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.

Tea on the lawn on a baking day with the hospitable Bleddyn and Sue Wynn-Jones of Crug Farm plans, the foreground a field of cows, the background the Llanberis Pass deep into Snowdonia. Every visit to Crug Farm is a revelation of plants I’ve never seen before. Sometimes I can spot the genus, very rarely the species: Sue and Bleddyn still don’t have names for half the plants they grow: just accession numbers. And the numbers grow lustily each time they do a trawl in South East Asia or Central America, two of their main collecting grounds.

They list over 60 trips on their website, starting in 1992. They collect friends, too: that day a Belgium nurseryman and his wife, every day a plantsman from near or far agog to see what new things they are offering in their little sales yard, or growing in their propagating tunnels, or displaying in their romantically intricate little gardens between the house and the old farm buildings.

Last year they saw an accidental fire as a chance to add yet another garden. An old cow barn went up in flames; leaving its walls and the remains of its roof as a sheltering spot for yet more new plants whose hardiness needs testing. It is a completely eclectic mix, from maples to lilies – all with a new twist – via fifty things I’ve never heard of. Scheffleras seem to be a Crug signature plant (they list 31 in their astonishing catalogue). Their elegant almost palm-like clumps and fingered leaves caught my eye several times, some with discreet yellow flowers, some with black berries. But it is absurd to pick on any plant, or even genus, in this garden of wonders. We left feeling as though the Creator had had another go with a new designer.

Plas yn Rhiw

I don’t suppose the name Plas yn Rhiw comes up very often at meetings of the National Trust Gardens Committee. If there was a period when a sense of horticultural correctness pervaded the Trust’s gardens, you wouldn’t know it here, around this remote little manor house on the Llyn Peninsula, stuck out in the Irish Sea. A sense of place certainly: it would be bizarre not to plant the sort of half-hardy things the benign sea air allows. Of fashion, inevitably: bright gardeners are always alert to new plants coming through the system. But, most agreeably, a sense of freedom to be as relaxed as befits minor provincial gentry happily hidden from the world.

The “Plas”, a word which in Welsh seems to cover anything from a mansion to a very minor manor, was restored by three spinster sisters from Nottingham in the 1940’s, encouraged by their friend Clough Williams Ellis of Portmeirion. Everything from the books in the bookshelves to the box-hedged enclosures of the garden bursting with exotic shrubs breathes an austere gentility in speaking contrast to tremendous views of sea and mountains.

The mansion “Plas” of these parts is Plas Newydd, the huge house of the Marquesses of Anglesey on the Menai Strait, in the front row of the stalls for the rock and cloud performances of Mount Snowdon. The National Trust plays this with a light hand, too: it is part family house, part museum (with Rex Whistler’s most famous mural as its USP). One formal terrace, the Royal Box as it were, is intensely floral: blue white and grey in the view that includes the mountains, hellfire red looking the other way.

Trees grow magnificently in its sheltered seaside policies. There is a macrocarpa avenue where these usually scruffy windbreak trees grow as straight grey pillars 80 feet high. Hydrangeas are a way of life in these parts; you pass walls of them hundreds of yards long on the way to the arboretum, with elephantine beeches supervising a jungle of visitors from down under: eucalyptus and every kind of southern beech.

Something we noticed everywhere this year: hydrangeas are ignoring all the colour conventions. The same plant has flowers from imperial purple to Cambridge blue. Is it the warm spring, the sunny summer, or just perversity?

The view from the woods, south to Cader Idris, Wales's Table Mountain

Forestry is inherently an untidy business. Each summer we come to North Wales to enjoy our woods, the mountains round and sea below, and each year another patch is a (temporary) eyesore. This year is a bad one: the February gales made a shambles of half a hillside, and fishing the trees out from the mess transferred a good deal of it onto the forest track. Ditches had to be scoured, surfaces scraped and loads of new stone spread and rolled in. Luckily we have a good roadstone quarry in the heart of the forest, but I have to admit there is a certain rawness about parts of the place just now.

It is easy to escape them. The bosom of the broadleaves is where I go, where a stream tumbles down white among the black rocks. There is a line of Milton’s that rings in my head here: ‘bosom’d high in tufted trees’. The distinct tufts of well-spaced oaks and beeches, ashes, some sycamores and innumerable birches pattern the hillside opposite like a gallery of portraits, now glowing gold in the evening sun. I have been surprised to learn how often a wood of originally close-planted trees needs thinning to make strong (as opposed to spindly) trunks: perhaps once every fifteen years. I wonder where Milton’s trees were.

Ashes are a worry: Chalara may well be on the way to kill or cripple them. But I never planted many; there are plenty of seedlings, and oak and beech are our favourites. The far bigger worry is the new threat to the larch from the dreaded Phytophthora ramorum that arrived in Wales from the southwest and seems to be heading north. Almost a quarter of our trees are larch, most of them planted in the 1960s and now close to full height. For ten years we have been thinning them; they need maximum light to form strong heads on their elegant bare stems, standing thirty feet or so apart. The forestry plan is to underplant them with beech or Douglas fir (or both), so when they are felled at full size the ground is already well treed. There is a good deal of volunteer oak and birch too. We hate the idea of losing the pale larch-green in spring and warm gold in autumn, born high above the hillsides on graceful poles.

It is strange to remember that larches only came to this country in the 1600s from the Alps. John Evelyn in his Sylva tells how his gardener threw out his first batch of seedlings in the first winter, thinking their loss of needles meant they had died. He goes on to rave about the quality of larch timber, as good as oak for shipbuilding, able to be sliced so thin that you could use it in place of glass in a window and light would shine through.

We visited the biggest larch in the Alps, in the Val d’Anniviers, this spring, a branchy, craggy monster supposedly born in about 1600. Some of Britain’s original trees still grow at Dunkeld, where they were planted 300 years ago by the Duke of Atholl. I can’t bear to think that we could lose all these lovely trees..

Rozanne is happy anywhere

There have been just enough hot sunny days to remind me how much I prefer the shade to sitting, or walking, or doing almost anything in the sun. Shade, that is, with sun round the edges; pools of shade, as under trees, for example. Perhaps I am like a camera film that renders the shady foreground well enough., but can’t handle the light beyond. The image burns out.

The garden is close to monochrome at present, or rather we have so many flowers in the hard-to-name purpley-bluey-lavender-violet range that it looks as though I’ve done it on purpose. They take their cue, it seems, from The Geranium of the Millenium – for surely Rozanne has earned that title. You see her everywhere, as you did Verbena bonariensis a year or two ago – and still do in this garden. Come to that their colours are both in the same part of the spectrum.

So are the little viola that spent the winter in a pot indoors and gamely flowers on in her summer quarters, Campanula persicifolia, a catananche I’ve taken up with, Tulbaghia violacea, which I’m told is rather charmingly known as Society Garlic, and the best of all summer clematis, Perle d’Azur. The newer and smaller Prince Charles is a relatively feeble version of the same colour.

I can’t be held responsible for the roses that flourish high above the wall on our neighbour’s side. They make sure no colour scheme of ours will survive. But do I mind? No, I’m not a schemer at heart, and their Wars of the Roses red and white is a splendid statement. Then on our side there’s Iceberg, Gloire de Dijon, Alister Stella Gray – all our old favourites – and Bantry Bay (perhaps a mistake; pink looks like a compromise).

What astonishes me is how much water everything wants. Above all, of course, the pots. When we go to Wales for two weeks I shall rely entirely on a newly-acquired Aqua-Pod dripper system. The landlord of the corner pub nourishes a lurid display with it. But then he doesn’t take a fortnight off in Wales.

Tilia petiolaris, the weeping silver lime, in flower at Kew

To Kew yesterday for a linden-bath. That was the sensation as I walked in through the Victoria Gate. Straight ahead stands a big common lime, taller than a church and as wide as two, covered with its pale drooping flowers – and pumping out essence of mid-summer.

Turn left past the little temple and a huge weeping silver lime, Tilia petiolaris, is spreading its skirts in front of you, embalming the air. Then they come thick and fast, one species after another, a whole wood of limes, from a squatly spreading small-leaved one, Tilia cordata, like a green and yellow cushion, to soaring T. petiolaris, surely some of Kew’s tallest trees. It is a curious fact about this splendid species (or is it a cultivar?) that it habitually divides into two, or often three, major trunks about twenty feet from the ground. They grow vertically with mighty vigour to make a majestic tower, their leaf-stalks (the petioles of petiolaris) twisting to show the silver-white undersides of the leaves. At this moment its cascading flowers are just opening and beginning to shower down their scent. More limes stand ready for their later flowering, with the fringe-leafed T. henryana from China coming last.

I headed towards the river from this honey-scented zone, skirting the sickly smell of the sweet chestnuts, to visit the Mediterranean area with its perfectly authentic-looking olive grove in a patch of rock-strewn garrigue and the cork oak forest next to it. A huge cork oak lies prone amid citrus and seedling pines next to a signboard exerting us to protect this threatened habitat. Threatened, among other things, by the spread of screw-capped wine bottles and consequent decline in demand for corks.

I’m not sure about this. The demand for wine-corks has grown exponentially in the past thirty or forty years, since wine became the popular drink it is today. In the past a relatively small proportion was given the dignity of a cork and capsule. Cork had many other uses, from buoyancy aids to insulation to high heels, but any increase in oak-acreage due to wine must be pretty recent. It’s not screw caps we should blame, but substitutes for its other uses. Let’s have more cork-tiled floors – and thank goodness we’re seeing the decline of the corkscrew.

Trachelospermum jasminoides

Christopher Bailes, the about-to-retire curator of the Chelsea Physic Garden, reckons his plants are between four to six weeks ahead of schedule this summer. Who knows how much of the advance is due to the mild spring after a mild wet winter, and how much to the famously privileged site, surrounded by buildings on the banks of the Thames? The garden has, after all, England’s biggest and oldest olive tree, a serious cork oak and scores of plants considered tender everywhere else.

Hoheria sexstylosa grows here to substantial tree size, palm trees include Jubaea and Washingtonia, the proper working tobacco plant, Nicotiana tabacum, has become a shrub, now in full pink flower. There is a fruiting grapefruit tree and an avocado…no, this is no ordinary London garden. Yet walking there through Chelsea I realise what a hothouse London in general has become. The plant of the moment, to the point of monotony, is Trachelospermum jasminoides , scrambling up walls, dangling from trees, often in its variegated form, a pale presence in expensive front gardens it seems almost everywhere.

No wonder. It is decorative, vigorous and now apparently reliably hardy. It casts a sweet but not obvious jasminoid smell about it. It only needs a little help from a trellis to reach ten feet or more. The cream-variegated form is less vigorous and maybe less hardy, but given a bit of care can be smart as paint.

What else do we know from its family, the Apocynaceae? Periwinkle is perhaps best-known. Also mandevilla and nerium, the common oleander. A trachelospermum in periwinkle blue (or in oleander pink) would be popular.