Top drawer: The teacher who has got botany down to a fine art

Behind a blank door off one of east Oxford's least prepossessing roads, and up a tiny staircase, is Sarah Simblet's garden. The lecturer's little flat is full of interesting things to look at. But clamber out of the hall window on to the roof terrace, and suddenly you are perched high over the city among daylilies, agapanthus and magnolias, forgetting there is a university there at all.

Simblet, 38, is exactly the kind of pre-Raphaelite blonde you would expect to inhabit this garden, but her day job is more earthly: teaching anatomical drawing, the detailed structures of the human body. She has drawn flowers since nursery school. "I drew so many hundreds, over such a long time, that they asked my parents to come in and talk about it." She is still drawing them today, believing that the world is seen more intensely when you draw it. "It doesn't matter if you can draw or not. If you draw something for an hour, you'll know so much more about it by the end than if you just stand there looking."

Using drawing as an observational tool is a practice Simblet is passionate about. She began her career writing a PhD about visual illusions in anatomical art – Vesalius's famous veined man, who stands jauntily in a landscape despite being flayed of all his skin: "Those fictitious, impossible images really fascinated me." Today, she is still fascinated by anatomy, and teaches it to a wide variety of students, some of whom are qualified surgeons. "They want to know how to draw an operation so as to be able to convey it to their patients," Simblet explains.

Then there are the medical students at London's Barts hospital, who draw huge skeletons on life-size pieces of paper; and the art students here at Oxford's Ruskin School of Drawing, who are the only students in the world to draw from real cadavers; and there's even a group of teenage remand prisoners at Feltham Young Offenders Institute.

I begin to wonder where Simblet finds the energy to garden, but she has a quiet, friendly, persistent quality that clearly serves her well. And she herself had good teachers. "My mother is passionate about plants, and we also had a gardener called Cyril Hallam when I was small – painfully thin, really tall, warty nose – who'd bend down and peer at me, and I used to follow him everywhere, asking where this or that came from, and I learnt a lot from him."

Her work today still shows her intense interest in plants, which twine their way across the pages of her books. Her Anatomy for the Artist, published in 2001, has become the standard on the subject, and still sells thousands annually worldwide, and four years ago she began work on a follow-up, this time on the structure of plants. "It isn't a conventional 'how to draw a rose' book. It's about plant morphology, how one plant looks different to another."

It took her 80 hours a week for 21 months to prepare the book: "It takes about 35 hours to draw the fungi for a double spread of pages," Simblet explains, ruefully. Just getting the source material from which to work seems taxing enough: "We would scour Covent Garden flower market at 3am, or take algae deliveries from Cornwall of a bucket of 10 seaweeds." The result is an outstandingly detailed, authoritative and delightful work.

Given she spent that amount of time on drawing plants, I am amazed she would come home after hours in the studio and look after the garden. But this rooftop oasis is a work of real love. For a start, she has had to lug all of its plants, pots and small trees up a flight of stairs and out a tiny window. How do you get such lushness without soil, I wonder. "I know!" she laughs, "it's really hard." Her secret is hidden discreetly behind a planter of thick green leaves, a Hozelock watering system that dribbles water into every pot, morning and evening.

Among the plants are surprising, successful mixtures: acers underplanted with wild strawberries, a bay edged with marjoram, and a bucket of basil at the base of a fig. Anemones are just starting to flower when I visit, and evening primrose are blooming in the centre of Simblet's sneaky rooftop "lawn" – a jumble of meadow wildflowers growing in what looks like a substantial container, but which is in fact two inches of compost on top of a cunning waterproof membrane.

It is her stunning plant combinations, though, that really make the garden. Wine-dark daylilies – a rare one called "Ezekiel", ordered at exorbitant cost but divided for friends ever since, and a yellow one "that came from a car-boot sale"; creamy foxgloves, acanthus and echinops, drooping yellow "Abraham Darby" roses, the lilac spires of perovskia and a creamy passion flower; each plant paying its way in terms of the delicious balances.

'Botany for the Artist' by Sarah Simblet is published by Dorling Kindersley, priced £25