Things are not looking too good in the garden just now. There’s a scaffolding tower blocking the outside staircase down from the ground floor, five feet of our Victorian garden wall has been demolished, the trellis with our laboriously trained trachelospermum has been removed and everything is covered in a thick layer of dust, The noise of drilling and hammering starts at eight and goes on till six; the noise of pumping water up from the basement depths goes on all night.

The roses and agapanthus, the geraniums and anemones, the fuchsias and salvias, the lemons and oleander, the clematis and solanum, the kirengeshoma, the thalictrums and my pretty ferns are all pretty in vain. Nobody’s looking. We don’t even open the door – except to dash to the shrouded greenhouse in hope of a ripe tomato (I picked my first Gardener’s Delight yesterday).

What’s it all about? This is London life in the wake of the Party Wall Act 1996. It’s your property; you can do what you like on it and the neighbours can just grin and bear it.

To do them justice the twenty Bulgarian workmen who assemble every morning at seven in our neighbour’s garden are a cheerful crowd and seem to work as fast and as tidily as they can. What they would do without a pretty big garden as their assembly point, canteen and depot I can’t imagine; there certainly isn’t room in the street.But the rage for basements is out of control. In the next street one terrace house is in a basement sandwich with hoardings up on both sides and lorries queuing to cart off their neighbours’ rubble. The Council? They are hamstrung, they say, by the Party Wall Act. They can’t disallow ‘development’ – or even police it. Not enough enforcement staff…. there are plenty of excuses.

We’ve visited a couple of basement developments. One consisted of eight featureless rooms, all painted this season’s grey, dubbed ‘Media Room’, ‘Gym’, ‘Office’, ‘Bedroom’, ‘Wet Room’, ‘Meditation Area’. Upstairs was a perfectly nice Victorian house – until they took our all the cornices, mouldings and everything that gave it character – and of course painted it all grey. Gardening? The developers opposite used a crane to heave four mature Italian cypresses right over the house to plant then in the diminutive back garden. They died, unwatered, within six weeks. And what happens underground in a power cut?

It’s all justified, it seems, by the price of any space in London. Add 1,000 square feet, even underground, and you’ll be in pocket for more than twice what it costs. Your neighbours can lump it; that’s what the Act says.

Thalictrum delavayii at Saling Hall

Thalictrum delavayii is my plant of the summer. I planted it last year at the top of the steps leading up from the kitchen door; so far there is only one main shoot, now nearly seven feet high, like an impossibly delicate tree with lilac leaves, each leaf a flower like a tiny nodding clematis. The cloud of flowers is at eye level as I reach the top step, and from the kitchen window it is as important as the trees beyond, a lilac filter for the view of the rest of the garden.

‘delavayi’ betrays its origin, the mountains of south west China, where Father Jean-Marie Delavay went to convert the heathen and became one of the most industrious plant collectors, with 1500 new species in his bag. ‘Meadow rue’ is the English name for the family; the finely divided leaves, in this case palely glaucous and scarcely significant in the picture, explain the name. The monster hybrd ‘Elin’ proves that bigger is not necessarily better, and the double one, ‘Hewitt’s Double’, that two is not necessarily a higher value than one. Of all the perennials I grow in our perpetual shade this is the most striking and rewarding.

I reported from the Hanbury Garden at La Mortola in February last year that I had seen a nasty new threat to box plants. Little did I think that only a year later it would have arrived – right here, in our garden and our neighbours’.

It’s called Cydalima perspectalis. In Italy they call it Piralide del Bosso. So far it’s only Box Caterpillar here. The caterpillars are green, yellow and black, up to 4 cm long, and the moth brown and white. But it’s the larvae, from yellow eggs laid on the underside of the leaf, that do most of the damage. They protect themselves with something like a fine miniature spider’s web while they’re at it. They eat whole box leaves or leave the skeleton; in any case defoliating the plant and ruining its appearance for – how long? In some cases there are signs of new leaves growing out, but one must presume that the damage is debilitating and could end in dead box.

At first it’s not easy to spot. There are often little brown patches after trimming a hedge. But look closely: the little webs are not hard to see. It spreads quickly; there’s no time to waste when you see it. Look at the RHS website; it’s not particularly encouraging.

There's just room for a little lawn

It was déjà vu all over again when I turned into Roy and Sue’s drive in Chandler’s Ford, in a leafy suburb just south of Winchester. I’ve seen so many pictures and heard so many stories. What feels familiar, though, on looking closer turns out to be almost subversively alternative. You know the genus but you never saw this species before. It happens again and again.

Roy is genial, passionate (we all know that from television) and loquacious. ‘Isn’t that a ….?’ you start. ‘Yes, I collected it with (half a dozen well-known collectors may feature here; very often Mikinori Ogisu, his Japanese chum) on Emei-shan’. (I’ve learnt that China’s holy mountains are honorically labelled ‘Shan’. Mount Fuji, come to that, is called Fuji-san; are they related? Gongga-shan, alias Minya Konka, seems to be Roy’s favourite peak.) ‘….It was growing with Abies delavayii, and a daphne, and a lily I’d never seen before, before breakfast one day outsde a village in Sichuan. I’m not mad on rice at breakfast. We’d just seen an extaordinary mahonia with coppery leaves….…’ Perhaps I parody, but Roy’s reminiscences are worth as long as you’ve got.

His selective eye has been focused on woody plants (and not only woody ones) since he worked at Hillier’s Nurseries nearly fifty years ago, I could never have written my own tree book without Hillier’s Manual – and didn’t even realise at the time that this astonishing compilation was newly hatched, the work of Sir Harold Hillier and Roy together. But I owe him for far more than mere knowledge.

He has a thing about mahonias – and things, for that matter, about a hundred other genera as well. I came away from his garden with ‘Roy’s Choice’, a selection of the cross made in the Savill Gardens in John Bond’s days between two Lancaster collections, M. gracillipes and M. confusa. The first has dangling flowers with red sepals and hard, armoured, almost scary leaves, the second an altogether softer look, far more and narrower leaflets and more conventional flowers. The 2013 Chelsea Plant of the Year, Mahonia ‘Soft Caress’ seems to be a variant of a subspecies.

I would never dare plant things as close together as Roy does. He packs a whole botanical garden into a typical suburban one. It looks busy, of course, but still in the way Loudon called ‘gardenesque’. One trick is persuading shrubs to climb the house walls, encouraging them with supports, then clothing them with a climber or two – underplanted with two or three other rareties. The monster honeysuckle, Lonicera hildebrandiana, has shot up and needs hacking back from a bedroom window.

I should know the Cotswolds better. My two lifelong favourite gardens, Rousham and Hidcote, are there, or at least en route. But I am always confused by the tourist-board names. This on the Wold, That on the Water, One on the Hill and Another in the Marsh. High-hedged lanes wander dementedly between clusters of cottages, all buff stone, many thatched, all painted in Farrow & Ball colours. One gets lost.

This excursion started at our favourite pub, the King’s Head at Bledington, with few clear objects, turning aside when a church tower, an intriguing signpost or a Garden Open sign hove into view. Sezincote and its Indian gardens were shut, as we learnt at the gate, but at Bourton the Hill we found Bourton House garden open. I had missed its election as HHS/Christies Garden of the Year in 2007 and was caught unawares by its quality, its mastery of palettes and idioms, from some of the coolest to quite the hottest borders I have seen, from severity in topiary to a box parterre like a nest of serpents, from placid pastoral to hothouse exotic. Does a bird of such bright plumage in the calm of the Cotswolds conform to the rule that gardens should reflect and interpret their surroundings?

Another chance encounter was Minster Lovell. The name on the signpost was somehow familiar, but at first the sign seemed a wrong steer; the lane led only to an isolated church – if a fine one. Behind the church, though, was a revelation: the pale ruined towers of a medieval mansion on the green riverbank of the Windrush. How rare it is to find an important monument these days uncluttered by signs and gift-shops, not a teapot in sight. This is the mansion the Coke family left behind to establish themselves in Norfolk at Holkham. It is not a violated abbey, just a great house left without a roof. Why?

English Heritage deserves a prize for its sign, a brief history of the house with Opening Times: “Any Reasonable Time”. Nowadays such a non-prescriptive notice is a rare sight.

I have never actually washed my garden before. It’s the ground you usually water, not the leaves. But the builders next door send over such a dust-storm that after weeks without rain every plant was grey with grime. I’m sure it was stopping them growing; flowers were fewer and fewer – and sadly dingy.

So I got out the hose and sprayed the whole place; drenched it all over – an hour’s work the other night. How effective my artificial rain was I shall never know; it’s been stair-rodding down all day.

Later: how does steady rain have a totally different effect from watering? You can fill pots to the brim regularly, but the day after a downpour they look quite different, more buoyant, ready to perform. Is it the atmosphere, the pressure, the humidity?

The visitors book proves it: I was last here fifty years ago – and nothing has changed. The broad white verandah still looks out over orange trees to the steep terraces of vines going down to the river. The house, verandahed all round, sleeps like a planter’s bungalow on any tropical station, lawns shaded by thick trees (in this case limes), screens closed against mosquitoes, the rooms complete capsules of times long past. Deep beige armchairs, faded prints, dusty books, the polished dining table, have not changed since the 1960s. Probably not since the 1920s, when the Gilbey family bought the estate, 150 acres of vines and the stone barns where Croft’s port has been trodden time out of mind.

The upper reaches of the Douro, a hundred miles from the sea through range after range of steep hills, are dry, hot and fertile. When we arrived the other day there had been four days over 40º, the conditions that make great vintage port. It’s a long time since we slept as past generations have, bare under a sheet hoping for a draught from windows open on both sides of the house, resenting the mosquito screens blocking the free passage of air. The thermometer drops to 30º at dawn: I get up to open all the doors to let the cool air in, and doze off just as the sun shoots its first shrivelling rays into the house.

The early morning is when the vines can get to work, photosynthesize and swell their grapes. In this exceptional summer veraison , when the grapes turn colour from green to red, is already under way. By mid-morning, vines that stood trim and gleaming have started to droop; their stomata closed; evaporation exceeds the power of their roots to find water in the parched soil. They look hangdog until evening, metabolizing nothing, losing time in the journey to ripeness. Fig trees show signs of the same stress, their big leaves limp. Olives, on the other hand, with their small grey leaves, seem immune to the heat. The agapanthus are unbothered, too, baking under the dry stone walls. And orange trees gleam on regardless.

I’m afraid I react like the vines, with the advantage that I can hide in the shade and dip my feet in the fountain. Our hosts’ labrador, on the other hand, has found a niche in a flower bed and lies between hydrangeas and agapanthus with a lime in his mouth for refreshment.

Quercus exceptional, a Berkshire native

It’s a problem photographing a tree like this one. But then there aren’t many. The trick is to ask a patient friend to stand by it and walk away until you can fit the whole tree in your lens. Communicate by shouting.

This oak is in a garden near Newbury in Berkshire. No one can see it without asking questions. How old is it? How wide? How high? But above all how? There are taller oaks, and girthier ones, but are there any so complete in their domes, with branches stretching so far in an uninterrupted circle?

Part of the answer may lie in the way it was evidently planted, it must be four or five hundred years ago, on a mound of earth. The sapling was planted on the summit of a mound some five feet high and perhaps thirty feet round. Why? The soil below is heavy clay; perhaps the gardener thought it would get away better on a hill of something easier for its roots to penetrate. There is another magnificent tree three hundred yards away on a similar mound. Yet curiously the woods around are full of strikingly tall straight oaks, big beeches, soaring Scots pines and lime trees of immense size. So why the mounds?

The rest of the garden, I should say, is in keeping. Many plants seem larger than life, and it is quite a collection, centred on a water garden round a large stone-edged water tank, ingeniously fed from a smaller and slightly higher tank in its centre so the water is always gently moving. The owner, Rosamund Brown, is a painter of memorable abstract landscapes; her sense of colour makes the planting sing – a tune that changes abruptly when you pass the door to the kitchen garden into a Mondrian world of primary colours and daring contrasts. Another smaller enclosure is planted entirely with cactus and sedums; the ultimate low-maintenance plants, but a startling display, and I guess a unique one in a garden.

Broad York-stone paths link the elements: formal to woodland to glasshouses and pools. And it is as though the fertile earth below resents their weight. Self-sewn volunteers push up between the slabs and are carefully edited, so that here verbascums, there agapanthus or daisies or campanulas embroider the grey stone.