The red borders and twin gazebos at Hidcote Manor

British gardens draw visitors from all over the world and in late July they are entering a second phase. As the roses are over, many private gardens have now had their open days, giving way to the public gardens of the English National Trust. Sixty years ago there was only one garden in the trust’s care, Hidcote Manor, near Chipping Camden in the Cotswolds. In 1949 it attracted a mere 600 visitors and took all of £30 during its first public season.

Nowadays Hidcote is famous worldwide as a masterpiece of design and planting. It is set to attract 150,000 visitors and raise nearly £1m from entrance tickets in this 60th anniversary year.

Hidcote’s garden was the brilliant creation of a quiet American in Britain, the shy Major Lawrence Johnston, who arrived to live on the site in 1907 with his widowed mother, the twice-married Gertrude Winthrop. Mother nature is seldom the only important mother behind a great garden. By 1948 Winthrop was long dead but she continued to deny her son free access to the family capital, having tied £1m and more into trust so that he could enjoy only the income after her death. She had seen and feared her son’s talent for expensive landscape gardening during their 17 years of adult life in Hidcote’s manor house.

By 1948 Johnston, ever unmarried, was in his late 70s and his mind was failing. He was beset by the postwar menace of socialist taxes in Britain and was wanting to transfer his domicile to France, where he had made a second great garden at Serre de la Madone near Menton. The London society hostess Lady Sybil Colefax was encouraging him to solve his fiscal problems by giving Hidcote to the National Trust and then emigrating for most of the year. Tentatively Johnston agreed but even his advisers were unsure if he would apply his wandering mind to the problem.

The National Trust’s unofficial secretary, James Lees-Milne, recalls the tense moment of transfer in his diaries. In a downstairs room at Hidcote the “conspirators”, Sybil Colefax and himself, held their breath as Johnston held the pen poised and a shadow crossed his face. They felt unable to rely on Johnston’s failing memory and so they told him that the crucial document before him was one of lesser importance. He signed and the trust acquired its first garden.

Twenty years earlier Johnston himself had had to have his ageing mother certified as mentally incapable. He had taken her to a rest clinic near Menton, where he began to buy the ground for his second superb garden, his alternative Hidcote in France. In 2002 I was shown an old sepia postcard-picture of his mother in the flower-decorated salon of this clinic, a touching record of their drama.

Having gained the garden, how on earth was the National Trust to cope with it? We all forget how old Hidcote’s landscape already was. Johnston had started to design it 40 years earlier in 1908 and had brought it to a peak in the early 1930s when he was already in his 60s. We think of him beside Harold Nicolson and Vita Sackville-West, creators of Sissinghurst, his garden’s only surviving equal. Amazingly, they never saw his Hidcote until their Sissinghurst was already mature; and they belonged to a younger, post-Edwardian generation.

Gertrude Winthrop, mother of and inspriation to garden creator Major Lawrence Johnston, with staff

By 1948 the maintenance at Hidcote had slipped during wartime as its owner’s powers failed. Newly found photographs even show lines of cabbages being grown on the fabled Theatre Lawn as part of the war effort. Johnston had placed great faith in the skilled plantswoman Nancy Lindsay but she had many talents, including one for taking offence. A Garden Committee was ill suited to dealing with her impassioned views and before long Lees-Milne believed that she had simply burned the crucial diaries and documents of the garden’s history.

By 1955 the task of reviving Hidcote had passed to the National Trust’s new gardens adviser, the great plantsman Graham Thomas. Rare paeonies and half-hardy plants had been lost by the dozen as the trust had neither the staff nor the cash to preserve them. Thomas knew he had to rationalise and to extend the areas of public interest. So he introduced new plants, new colours and a Mediterranean Bank where Johnston had had nothing of the sort. Hidcote survived to become an international icon because Thomas saved its superb structure of avenues, mixed hedges and evergreen rooms. Johnston himself had referred to it as a “wild garden in a formal setting”, the most difficult legacy to maintain in style.

Since 1949 the public has innocently been admiring Thomas’s flower-plantings, not the Major’s own.

The central red borders have become world famous but it was Thomas who added the emphasis on purple and dusky brown foliage. When the important designer Russell Page visited Hidcote in 1934 he recorded borders of red and orange flowers and a matching haze of deep blue. He also hailed the half-hardy plants in the summer house as a “gay museum”. Thomas and the trust had lost them and pulled the house down.

I have just sat in the trust’s small archive room at Hidcote with the present head gardener, Glyn Jones, and discussed the puzzles of Hidcote’s past. We talked of the memories of a former young gardener, Jack Percival, who only surfaced as a source in the 1990s. Invaluably he remembered constructing Johnston’s lost rock garden with truckloads of sawdust and chippings and a stream below them to simulate the snow-melt in Alpine mountains. The site has now been re-excavated and pools and water-basins have been rediscovered exactly as Percival had remembered after 60 years.

At the Royal Botanic Garden Edinburgh lists have also been found of plants from foreign collections which were sent to Hidcote across 20 years. They help to focus our ideas of the amazing range which Hidcote once tested and held. Jones then brought me up in shock by asking if I would like to see the “old diaries and notebooks”. Diaries? Notebooks? They were all supposed to have been burned or lost.

Out of a padded envelope from a locked drawer the head gardener produced three hardly believable survivors: little hard-backed blue note and day books. In 2002 Jones had started to give talks about Hidcote’s lost early history and to appeal for memories from his audience. A few months later an envelope arrived for him, postmarked in Somerset. There was no letter inside, only Johnston’s personal daybooks for 1929 and 1932 and a similar book of his sketches and notes between 1925 and 1928.

Who ever had hung on to them so long and quietly turned them in? My hands shook as I looked at page after page of pencil-observations and sketches by the enigmatic genius of garden design, a man from whom almost nothing else has survived in his own hand. They do not contain revelations about a private life or longings. They are far more interesting. They are classic records of “house and home”.

The red borders and twin gazebos as they were in 1930

On page after page Johnston has noted down the plants in his mind, interspersed with the day’s guests for lunch or for a game of his beloved tennis, the addresses of useful craftsmen and recipes for anything from sleeping draughts to poisons for slugs and rats. The novelist Edith Wharton is noted for lunch and overnight visits and then there are lists of menders of broken china or sources of antique furniture in Paris or trousers in Warwick. Above all there are the plants, densest in springtime, in years when Hidcote was at its height. Ceaselessly the Major’s pencil was listing scores of plants whose names I have never read before. Here are his notes for this very day, 80 years ago: “Wulfenia amherstiana, Kew. Lavatera. Agie to tea. Rose Penelope. Linaria macedonica. Halesia carolina monticola [the Snowdrop Tree, of which his plant later won a Royal Horticultural Society Award]. Godetia Mauve Queen. Omphalodes linifolia,” the latter being a lovely annual which we should scatter more often in the front of flowerbeds. How many of you garden-readers will have noted a fraction as much by teatime today?

The discovery of the notebooks has offered Hidcote a chance and a challenge. Inspired by them, an anonymous American has given the garden £1.6m, to be matched by the trust (£1.2m of the necessary pledge has already been raised). Crucial work on paths, the famous pool and the drains has been carried out and the old summer house has been replicated together with the former house for sheltering rare alpines. They might surprise visitors but they are Johnston’s ideas restored. Now the trust must take its collection of plants up one more notch, as we know the very things which appealed to the Major in the garden’s golden years. The challenge should be thrilling.

It has certainly thrilled me. “But isn’t it all so long ago?”, a non-gardening academic asked me after patiently hearing my excitement out. On the contrary it is still with us, especially on evenings as heavenly as those we had recently. I walked up the greatest central vista in British gardening, the design I regard as the greatest in British art in the past century. It was the work of an American genius, building an un-American garden of his own. It passes between the red borders, where I now want to root out the brown leaves and purple hazels and even restore the Major’s ornamental grasses and the pampas grasses which photos show to have stood there instead.

I climb the fine steps past the twin gazebos with pointed roofs which are not to be traced to any one source, neither Dutch nor English. On either side of the steps I bless Thomas for the velvet-red Clematis Royal Velours. I then walk on between the double blocks of clipped hornbeams edging the upper terrace, whose proportional relation to the avenue convinces me that the inspiration came from France.

I do not believe the fashionable guess nowadays that the Major was influenced by books on contemporary design by Britain’s Thomas Mawson, an altogether commoner mind. Beyond their bare trunks run the beds in which Johnston planted heavenly blues and anchusas. Then, two elegant brick pillars hold portrait-busts as if in Italy, reminding us how Johnston had travelled widely in Europe as Mawson had not.

Between them two fine gates of pierced iron seem from the foot of the avenue to be giving entry into heaven above. Will the garden-angels let me in?

I remember Johnston’s own conversion to Catholicism and how in France I have seen his lifelong prayerbook, marked with the same pencil script as his notebooks and stained on its pages by his earthy gardening hands. Only when you pass through the gates do you see that they give on to earthly paradise instead: grass fields, old evergreen oaks, sheep beyond the hidden ha-ha and, further still, a view across what Henry James once described as the “pastoral genius of England”.

On the evening when I read Johnston’s long lost notebooks the view across this heaven was lit with a golden light. It was as if their faithful author was approving in spirit that his life’s masterpiece could give such joy and still be understood.