For the last year she has been spending even more time on the pots and beds in the shallow area that divides her basement flat in Camden Town from the main railway line out of Euston, at the point where the InterCity expresses gather speed for their journeys north-west. As well as tending the garden she has been writing about it and painting pictures of the plants. The resultant book, catchily titled Blooming Small, is published this month.
In late spring, new gardening books are as invasive as greenfly, and some of them just as irritating. This one, though, is individual and exceptionally charming, not just because Ms Jackson's watercolours displace the usual photographs, but because her elegant prose is narrative rather than instructional. And she has, in gardening terms, a remarkable story to narrate.
An illustrator and retired television costume designer, she had just finished a book on costume for the Herbert Press when she showed David Herbert an article called 'Smaller than Small' that she had written for the magazine of the Hardy Plant Society.
'I thought there might be a book in it. David said he didn't want a garden book that told people what to do because there are millions of those. It's really - and this sounds terribly conceited - much more like Vita Sackville-
West, who wrote because she loved the garden. And I love my garden in its little way.'
Even given that love pulls people in peculiar directions, there did not seem much scope for a passionate relationship when she moved into the Camden Town basement 29 years ago. Sissinghurst it decidedly was not. The yard was covered in cracked concrete except for an area of earth that had become a dump for builders' rubble. And there were the incessant trains, which she has now managed to block from her hearing: 'I could spend the whole afternoon out here and swear there hadn't been a train at all.' Her visitors are not so lucky; but even the railway has its advantages, for it means there are no tall buildings to block out the sun.
The first constructional work after moving in was the creation of a raised bed, edged in brick, where the rubbish dump had been. The soil was poor but over the years she has enriched it with compost. For the rest of the garden she has collected scores of pots and other containers, notably some dramatic chimneys.
'What is so nice about pots is that you can make the shapes and colours change all the time,' she says. From such unlikely beginnings, she created a mature garden that attracts hundreds of plant-lovers, who crowd in when she opens it to visitors a couple of days each year.
You go in down the steps at the side of the house. The first thing Ms Jackson does is to apologise for the cluster of planted containers around her main door. 'I don't actually count this area,' she explains, although to a visitor it seems more than presentable, with a variety of plants including an endearing clump of unusual white honesty (Lunaria alba) and an Arum italicum with intriguing patterned leaves.
The yard at the side, near the entrance to the main garden, is dominated by a Pyracantha coccinea, bent and twisted with age, giving it the appearance of an oversized bonsai tree. Beneath it is a Japanese pot on a platform made from granite blocks acquired when the road outside was dug up. The garden is full of such found treasures, from central heating bricks to a Victorian saucepan stand.
When you get into the main area you understand why Ms Jackson plays down the modest display by her door. You are confronted by an impressive mass of shrubs, grown hugger-
mugger, each vying with its neighbour for space. Fleetingly you assume they have been put into their pots and beds at random, but it does not take long to detect that a designer's eye has been at work.
The pleasing variation of height is an early clue. With lateral space at a premium, Ms Jackson has made use of the vertical. 'The main thing I've tried to do is with the height. It makes the garden so much more interesting, because you can put things into clumps and shapes. If you've been trained as an artist you do have a start in life with things like that.'
The chimney pots are the best example. Apart from some standard three-footers she has four twice that size standing in a line near the back. They were removed from the Victorian house when they became unsafe. They are grey, on a square base and encircled by a pleasing moulded feature half-way down, making them look like small Roman columns - an impression she enhances by looping Clematis armandii and winter jasmine from the top of one chimney to the next, garlanding the pots of ornamental grasses and other foliage plants that she places at the top.
Other climbers and spreading plants - roses and ivies, artemisia and a tall mahonia - help give the variety of height she seeks, while lower-growing shrubs in pots make use of artificial aids including the saucepan stand, bought for 25p at a junk shop, and an ornate wire staging given as a retirement present. Among the most prized features of the central bed and the islands of ground-level pots are a number of silver-leaved plants: hostas - which she manages to keep free from snails and slugs - hellebores, corydalis, euphorbia and a tall bamboo. Her workroom looks out over the area, and just outside the French windows she keeps scented shrubs such as honeysuckle and witch hazel.
She has also managed, by the deft use of saucers and pots, to create a miniature water garden, and a woodland garden on the poor soil right by the railway fence. She has even devised a shallow glasshouse that abuts on to her bedroom window frame. Tender plants are over-wintered and seeds raised in it.
When she is asked by visitors how so many plants can thrive so tightly packed together, she points out that this is how they generally grow in the wild. But there is a cost in terms of effort, especially in the summer.
'Watering is horrendous,' she admits. 'If I do it properly it takes a good hour-and-a-half. Getting down to water each plant is very bad for your back and sprinkling is no good because this water doesn't get into the pots. Things have to be cut back a lot to stop them getting out of hand - that's one of my secrets.
'People say to me that it doesn't look like a small garden but the corner of a very large one . . . but I just hope it isn't prissy. I would hate it to be at all prissy.'
As the umpteenth train roared by a few yards from our ears, I confessed that 'prissy' was the last word I was thinking of. Gardens say as much about their creators' character as taste, and this one represents a triumph over adversity. The adjective I would choose is rugged.
'Blooming Small' by Sheila Jackson is published this month by the Herbert Press at pounds 16.99