The most celebrated gardens in Northumberland are without doubt those of Alnwick Castle. As it happens, they’re also among the newest: a slightly hyperactive television series a few years ago brought their creation to public attention and they’ve been a big talking point ever since.

Designed by the Belgian landscapers Jacques and Peter Wirtz, and enabled by the dynamic, lively and therefore not-very-Duchess-like Duchess of Northumberland, they are among the most exciting gardens in the country. I loved the mix of the historic and the very modern, the celebration of plants, and above all how much sheer laugh-out-loud fun it all was.

At the heart of the garden is water. This is the only garden I’ve come across which takes such great delight in reviving the 500-year-old Italian Renaissance sport of Giochi d’Acqua – garden water games. The massive central cascade that dominates the garden is highly dramatic at the best of times – but when it plays, on the hour and on the half-hour, it’s enough to make you gape in wonder.

The garden luxuriates in water: it’s just everywhere. I loved these funky swirly rills…

…and the way this fountain pours down the steps from the European Garden at the top.

Best of all there are no ‘keep off’ signs here: the kids are positively encouraged to get wet (you’re advised to take a spare set of clothing for them in the brochure) and my girls couldn’t believe their luck.

Now in case you’re wondering where all the plants are…

The Ornamental Garden at the top of the cascade is on just as grand a scale as the rest of the garden. It’s the sort of place where (at the risk of sounding like an M&S advert) you don’t just have a pergola: you have a hand-built, 20-foot-high pergola swagged with roses around a bubbling pool. You don’t just have a seating area: you have a staggeringly high dome of elegantly-wrought iron clothed in rambling roses and Clematis montana and get a crick in your neck looking at it. Well… you get the general picture.

The formal structure here is just marvellous, and also breathtaking in its scale. Parterre after parterre clicks into place like pieces of a jigsaw puzzle, defined by crisp box and secluded by pleached crabapples. Like the water cascades, it’s a modern design celebrating ancient traditions.

The middle parterres were filled with a froth of roses – this one’s ‘Just Joey’, and the scent was heavenly.

The surrounding beds, along the mellow old brick walls, were where the plantsperson’s delights were. Now, at the risk of sounding like I’m carping, it wasn’t the most inspiring planting I’ve ever seen: but in their defence, I think I probably caught the borders in their mid-summer slump, and I suspect they would have looked less weary earlier, or indeed later in the year.

Which is not to say there weren’t some choice finds in there.

Don’t you just love this blue? The bluest blue salvia I’ve ever seen, labelled as S. patens ‘Blue Angel’.

Bees were everywhere: here squeezing the last few drops of nectar from Anthemis tinctoria ‘Susan Mitchell’. I liked the way they left the heads on instead of dead-heading (my usual knee-jerk reaction to anthemis at the end of its season): they looked like palest yellow pincushions dancing in the breeze.

And last but not least, I discovered this extraordinary hydrangea. It’s H. sargentiana, with giant leaves the sultry rough green of old velvet and flowers the slate-purple of a thundery afternoon. Big and moody: I loved it.

For more about the garden, and the many new projects the Duchess has planned for the coming years (including a skating lake – worth a return visit for that alone) there’s a website to visit; or you can come back here tomorrow for part two.