower power moderates, the trees thin out, and glimpses of sparkling water allow you to see white sails gliding by. You are, after all, on the Beaulieu River, the Solent is round the corner, and there is that faint smell of salt in the air.

You see the plant you’ve been looking for growing lustily in a big pot in a nursery sales bed. You pay the 5-litre premium, you take it home, dig the hole, add the fertilizer, knock the plant out of the pot – and a meagre little root comes out in a cascade of fresh compost.

It happened to me again yesterday. The guilty nursery was Wisley. There should be a decent delay after repotting for plants to fill their pots again before they are offered for sale.

.....a green shade at St Paul's

To St Paul’s Walden Bury on one of the rare fine evenings of June to wander round one of England’s most romantic gardens at that moment when the very air seems charged with chlorophyll. It must have been on such an evening that Marvell wrote ‘a green thought in a green shade’. The oaks and ashes are joyfully green against an azure sky. Every plant is in improbable perfection; towering forest trees, magnolias in the infant innocence of pale flowers,  rhododendron of every hue, billowing white clouds of Siberian malus, ivory flowered dogwoods and wild service trees the size of oaks.

But this is a woodland garden disciplined by calm grass rides, arrow-straight, turning your steps towards a statue, a pavilion, a grassy theatre overlooking a simple fountain. At one moment lilies distract you, at another the perfume of azaleas: all the spring garden pleasures are there, all the more intense for the calming effect of geometry and proportion, measured out in straight beech hedges.

Behind it all are two indefatigable treasure-hunters, Sir Simon and Lady Caroline Bowes Lyon.  Laid out this evening on the billiard table in the house were a hundred fascinating photographs they had taken in the previous weeks in Bhutan, many of rhododendrons planted or to be planted in Hertfordshire. Such enterprise, such order, and such an evening are really the summit of gardening.

Some veterans persist

Each year we try to time a visit to our Welsh woods to see the bluebells in their glory – hopefully in concert with the blue rhododendron augustinii we have infiltrated among the beeches. Last weekend was almost the bluebell climax; they still had a little way to go. The rhododendrons, on the other hand, seem to be better at time-keeping: they were perhaps a week past their peak. We call them blue, but the colour of French lavender is a closer match.

The Snowdonia spring, our neighbours tell us, is three weeks late, or (seeing it in a positive light) three weeks longer than usual this year. It is the most beautiful ever : the sky is azure, the opening leaves of oak and beech olive, russet and that most tender green, scarcely shade the woodland floor where bluebells in millions and clumps of primroses compete with the fresh fronds of ferns. In damper shade the little white stars of wood sorrel are like timid wood anemones. Locals say there is a relationship between wood sorrel and copper – which rings a bell at Cae Gwian: a projected gold mine here in the 1840s (we still use the old grey stone building) turned out to hold nothing but copper after all. What flowers would gold has favoured?

The next valley to ours, running north-south down to the estuary of the Afon Mawdach, has been bought by the Woodland Trust. Its name is Cwm Mynach, the valley of the monks (of the Cistercian Cymer Abbey at Dolgellau).Its secluded meadows, hemmed in almost completely by steep hills and dominated by the cliffs of Diffwys, a 2000-footer, must have been some of the loveliest in Wales before the foresters arrived. The Woodland Trust is dedicated to restoring the ancient oak woodland now brutally invaded by conifers. They planted spruce, larch, douglas fir and the hideous (and useless) lodgepole pine under the oaks with the object of shading them to death. Often they succeeded, but veteran oaks still persist, and in the remaining open meadows show how magnificent such sessile oaks can be.

We started the rescue and restoration of our own oakwoods at Cae Gwian nearly 20 years ago now. It’s a long job, but hugely rewarding.

I was nervous about going back to Saling Hall. Our successor there is a man of action, and I knew he would waste no time before tackling his new project. But what I saw when I went back last week astonished me – and made me realise how long I had let things drift.

Judy and I had often talked about felling the long file of Lombardy poplars that flanked the front of the house, memorable trees (they were planted in the 1930s) that contained the front courtyard, separating it visually from the churchyard next door.

Last week they had gone. A loader was shifting their immense logs onto a pile bigger than a bus. The sense of light and air around the pink brick façade of the house was extraordinary. The rather gloomy presence of the towering trees was replaced by the broad green dome of a wild service tree I planted in the churchyard in 1973, looking in perfect harmony with the grey flint and gothic windows of the church. It was our predecessor, Lady Carlyle, who wanted to hide the churchyard from the bedroom window (so we were told). Her poplars had gradually become the main feature – and to see them gone gave me a gush of relief, delight…. feelings I could have experienced years ago if I had been more resolute.

‘The axe is my pencil’ said Humfry Repton. Knowing when to fell trees is as vital as knowing which ones to plant.

Not only the poplars are gone. Boring Lawson cypresses I should have condemned twenty years ago (but didn’t, on the pretext that there were shelter from the cold east wind) are there no more. Old pollard bat willows have gone from the moat (I’m not so convinced about these); a dozen old friends – or at least acquaintances – are piles of firewood. There is less muddle, and less mystery too. Over the years our gardens create their own untouchable auras. Nostalgia feeds on inertia and vice versa. Seeing radical change, and knowing it was necessary, is exciting, surprisingly emotional but hugely positive.

It did well in the heats

Of all plants perhaps the wisteria looks most obviously pregnant before it bursts its buds. Just now the competition is fierce among the countless wisterias of Kensington. There are veterans bending railings and wrenching down-pipes,  infants reaching wildly for their first grip, and dozens of perfectly-pruned, proudly, displayed specimens adorning the fronts of terrace houses. Teamwork has done wonders. Just round the corner three householders evidently work on their marvellous plant together. Its gnarled spurs, now starting to dribble their purple down the masonr, cover sixty or seventy feet of façade.

What about a wisteria championship? Who would like to organise local and regional heats, with points for size, coverage, tidiness, colour, and above all number of tassels? Just a London one, perhaps, to start with. I can see The Evening Standard under its feisty Russian proprietor making a go of it.

Size is not everything. The world’s biggest is apparently in California and covers an acre of ground. In the forests around Kyoto I have seen the tree canopy across a valley mauve with wisteria here, mauve with paulownia there. No, this is not that sort of contest; more a concours d’elegance. The time to start is now and the place, judging by what I can see from the café on the corner, London W8.

To Strawberry Hill to see the progress of the restoration of Horace Walpole’s riverside summer house. Is any house more famous and so little known? The reverend fathers who took care of it for so long loved it dearly and defended it well, but they had no money to restore its glories. Now some inspired fund-raising, boosted by a handsome Heritage grant, has set the wheels in motion. The results, sticking as faithfully as possible to Walpole’s plans, tell us almost as much about him as reading his irresistible letters.

Riverside, alas, the garden is no more, although the river has not gone away. The 200 yards between Walpole’s raised terrace walk and the Thames have inevitably been filled with houses. Of the 40-odd acres of garden and park, originally in open countryside, some four remain. We know enough about Walpole’s planting to reproduce much of it: young lime trees in serried rows already begin to form the

patte d’oie whose alleys lead to his favourite bay window. One oak survives from Walpole’s time on the terrace walk. More trees serve to screen the college buildings that could easily be uncomfortably close neighbours.

He achieved a truly wonderful deep brilliant guardsman red with a wall-covering of silk mixed with the wool of a particular Cumbrian breed of sheep. The wool is so springy that even in a weave there are no reflecting surfaces. The scarlet in unremitting purity has no highlights to help your eye to focus. Your gaze buries itself in pure colour.

All this red is set off by intricate gilding: a throne-room could hardly be more dazzling – and there are more intimate rooms where gilding traces a sort of Medieval allegro above your head.

There is not yet much furniture, nor very many books in Walpole’s famous library. The greatest want, though, I felt, as we explored, was the voice of the man himself. Perhaps an actor with a suitably camp voice could pronounce Walpole’s commentary as he opened each door to reveal his latest jeu d’esprit.

“Gloomth” was famously one of the effects Walpole aimed to create. He loved to pass from gloomth to brilliance. Where you do, from the grey Gothic corridors to the long gallery upstairs, you blink – just as he intended.

There are cars circulating in London at the moment scattering petals as they go from the accumulation on their roofs. The odds are they come from Kensington. The Royal Borough is blossom-crazy, with sometimes spectacular, sometimes frankly garish results. I hadn’t realised, I confess, before it became my borough, what an arboretum of street trees it contains, or what a show they make in April and May.

The theme tree in our street is our native double white cherry,  Prunus Avium ‘Plena’. It was one of our favourite trees at Saling, a sumptuous white cloud in April and as prettily motley as cherries get in autumn. There must be forty in the street here, ranging from craggy old veterans to novices only planted last year. The disadvantage of cherries in pavements is the way their roots emerge from the ground as writhing monsters – as gardeners know only too well.

I wrote a while ago about the ludicrous planting of the Avenue de Champagne, that noble address in Epernay, as an arboretum. When local authorities hire enthusiasts, people who love trees too much, I fear this is the result.

When I first saw the bare weeping tree in our neighbour’s garden I took it for another cherry. Now it is in leaf I am thrilled to see it is the exceedingly beautiful and rather rare weeping Katsura, Cercidiphyllum japonicum “Pendulum” – so pendulous, indeed, that its branches would reach the ground without judicious pruning. Passers-by walk under a beautiful green parasol over the pavement. Its little heart-shaped leaves are now a brilliant tender green. In autumn it will turn everything from cream to scarlet – and the street will have that warm sweet elusive scent of strawberry jam.

There are some almost-avenues in neighbouring streets, with the trees correctly uniform on both sides. Ours, though, is a bit of a muddle. There are two old planes, a couple of limes, two hornbeams, a scattering of the default street tree today, Pyrus “Chanticleer”, and (horror) what looks very like the dreadful Prunus “Amanogawa”, that pink scarecrow from Japan.