Passion portraits: A stunning new exhibition reveals the delicate beauty of Mary Delany's 'paper mosaiks'

There is hope for us all in the knowledge that Mary Delany, whose intricate paper flowers are the highlight of an exhibition opening this month at Sir John Soane's Museum in London, only began making these collages at the age of 73. All her life she had sketched and painted and embroidered in the polite, desultory way common to many women of the 18th century, who had no need to work for a living. But in a letter to her niece, written on 4 October 1772, Delany suddenly announced that she had "invented a new way of imitating flowers". During the next 10 years she produced almost a thousand "paper mosaiks", as she called them, astonishingly detailed plant portraits built up from coloured tissues stuck down on a black background.

Few people copied the technique, and, looking at the exquisite passion flower she made while staying with Lord Bute at Luton in 1777, I'm not surprised. The mad, waving tendrils of the flower are cut from at least a hundred bits of paper, each as fine as thread. How did she ever have the patience? Her eye for colour is not so surprising. She uses a dozen different shades in a single petal, but she was doing that in the embroidery that occupied her in the unhappy years of her first marriage. She designed some extraordinary court dresses for herself, scattered with flowers so real you want to pick them and put them in a vase.

The portrait that Thomas Lawrence painted of Mary Delany towards the end of her life shows a beady old trout with an extravagant line in hats. By then she had outlived two husbands as well as her great friend and patron, Margaret Bentinck, the Duchess of Portland. The Duchess was a terrific collector of plants and it was at her house, Bulstrode in Buckinghamshire, that Delany first took up her scissors and started cutting.

What I find extraordinary is that she didn't first draw the shapes she needed. She'd dallied with silhouettes, as many people did (it was a fashionable preoccupation at the time), but building up her flower pictures was a far more complex operation. According to contemporary reports, she always had the flower she was copying, live, on her work table. Behind the flower she slipped a piece of black card, so the outline of leaf and petal showed up more clearly.

Cleverly, she often used the different surfaces of her papers to suggest various textures in foliage or flower. The handmade paper she used was ridged in various ways with the lines of the racks on which it was dried. The ridges might become leaf veins, or indentations on a stem, or puckers in a petal. She created an almost 3-D effect by building different coloured papers up in layers, fixing them with a simple glue made from flour mixed up with water.

None of this could have happened if Delany had not also been an exceptionally observant person. Her letters, especially those to her sister, are full of delicious detail. She writes about a dress she is getting made for a wedding in 1734, "a brocaded lutestring, white ground with great ramping flowers in shades of purples, reds and greens. I gave 13 shillings a yard; it looks better than it describes, and will make a show." At the Prince of Wales's ball in February 1741, she wore a dress she had designed herself, but found her outfit comprehensively trumped by the Duchess of Queensberry's. "I never saw a piece of work so prettily fancied, and am quite angry with myself for not having the same thought."

In 1734 she writes to her sister that she has "a new madness. I am running wild after shells. This morning I have set my little collection ... in nice order in my cabinet." Only occasionally does she "feel a consciousness that my time might have been better employed". When, after the death of her second husband, she began to spend more time at Bulstrode, it was.

The Duchess of Portland was very much more than a dilettante. She had plenty of money and she spent it on building up an unparalleled collection of plants as well as a museum of curiosities. She was serious about her vocation, studied seaweeds, fungi, grappled with Linnaean nomenclature. Delany described the breakfast room at Bulstrode in 1769, "now the repository of sieves, pans, platters ... spread on tables, windows, chairs, which with books of all kinds (opened in their useful places) make an agreeable confusion; sometimes, notwithstanding 12 chairs and a couch, it is indeed a little difficult to find a seat."

Many of the flowers that Delany copied came from the garden and glasshouses at Bulstrode: a red morning glory, a fabulous sea daffodil (Pancratium maritimum) which must have tested her patience to the limit with its whirling white head, a West Indian frangipani, Plumeria rubra which she painted on 7 August 1776. Most of her pictures are dated, and this helps to fill out the picture of who was growing what in 18th-century England. The history of garden design is easier to trace than the history of garden plants, which is generally less well documented.

Since the Duchess was so firmly stitched in to the world of the plantaholics, Mrs Delany's "mosaiks" soon became famous. A Canadian bloodroot was sent down to her from the Royal Botanic Gardens at Kew together with a West Indian shrub with elegant, sweetly scented funnel flowers, named Portlandia grandiflora after the Duchess herself. Other plants came from the famous west-London nursery belonging to James Lee, who had plant collectors working for him in America and at the Cape. "Went to Lee's at Hammersmith, in search of flowers," wrote Delany on 11 May 1780, "but only met with a crinum, a sort of Pancratium; from thence returned to Kensington, bought cheesecakes, buns, etc, a whole 18 pennyworth." She copied the flower the next day.

By this time, Delany was nearing the end of an intensely productive period which had lasted for four years. In a single month (October 1777) she completed 30 plant portraits. They are fragile pieces of work, though paradoxically not so fragile as the real flowers themselves. After a December visit, Delany's bluestocking friend, Elizabeth Montagu, remarked how nature bloomed in the Delany drawing room "when it languishes in gardens". We have the same happy chance to see nature blooming in an exhibition constructed around Mrs Delany and her circle which opens on 19 February at Sir John Soane's Museum, 13 Lincoln's Inn Fields, London WC2. The show runs until 1st May and the museum is open Tuesday-Saturday (10am-5pm). Admission is free. Read all about it in Mrs Delany & Her Circle, edited by Mark Laird and Alicia Weisberg-Roberts (Yale, £40).