Well, perhaps there’s room for one or two more next year. Their delicacy and intricacy, the distinctive rime of their leaves and petals, I find entrancing. I admire them and pick over them rather like Fagin sifting through the jewels in his hidden casket. It may have something to do with the fact that they are all growing in handsome 4in terracotta pots and I can arrange them neatly in a cold greenhouse, where they are protected from the weather, but not offered any heat.

I would not describe myself as having gone that far, but I do enjoy 25 pots of individual varieties of auricula, for instance, which have given me enormous pleasure this spring. What are my own enthusiasms? (Neatly side-stepped, you see: enthusiasms rather than obsessions.) Obsessions do imply an uncontrollable urge to accrue yet more and more of a single genus or species and an unease if one particular type is elusive and remains beyond our grasp. I hesitate to name names, but I have met the female of the species with an unrivalled knowledge of alpine plants, irises, ivies, silver-foliage plants and… well, the list goes on. Some of the best specialists I have encountered in my gardening life have been women.

It would be easy to assume that an obsessive nature, where plants are concerned, is the province of the male of the species. Those invaluable members of Plant Heritage who cherish national collections of one particular species will understand exactly what I mean. It is impossible for each and every one of us to grow everything in our gardens, but when all of our individual collections are taken into account, the biodiversity becomes reality.

It seems to me that a balanced view of biodiversity is what makes sense. Although turning our backs on a comprehensive plant collection might lead to neglect of the bigger picture, the reality of the matter is that there are enough of us with interests in particular groups of plants to contribute to the wellbeing of the whole. Yet, without specialists, without those who become obsessed with their charges, we would not accrue nearly as much information about particular groups of plants and neither would we enjoy such diversity within their ranks, for it is the specialists - the obsessives - who notice a particular change in plant form, colour or habit and do their bit to help them survive. When does a passion become an obsession? Are the two things inseparable? I remember, when working as a student at Kew Gardens in the late 1960s, the elfin-like gardens supervisor in charge of the cactus and succulent department wagging his finger at those of us who showed a predilection for one particular genus of plants and warning us that ‘Specialisation leads to extinction!’ But then, I suppose he was speaking from personal experience. I'm not an obsessive, pleads Alan Titchmarsh, but just an enthusiast - and he tells us which plants he's most 'enthusiastic' about over the course of a year. Country Life's Top 100 architects, builders, designers and gardeners.

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