Golden oldie: Anna Pavord is entranced by the mesmerising display of flowers at Forde Abbey

Surprises aren't supposed to work more than once, but Forde Abbey on the borders of Dorset and Somerset surprises me every time I turn up there. I know the way pretty well, and could give you directions for finding it quite easily. Yet when I arrive there, I'm never prepared for the improbability of this vast, low complex of buildings growing out of the greenery.

Partly, it's the lanes that disorientate. Forde isn't far from Chard. Or Crewkerne. Or Beaminster. But the lanes leading to it intersect and curve, present endless choices, all the while swallowing you up in billows of cow parsley and campion. Driving through them is more like being in a boat at sea, than in a car on tarmac. So when Forde's stone entrance pillars loom up, announcing the drive down to the house, it's like being offered an escape from a great floundering maze into a different world. Space. Order (but not too much). Long views. And rootedness. Forde is a very rooted place.

For more than a hundred years, it's been in the hands of the Roper family. Long before that though, Cistercian monks settled there. They always bagged the best places – think of Tintern alongside the beautiful Wye river. At Forde, they diverted a stream for more than a mile to create fish ponds and a huge head of water – now known as the Great Pond – to drive a corn mill. Their waterworks still provide the underpinning of the garden's design, with water from the Great Pond dropping via a canal to the Mermaid Pond with its great fountain and, lower down still, the imposing rectangle of the Long Pond. "You're lucky it's raining," said owner Mark Roper as we admired the busy beauty of the cascade spilling out of the Great Pond down to the canal below. "The cascades only run like this on about 20 days each year."

There's a fine line, though, between the cascades running well and becoming so over-exuberant they flood the house below. It happened last year, when water gushed through the chapel three-feet deep. Roper doesn't fuss about that. Nor about the storm that brought down three dozen trees in the park in one night. Nor about the sudden fall of wet snow in March last year that stripped whole branches from camellias more than a hundred years old. "We had a bonfire going for five days here, burning up the rubbish," he said, pointing to a patch that's now covered with blue-flowered poppies – the difficult Himalayan kind.

I suppose it's the rootedness that makes people like Mark Roper sanguine. He's working in a place where people have worked for more than 800 years. Both house and garden have been endlessly overlaid: new owners, new ideas. The series of cascades and lower ponds were laid out in the early 1700s by Sir Francis Gwyn, Secretary to Queen Anne, as he proudly announces himself on a lead drain head on the south front of the house. An unmistakable Victorian patina – monster conifers in the park, vast ranges of glasshouses – was the work of Herbert Evans from the 1860s onwards. But by the time the Ropers arrived in 1906, the place had drifted into obscurity. Everything that had been done in the past needed doing again. The Long Pond was dredged, the rockery rehabilitated, the kitchen garden brought back into production.

And there's another reason I love Forde Abbey. You come into it the right way, through rows and rows of vegetables laid out in the kitchen garden. The back of the house shambles about in a domestic way, not needing to impress. Low, bulging edges of dwarf box barely contain beds of black earth planted up with potatoes (Maris Piper, Pentland Javelin, Winston), carrots, leeks and the mammoth onions that are such a riveting attraction in autumn, laid out to dry on the floor of one of the glasshouses.

Every summer Charlotte Roper, Mark's sister, conjures up a promenade of flowers to line the central path through the kitchen garden. This year she's set out lines of a gorgeous African daisy (Arctotis 'Zulu Prince') with complex cream daisy flowers, finely ringed in the centre with orange and brown. Behind are mounds of the silvery-leaved daisy Argyranthemum 'Jamaica Primrose' with dahlias at the back. I remember the mesmerising display they created in September last year, a triumphal preparation for what is to come.

Half-hardy plants play a big part at Forde, providing great ribbons of colour (heliotrope, white-flowered Paris daisy, blue salvias) around the front of the house and ensuring the wow-factor that Mark Roper thinks is so important in a garden that's open all year to visitors. "We're in the entertainment business," he said robustly. "We've got to provide a good show."

Different parts of the garden peak at different times. The exuberant bedding along the front will pick up next month and go on until autumn. The double herbaceous borders also roar into action from July on. Just now, the meconopsis in the shrubbery on Blacksmith Hill are at their peak, flowering in huge pools among the rhododendrons. They grow well here because the acid soil, though thin in most parts of the garden, is deeper on the hill. It's a rich, jungly place, lovely this month but with plenty of promise for the rest of the summer: show-stopping cardiocrinums taller than I am, martagon lilies, evergreen eucryphias.

If you push on through the intimate paths of the shrubbery, you come out above the Mermaid Pond with the bog garden and the Great Pond ahead. June is the bog garden's most spectacular moment, when vast swathes of candelabra primulas (mostly P. bulleyana, P. beesiana and P. alpicola) wind alongside the various streams, between the hostas and the ferns. The green is a necessary buffer because the primulas are psychedelic – sunset shades of pink and orange. Used in this fantastically generous way, they are unforgettable. The noise of the water soothes. The colours excite. It's the best thing I've seen for ages.

The garden covers 30 acres and I've described only one part, lying along the western boundary. To the east is the rockery, a magnificent lime avenue planted when Mark Roper was born, and a huge arboretum. His father, he reckons, planted at least 350,000 trees at Forde.

And on any visit, you've got to leave time for the nursery, run by Paul Bygrave and Peter Sims in the old walled fruit garden. There's a separate entrance so you can get into it without a garden ticket and going there is one of my top treats. Even if I don't particularly need anything, I never come away empty-handed. Herbaceous perennials are the speciality but the owners only propagate small amounts of each plant so, like the garden, the nursery changes each time you visit. It's beautifully kept, beautifully ordered and because nothing hangs around long, no plant ever looks tired of life in a pot.

The Abbey Nursery is open daily (10am-4.30pm) until the end of October. The garden itself is at Forde Abbey, near Chard, Somerset TA20 4LU and is open daily (10am-4.30pm) all year. Admission £8.50