The plant of the moment at Tresco, if I had to choose one, was the echium from Madeira, Echium candicans. When it was accompanied by the monster geranium from Madeira (G. maderense is happily naturalized on the islands) the sight was one of the most splendid in gardening.

I believe I’ve hinted, over the years, at my faiblesse for anything blue. Unconsciously I seek out blue flowers in gardens. In nurseries I head straight for the blue plants. It is a minority colour; there are ten pink/mauve/reds and maybe twenty yellows to each blue – and true blue, in the range you might call French, butcher’s or sailor’s, is rare. Ceanothus gives us some pretty punchy deep blues; hydrangeas tend to wobble between blue and pink. Borage is a clear lightish blue…. but its subtropical cousin, Echium candicans, is the real deal.

The Scillies are mild enough to make a shrub of it. It would struggle in most places on the mainland. Its cousin E.pininana can get close, as an almost ridiculous spire, for a year or two, though often wandering from blue towards red. Its other equally lofty cousin, E.wildpretii from Tenerife, lets the side down by being red, thus of scarcely more interest to my blue-besotted eyes than a red delphinium.

On a visit to Cambridge the other day, though, I had a 60 volt shock. One of England’s most photographed walls is suddenly under siege from with echiums. The splendid sandstone flank of Clare College, the left hand part of that view of King’s College that symbolizes King’s, Cambridge, and sometimes even England on calendars, is a frenzy of flowering spires.

The enterprising head horticulturist at King’s College, Stephen Coghill, spotted a perfect south-facing site for his seedlings of E. wildpretii and E. pininana, the soil essentially gritty hoggin, and seized the chance. Almost incredibly, 18 months later, the seedlings, planted out in April last year, tower over seven feet high. Maybe one day E candicans?

Tresco: shadows in a corner

An ambition achieved: for years I have wanted to visit the legendary island of Tresco, to see its garden of all the things we can’t quite grow anywhere else in Britain. Somehow the journey to the Scilly Isles always felt like an insuperable obstacle. Resolve arrived this spring. The ‘transfer’, as a travel agent would call it, was an entertainment in its own right: 7. 06 at Paddington, 9.45 at Exeter St David’s: an hour to visit the startingly beautiful cathedral (which by itself made the whole journey worthwhile), 11.45 take off from Exeter Airport in a Skybus Twin Otter, 12.20 land at St Mary’s and 1.15 catch a launch to Bryher.

Bryher is the nearest of the Scillies to America, England’s westernmost habitable point, a tiny island just west of Tresco. At night its rocky reefs and peaty downs are swept every half-minute by the beam from the lighthouse on Bishop Rock, the starting line for liners challenging for the Blue Riband of the Atlantic. Bryher also boasts the Hell Bay Hotel; extreme comfort roguishly named after a sailor’s death-trap, a reef-rimmed lee shore notorious for shipwrecks.

The islands were serene, the sea a glassy azure, the air cool and the sun bright when we took the five-minute boat ride to Tresco’s tiny village. No sign of the famous garden except, peeping over a low hill, tell-tale tattered tops of tall windbreak trees. Only long-serving Monterey cypresses have that ragged silhouette. Twenty minutes walk brought us to the garden gate – and another world.

We were in the horticultural equivalent of a fancy dress ball. The climate is sub-tropically benign, insulated by the Gulf Stream. It can very occasionally break down, as it did in 1987, when frost and snow destroyed all the tender plants, and in January 1990, when a hurricane felled most of the 130-year-old shelterbelt and almost all the biggest trees. The epic story of recovery from these disasters is told by the head gardener, Mike Nelhams, in Tresco Abbey Garden, a Personal and Pictorial History. I don’t often give ISBN numbers, but it’s worth it for this eye-opener: 978 185022 200 2.

I found myself floundering with many of the genera, let alone the species, of the plants in his lists. Aeonium with its varnished maroon leaves, Banksia, Beschorneria leaning lasciviously, Bomarea, Carpobrotus, Coleonema, Doryanthes, Dryandra, Fascicularia, Furcraea, Kunzia…. The impact of so many strangers at once is overwhelming. It was almost a relief, after an hour or two of wandering through such a brilliant palette, to emerge to the quiet beauty of gorse and campion and bluebells and wild garlic lining the island’s narrow lanes.

Tresco Abbey Garden, we learned, was begun in 1834 by Augustus Smith from Hertfordshire, the heir to a fortune, who leased the island from the Duchy of Cornwall and became Lord Proprietor – as his great great grandson Robert Dorrien-Smith still is today.

When the Smith family arrived they found the remains of a Benedictine abbey in the most sheltered spot on the island and built their house beside it. It remains a sequestered place, shaded by New Zealand tree ferns, while the dazzling profusion of the Mediterranean, South Africa, Australia, Mexico and Chile spreads up the hillside above in a pattern of paths and steps and pools reminiscent, in places, of its Riviera contemporary, the Hanbury Garden at La Mortola.

The Dorrien-Smith family have persevered for nearly 200 years, living on the island they rule, loyal to a marvel of horticulture which is entirely their invention, and still bursting with innovation, new ornaments and new plants.

The depredations of the boxtree caterpillar, Cydalima perspectalis, have reached such a pitch in Kensington that Rassells Nursery has stopped sellling box plants for hedges. Instead they are offering a miniature holly, Ilex cornuta ‘Luxus Globe’,

Its leaves are smaller and darker than box. Its paler new growth at this time of year is quite pretty, but no one can pretend it will ever be a substitute for the mainstay of garden design for, literally, thousands of years. The Romans relied on it. Pliny the Younger cut it into extravagant figures at his villa by the sea. Populations of box trees are often evidence that Romans colonized a district. Where we lived in the Auvergne the outlines of a Roman town were still just visible, but the valley below was thick with ancient box trees. You didn’t find them anywhere else near there.

We all know its qualities as the trim and malleable friend of gardeners in the European tradition. Not everyone likes the evocative small of a box parterre on a dewy morning in summer, but the thought of a unique tradition chewed to oblivion to feed a nondescript little moth is hard to stomach.

We are defending our little hedges with whatever treatments are allowed. The caterpillars last year arrived in August ; in this strange spring they became obvious in chewed leaves and tiny webs in late March. The tiny caterpillars are hard to find but munch alarmingly fast. Even with daily inspection some survive – and turning your back to go away for a week is seriously ill-advised.

Ash trees, even elms, can be replaced with other trees, however we may miss their familiar silhouettes. Box, alas, has no real substitute. At Wisley there is a bed planted with possible replacements, trmmed as low hedges ; none, I fear, really cuts the mustard. Perhaps the best is Teucrium x lucidrys, or by its medieval-sounding name of hedge germander. The Prince of Wales evidently thinks so ; he has replaced box with it at Highgrove. Its chief drawback is its spikes of pink flowers, but I’m told hard trimming avoids them. Meanwhile, pray for a predator keen on cydalimas.

The gaudy gorse only stops at the Solent saltmarshes

A diarist must beware of mentioning the weather too often. It was admittedly half the point of Gilbert White’s diaries, and there are times and places where it simply grabs the limelight. I was probably pretty boring about the drought in Essex (and most parts) in 1976. Watching the sky became an obsession, and dragging hoses my principal exercise. The aftermath called for comment, too. The deluge killed plants that had survived the drought. Venerable beeches that had managed to make deeper roots to find moisture (their customary ones are near the surface) drowned when the floods came in October.

There was no ignoring the great gale of October 1987, either. But what about this spring? For the second year running winter has failed to show up. The first quarter of the year has been consistently mild. Phenologists (those whose bread and butter is recording dates of budding, leafing and flowering) say that one degree centigrade above average means one week earlier leafing. It sounds too tidy, but the average means one week earlier leafing. It sounds too tidy, but the Kensington wisterias are saying three degrees – or at least are in full bloom three weeks earlier than last year (which was early, too). The result has been what amounts to a summer Easter (Easter being late this year).

The resulting coincidences of flowering are alternatively a) charming b) bizarre c) disturbing or d) the writing on the wall. No scientific evidence is needed to know that, for example, London now has a pavement-café life that would have been unthinkable twenty years ago. Wine-growing in the south of England is no longer an optimistic eccentricity. It is making good money. Nor is it only England; growers in Champagne are nervous about losing the racy acidity in their grapes that gives champagne its edge in every sense.

Meanwhile Beaulieu Common in the New Forest is an extraordinary sight. The gorse has gone mad, painting the whole landscape a brlliant searing yellow. It is all in flower at once (and the smell, of vanilla and coconut and honey, is intoxicating). The vast gorse mounds where cattle and ponies shelter their hind-quarters in winter storms, turning their heads to the wind, are ramparts of a Buddhist colour. I fancy artists call gamboge, relieved here and there with the while of blackthorn. Whatever happened to the traditional blackthorn winter?

A morning at the Garden Museum with its ebullient director Christopher Woodward. He has much to ebull about. The redevelopment that has kept the museum closed for 18 months is nearly finished. On May 9th Friends will see the result and be amazed. It has been transformed, inside and out.

The main body of the museum is the nave of the gothic St Mary’s, Lambeth, but now it has an ingenious mezzanine with room for collections of garden gear; tools and paraphernalia in wonderful variety, including many of the sort of charming horticultural paintings you know must exist but would rarely be able to find. I intend to spend happy hours enjoying them.

The floor of the nave is a generous space for gatherings and lectures. The restaurant that was one of its most popular features, with home-made food that always manages to seem relevant, now has an airy space of its own in the new building that forms a cloister, surrounding the inner garden of 17th century plants designed by Dan Pearson, and its two famous tombs, of John Tradescant and Admiral Byng.

I was amazed by how many rooms and activities have found space in the new development. They include a substantial classroom for students and school-children, a studio, a teaching kitchen, and the Tile Wall which is already shaping up as a memorable feature. One of Christopher’s many snappy fund-raising ideas was to invite gardeners to adopt a ceramic tile made from their favourite garden photograph. 200-odd photos could keep you browsing for a long while. Another of his enterprises was to borrow back from the Ashmolean Museum some of the objects from John Tradescant’s collection that originally formed its core – and to swim from Oxford to London to raise the necessary money. They have their own gallery.

All this, and a surprisingly big outer garden, designed by Christopher Bradley-Hole and maintained by volunteers, seems to have expanded the whole enterprise by a factor of three or four. London’s museums have a serious new recruit.

Christopher’s blog on the museum’s website is well worth reading. www.gardenmuseum.org.uk

Blatant but not blowsy: Magnolia 'Star Wars', Kew, 31 March

Has this been the best year ever for Kew’s magnolias? It doesn’t really matter; last Friday they were sublime, under an azure sky with the proper proportion of fluffy white clouds, and an improper one of big shiny aeroplanes. The air was balmy, the breeze just enough to dislodge a petal here and there to brush past you on its way to lie at your feet. You must hold one to your cheek; there is nothing softer or cooler.

In the woodland garden the breeze jostled the white erythroniums from Oregon. Some pointilliste had filled a meadow with purple and white fritillaries. In the alpine house Tulipa sosnowskyi from the Caucasus was in orange and yellow flame. By the lecture rooms the huge bones of Eucalyptus dalrympleana shone white in the sun. Congested green flowers burst out of maples, embryo catkins from wingnuts, and tiny green points of leaf everywhere. All too soon? When times are out of joint like this is it is difficult not to feel a little surge of panic. Is this thrilling performance the swan song?

But then Kew induces the sense of a Grand Order of Being. Every plant is listed and assigned a place. At Kew God proposes, man disposes.

Fleshing out the bones

Another weekend in the New Forest, inching forward with the design of the new garden. We are building an ample stone-flagged terrace outside the kitchen-conservatory, facing pretty much due south. March days with clear skies have given us a chance to road-test it in perfect conditions, so the first question is, when summer comes and heaven shines in earnest, what do we do for shade?

We have outlined the terrace with brick pillars at the corners, supporting an oak beam that frames the view down the walled garden and will carry swags of climbing roses. Roses, though, are scarcely shade-trees. It can be a windy spot, too, on a hill facing southwest and only three miles from the sea, so parasols have limitations; could end up fields away in fact.

Our new scheme is more nautical; to step a ship’s mast in the centre of the terrace and rig it to the house, the garden wall and the brick pillars to carry an assortment of sails. I first met this idea in a restaurant overlooking the port of Sète on the Mediterranean. The white triangles of jibs (and I suppose staysails) caught the eye invitingly from the quayside below. Rigged precisely to intercept the rays of the sun crossing the sky, they kept at least some of its heat and dazzle off the restaurant. Could we make it work at this slightly higher latitude? It will take a lot of seamanship, but I love the idea of unfurling a jib for lunch and another for tea.

I see the walled garden as a stage set, with the terrace as the stalls, the pergola the proscenium arch, and the wings as, on the left, the perspective of a long border, and on the right a pleached hornbeam hedge half-hiding a lower level of garden, and beyond it the wide valley view. The lower garden is the sports section: ‘short’ tennis and croquet, the upper part the leisure department – apart, that is, from the gardener’s point of view.

Water flows laterally across the garden from the upper wall on the left, reappears as low fountains in a central rectangular basin, and makes a third appearance splashing into another tank at the lower level. You’ll hardly think you’re in the Shalimar Bagh, I’m afraid, but this is Hampshire..

How you cover the walls is absolutely key to a little hemmed-in garden like this one. Intense competition, for root-run as well as for light, is taken for granted; the question is how many place-earning climbers can you persuade up the walls and trellis. The total length of our walls is 2 x 55 + 17 = 127 feet. (The house end is all paved; so no soil to plant. We collect ferns in pots in the permanent shade). I count 21 climbers, or wall plants, so far, that’s six feet per plant, which sounds pretty generous, but they have no inhibitions about invading each other’s space.

One side is entirely dominated by ivy. There’s nothing I can do about it except cut off its shoots. I don’t even know where its trunks are. It shares almost half the west-facing wall and the trellis, to a height of 10 feet, with a climbing hydrangea, whose bright green shoots are just waking up against the dusty green of the ivy.

Two roses and two clematis share this space; the clematis doing better than the roses. C. alba luxurians in particular is never short of energy, and sends its exiguous shoots way up in the ivy to splatter it with its green and white flowers all summer.

I always think of clematis as high-risk, though. Last year two started lustily before they collapsed; a highly-prized C montana ‘Wilsonii’ and a C. tangutica I intended to fill a yellow-and-green Canariensis ivy with inappropriate blossom. The evergreen Clematis armandii, which I planted on the west-side trellis to obscure activities next door, grew strongly from April to June, then stopped, then started again in late September in time for its new growth to be blasted by an early frost. What signal of temperature or day-length can have pressed “Go’ as summer was winding down? This year it shows no sign of growing on from the height it achieved last year: it has started again from almost the bottom.

All this makes me apprehensive when I give a viticella the statutory February chop down to two feet.. Those thin paper-covered stems show no hint anywhere of incipient buds. Then suddenly a fat red shoot appears from under the paper covering and away it goes..

Of all our climbers the most fragile-looking is Eccremocarpus scaber (or ‘Chilean Glory Flower’, though I’ve never heard anyone call it that – and glory is slightly hyperbolic for flowers barely an inch long). A tiny wisp of a seedling (they are evergreen) sat under the wall in deep dank shade all winter, in a slug safari park, and put out its tendrils no thicker than a daddy-longlegs’s legs when it felt the first breath of spring warmth. I was lucky enough to inherit the yellow-flowered version; in fact it’s the colour of clotted cream. It will scramble up the bare trunk of a Viburnum x burkwoodii trained against the wall. It will flower all summer and scatter fertile seedlings; there’s one just coming up in a camellia pot right now.