The Queen’s Green Planet director describes what it was like working with two "kindred spirits"

• “She is very unsolemn” – David Attenborough reflects on filming with the Queen
• How the Queen is putting politicians to shame with her Commonwealth Canopy project

The meeting was the culmination of a year’s filming that also involved her grandchildren, Prince William and Prince Harry. I’d observed the Queen on several occasions across the year handing out Queen’s Commonwealth Canopy certificates to Commonwealth heads, and had admired her unflustered and inquisitive manner, often broken with a broad smile. But I’d never seen her so relaxed and delighted at being outdoors in a summer garden with someone her own age, similarly revered and arguably just as famous. Her only irritation was saved for overhead aircraft noise. “Why do they go round and round when you want to talk?” she pondered aloud. “Sounds like President Trump or President Obama.”

Her plan to plant and protect woodland around the Commonwealth reflects both a very private passion for trees and a very publicly observed one. “It’s something we do as a family,” quipped Prince Harry when we filmed with him in the Caribbean, where he was acting as a tree-planting ambassador for his grandmother. “I must be up to my half-century of planting trees, but the Queen is up in the thousands.” The actual figure is 1,500, and all over the world. It’s a sort of royal “I was here”.

But it’s not only ceremonial trees that the Queen cares about. Now that she no longer undertakes foreign travel she has more time to spend in her gardens where, like her great-great-grandmother Victoria, she has kept up a family tradition of planting trees – right back to the early 1600s when James I introduced mulberries to undercut the price of silk.

With the cameraman and sound recordist positioned outside, Sir David and I waited for our personal royal tour in the cool of an inner palace hall with the Queen’s Assistant Private Secretary Samantha Cohen. We were prepared with a garden plan, a proposed route, a few scripted words describing the family and some historical references that the Queen would explain. I had been told that the Queen would kindly listen to my direction, but we didn’t really know what to expect. A buzzer sounded and no one moved. Exactly a minute later the Queen arrived, emerging quietly from a small lift.

Beautifully attired in a pale-green floral silk dress and wearing pearls, the Queen was obviously pleased to see Sir David – and happy for me to hold her script and glasses should she need them (she never did). We headed off to look at two enormous London plane trees that were planted by Queen Victoria and Albert, Prince Consort. As the Queen can trace her ancestry through the 1,400 odd trees in the Buckingham Palace gardens, I was anticipating a history lesson.

“Marvellous trees to climb…” murmured Sir David. The Queen stared forwards, looking slightly quizzical. Then her face broke into a big smile. “I’ve never thought of climbing a plane tree… Fir trees you can! It’s amazing watching the people who clear the branches, they spread up on ropes and go right up to the top and…”, she paused, “it makes one feel quite ill.”

And then they were off – two 91-year-olds who were born within a month of each other displaying a complementary sense of energy, adventure and fun, and on occasion it was hard to keep up.

Our first stop was scheduled to be one of the mulberries, but a younger tree had caught the Queen’s eye, a rather forlorn squashed sapling. “This one we won’t look at because it doesn’t seem to be doing very well… Somebody sat on it, I think, at the garden party.” Sir David doubled up in laughter before rounding a corner and spotting a potential problem: “Aha! A sundial neatly planted in the shade!”

The Queen chuckled: “Had we thought that it was planted in the shade? It wasn’t in the shade originally?” She looked incredulously at her off-camera Head Gardener before brightly asking, “Maybe we can move it?” Sir David responded: “Depends if you want to tell the time or not!” I believe the sundial is now being moved.

The Queen pointed towards the oaks planted for her four children, Prince Charles, Princess Anne, Prince Andrew and Prince Edward. Seemingly with all the time in the world, Sir David gallantly offered to read out the plaques for her.

While he boyishly bent his knees to take a closer look, the Queen pored over a large chestnut leaf as if inspecting it with fondness. There was a momentary difficulty in reading the plaques – was it Andrew or Edward? “I don’t think he believes me,” remarked the Queen before they strolled off to talk about foxgloves and the jars of honey produced from nearby palace hives. The Queen admitted that the bees fly over to Hyde Park. “I’d still put Buckingham Palace on the label,” whispered Sir David. “We do,” the Queen replied, smiling knowingly.

A blackbird began singing and the pair stopped to relish the sound, then the Queen spotted a “face” on a ball of bark on a plane tree planted by her great aunt, Queen Mary. Sir David remarked on the character of trees and how the horizontal branches “make you want to sit on them and swing your feet as if you’re a monkey”. The Queen laughed robustly.

Turning back to the job in hand, the Queen led us through a jewel box of a rose garden. “My birthday last year was very useful, you know. I’ve been quite difficult to give presents to, so people said: ‘Let’s give her a plant or a tree, which is very nice to have!’ Obviously, roses were top of the list. Which colour do you like?’

“Blood red,” replied Sir David, affectionately.

And on to conkers. “The dogs hate them, they are prickly.”

“Oh, but they’re so handsome inside,” remarked Sir David. The Queen paused: “Wasn’t it recently that somebody tried to stop children playing with conkers?”

Sir David looked annoyed: “On the grounds of health and safety? Really. You’d think they’ll stop people breathing.”

For 90 minutes we strolled, camera in hand, and the informality and delight shared between them was a joy to behold. Of course, I was working with two consummate professionals. One couldn’t hope to meet two people more used to walking in front of a camera than the Queen and Sir David, but they seemed to be kindred spirits – even more so now that the Queen has signed up to this campaign that will save an area of trees of global significance for future generations. Sir David was suitably impressed.

“A global network of forests, a sanctuary for the whole range of indigenous fauna. It might change the climate.” He nodded admiringly. “There may be all sorts of new trees growing here in 50 years?”

“I won’t be here then,” she said, with a smile.

“Well, it’ll be a wonderful legacy.” His voice soared with emotion as it does in commentary, but there was no doubt that he truly meant it.