It can be hard to spot a Lancelot Brown landscape simply because it is the landscape. Petworth and its deer park are his, as is the Grecian Valley at Stowe and the Blenheim cascades. Then there are Longleat, Warwick Castle, Chatsworth and Alnwick. In a frantic 30-year period from 1750, Lancelot Brown crisscrossed the country, remodelling more than 150 gentleman's parks, smoothing and raising, diverting and draining as he went. Every time you tramp around a National Trust estate or look at a Turner painting, there is a chance you will be faced with Brown's signature serpentine lakes, clumps of cedars and gently rolling lawns. There wasn't anyone to touch him. If you could afford him, and even if you couldn't  non-payers could be a problem  you booked him.

The "Capability" bit comes not from Brown's landscaping skills, prodigious though they were, but from his habit of telling prospective patrons that their grounds had "great capabilities". Even the most unpromising bit of brownish field or straggly copse could be coaxed into elegance by strategic digging or planting. Water was Brown's speciality; he was particularly good at turning sullen ponds into purling streams, helped by the heroic pockets and patience of his patrons. His work at Blenheim, which involved rerouting the river Glyme to create the 45-hectare lake over which Vanbrugh's bridge still floats as if by magic, cost more than £15,000 and took a decade to complete.

The only wonder is that the English gentry and aristocracy, with their childish greed for novelty and "distinction", should have followed Brown so sheepishly when he told them what to do with their birthrights. Wasn't there just the tiniest risk that everyone's park would end up looking the same? Brown's great knack seems to have been allowing his clients to think the bright ideas were theirs. Those ideas had often been gathered on the Grand Tour, that 18th-century equivalent of the gap year in which red-cheeked country squires briefly worshipped at the altar of high art. Trooping round Florence and Rome, they encountered the mellow pastoral landscapes of Claude Lorrain and wondered whether something similar might not be achieved in their particular corner of the English shires. Finicky fountains, fussy gravel walks and walled gardens suddenly seemed passé. Instead what was wanted was something nearer to nature, but nicer.

Under Brown's expert eye, then, the stately homes of England increasingly sat in a subtle sea of unbroken greens and browns. The parkland flowed into the surrounding agricultural land with no clear distinction between elegance and utility. Cows and sheep, instead of being tidied away behind the scenes, were now encouraged to become part of the view, manoeuvred into position by ha-has or steep ditches designed to steer them clear of the vegetable garden.

All this makes you think Brown must have been a smooth talker, adept at tickling his wealthy patrons into giving him bigger jobs than they really wanted. In fact, he appears to have been the opposite, happy to draw up plans and let other, cheaper men execute them. His childhood on a Northumberland farm served him well when it came to standing up to the gentry without cheeking them. Apprenticed as a gardener, he rose through the ranks by dint of hard work, discretion, honesty and yet more hard work. His was a trajectory far more possible in the mid-18th century than it would be a hundred years later, notwithstanding the Victorians' self-congratulatory talk of "self-made men". As George III's master gardener at Hampton Court, and with a grace and favour house there, Brown could often be found chatting happily to "Farmer George" about everything from plantings to politics, his tongue emancipated by the freemasonry of the soil.

Jane Brown's biggest problem in this biography is that her namesake was not the most interesting of men. While the lords and ladies for whom Lancelot Brown worked got divorced, wasted fortunes and went mad, their landscape architect led a more humdrum life. Nearly always working, he had little time for high jinks, let alone recording them. Virtually no written or spoken word of his remains, although Brown makes excellent use of his Drummonds bank book. She fills the blanks with speculative touches along the lines of "he must have felt", playing straight into the hands of critics who declare biography is nothing more than lumpy fiction with ideas above its station.

In the circumstances, the landscape remains the star of this book, and Brown shows herself an adept reader of all those ice houses, belted trees and shady pools. So it is a shame that her text should be so bereft of illustration: two clumps of eight pages are all she gets, and the images are dull ones. In some cases you can get away with it, using imagination to supplement the text. But this is not one of those occasions. Lancelot Brown may have been able to see pictures of perfection in his head, but the rest of us need a bit of help.

Kathryn Hughes's The Short Life and Long Times of Mrs Beeton is published by HarperPerennial.