Ihave never felt best pleased when this newspaper describes sacked directors as going off on “gardening leave”. It sounds like heaven to me. But what do we feel about an outgoing prime minister? Can you really imagine Tony Blair, that apostle of organic recycling, applying rooted compost in a cold wind? Somehow I cannot picture Cherie with a pair of secateurs that she really knew how to use. Hang on, you may be thinking, this is not a fair goodbye. Politicians simply do not have the time or training to look after collections of plants.

But you are wrong. There is one British ex-politician who has made a titanic mark on the landscape and is now growing far more types of plant than I can identify. When Blair came into power in 1997, Michael Heseltine went out. A large part of him went back to his grounds and gardens in south Northamptonshire, where he has intensified their scale and scope. How can Tony learn to do anything so worthwhile?

I have just been to Lord Heseltine’s home shrubberies to find an answer. Curiously, I have known his house and grounds for more than 50 years. I grew up nearby and rode regularly through the park and woodlands when they belonged to a former Conservative MP. They were a forgivable muddle. The historic lake had silted up. The 18th-century plantations were thick with unwanted scrub and sycamores. It was excellent news for foxes, which we used to chase from the lower woodlands to the brick-walled kitchen garden, which was so full of nettles and wild artichokes that the hounds could never force their prey out. I went only once behind the house’s superb façade and, even as a child, was puzzled to find the same brown paint as appeared on the background of the signs for railway stations in the 1950s.

Nowadays, Thenford House near Brackley is unrecognisable. The old lakes work and new lakes have been cleverly landscaped in the greenest of views from the front rooms. The old kitchen garden has an amazing design of flat mirror-pools of water, fountains, elephant sculptures and immaculate greenhouses. The grounds have become an exceptional arboretum, which include excellent acers, rare Chinese junipers and almost every family in a grand nursery’s catalogue.

“We now grow 3,500 different types of trees and shrubs,” Heseltine told me as he relaxed in embroidered slippers in his winged armchair. “How many acres of garden do you have?” I asked innocently. “Sixty-four,” he fired back at someone who struggles to keep two. “And how many gardeners?” “Ten,” he answered.

So, dear Tony, if you are going to take your gardening leave seriously you will have to do something about your finances. To prove the point our former deputy prime minister crammed me into his motorised buggy and we sped uphill to his garden’s main propagating house for hundreds of seedling trees. Jamming on the brake, he pulled a young walnut seedling out of his cold frame and asked me if I knew which one it was. “Of course you don’t,” he answered for me. “It has just come in from China and nobody else has grown it in Britain. It’s juglans strigillata.”

At this point, I fear some of you may be thinking about that old Tarzan nickname. So did I, for a moment, as we hurtled downhill in the buggy through acres of good waterside planting, steamy irises and overhanging types of oriental tree. Heseltine has laid out thousands of plants along the banks of streams he has diverted and the river that he has restored to life. Big hostas, purple-blue Japanese irises and hundreds of astilbes lined our route, but the overriding impression was tree after tree of rare greenery, hard to identify at 20mph. We returned safely and I realised I had seen the most extraordinary arboretum, created during a life in and out of politics over the past 30 years.

I would like to boil the principles of all this gardening down to four lessons in order to help Tony realise his green rhetoric in the world. I agree with Heseltine that gardeners ought to start early. He remembers beginning aged 12, as a boy at prep school, when he was given one of those individual garden plots. In those days boys sowed innocent annuals and Heseltine grew a super crop. Here is the first trick you missed, Tony. Gardening should have replaced “personal development” on your national curriculum.

The next Heseltine guideline ought to interest you. He advises us to turn to gardening in times of career stress. In 1962, his future publishing empire was financially wobbling but the young Heseltine would go out on Saturdays into the gardens of his rented cottage in Wiltshire and slash the weeds for relaxation. That is one reason why this business newspaper still prints this column at weekends.

The third principle is to begin with the best advice. After buying Thenford in 1976, Heseltine turned to the king of trees and shrubs, Harold Hillier. He even remembers with pride how he put up Hillier for his richly deserved knighthood. I wish Sir Harold could see the scale and rarity of his pupil’s plantings nowadays. If Tony Blair starts quickly he, too, may find that he is planting on his own initiative after the first 10 years.

The fourth principle is the most ingenious. Apply the logic of private equity, parcel up your arboretum and run it as part of your company. Heseltine’s Haymarket Publishing is Europe’s biggest publisher of horticultural trade magazines. It runs Gardeners’ World Live in association with the BBC and RHS. It manages the Thenford Arboretum as a business showpiece and a trial ground, which has the added advantage of impressing and teaching its horticultural business clients. Hence the 10 gardeners. Tony will have to do some careful thinking here. I can picture him starting an eye-catching initiative called Blair Care with a rotary mower in the van at Connaught Square. If he is going to bring in the wages he will have to do more than issue Asbos against clients’ overgrown shrubs.

What does gardening do to a politician? The crucial themes here are delivery and truthfulness. I stood with Heseltine beside the long herbaceous border he had designed five years ago with the help of his RHS dictionary. It is my favourite part of his garden because it strikes me as the most personal, where the owner himself has worked out his own plan. The challenge was to match it to the long, high brick wall of a historic kitchen garden and his chosen heights, colours and proportions are remarkably good. He is not afraid of strong yellow or penstemons in shades of red and purple. His giant fennel is much better than mine and I admired his own touch of design, what we agreed to call Heseltine Balls. They are clipped balls of yew on standard stems, sitting on the lines of yew hedging that divide up the long bed.

As we walked out I asked casually if he grew any types of magnolias. “Hundreds,” Heseltine replied, and as we walked back I could see he was right. When politicians take up gardening they even start to give out direct answers. If Tony Blair does start to learn my noble art, he may even revert to telling the truth.

More gardening articles at www.ft.com/lanefox