For gardeners, a half-century birthday often marks a brilliant career that is still in its infancy. Practically all of the eminent gardeners portrayed in this handsomely produced book have long been eligible for the free bus pass, with many still sprightly in their eighties and nineties.

Borrowing the title of a famous 16th century tome about horticulture, A Gardener’s Labyrinth partners a current exhibition at the National Portrait Gallery. By its own account it opens ‘brief windows onto fifty or so plant-obsessed lifetimes’.

These include Alan Bloom, whose portrait peers, unkempt and with earrings flashing, through a garland of beech leaves. The interview with the late Graham Stuart Thomas is characteristically revealing of the man. Dr David Hessayon robustly defends his workmanlike approach as author of the ‘expert’ books. There is also the odd wild card, such as Andy Goldsworth who, although not a gardener, works outdoors and is a safe choice for bringing ‘acceptable’ conceptual art into the frame.

As a portrait of early 21st century gardening in Britain it is revealing. There are plenty of lyrical views of gardened meadows and wayward herbaceousness, or close-up drawing out the simple beauty of wildflowers.

For all its considerable charm, this exercise could have been more daring. More than a third of the subjects are regular writers and journalists with already well-aired views. Where was the Duchess of Northumberland, who is controversially creating ‘The Versailles of the North’? Or the young orchid hunter Tom Hart Dyke, held hostage for nine months in Colombia?

For all that, this enormous book is an interesting gathering of some of horticulture’s National Treasures.