'Stumbling on melons, as I pass, Ensnared in flowers, I fall on grass." This image from Andrew Marvell's lovely poem Thoughts In A Garden is all very well, but what if, having fallen, you can't get up? Which, now that I am over 90, would certainly happen to me. So I can't pretend that in advanced old age active gardening is something you can do. On the other hand, the 20 years leading up to one's 90th count as "old" to those who haven't yet reached them, and during those years quite a lot can still be done; added to which, the garden still goes on even after one's active gardening has stopped.

The aunt whose garden is now in my care did, in fact, push it past 90, insisting, "I'm all right so long as I've got my hoe and my bucket." Her hoe was short, just the right height to be used as a prop to her wobbly steps, and her bucket was a little plastic pail she kept in front of her once she was on her knees doing her weeding, so she could lever herself back on to her feet by leaning on it. Normally scrupulously honest, she didn't hesitate to promise family and friends that she would never do any gardening if alone, without for a moment intending to keep this promise; and I know why. If you are someone to whom getting your hands into earth means a great deal, you are not going to give it up until you absolutely have to.

I don't know why it should mean so much, only that when, in my early 60s (I was a late starter), I planted something for the first time in my life and it actually grew, I became hooked, and hooked I remained, although I don't consider myself a proper gardener. This is because I never did any serious propagating, just poured a lot of money into nurseries, either pottering round them or by mail order, after brooding happily over their catalogues. Nor do I share the proper gardener's passion for the Perfect Lawn: all I want is a green space for lying on, or to serve as an important part of the garden's design, aesthetically speaking. If it consists largely of moss, so what. Though I do draw the line at buttercups. Our lawn developed a crop of them last spring, when the mower had to spend almost a month being repaired, and one school of thought was, "Oh, how pretty - let's keep them!" But I discovered enough of the proper gardener in myself to be shocked.

There are two kinds of pleasure in gardening, to my mind. One is making a place that looks beautiful (in your own eyes, anyway), which is not unlike the pleasure of painting or any other creative activity: it wasn't there before, and now it is, and you made it! That is a satisfaction that remains intense however old you are, long after other kinds of delight have ceased to be part of your life. I often sit in the garden just looking at some part of it that turns out to be performing as well as I meant it to, or perhaps even better, and feel while doing so perfectly happy - a condition that is not common in advanced old age.

Against that there can be disasters. This year my precious crambe, a vitally important part of one corner, failed to flower, and even worse a valuable viburnum turned up its toes and died (it has been a year in which many sudden and mysterious plant deaths have been reported). But garden disasters do leave spaces in which something new can be tried, so you recover from them quite easily - and that, too, is rare in old age.

The other pleasure is simply plants. I can no longer remember what it felt like not to be fascinated by them, though I know that for years I just took it for granted that many flowers were beautiful and some smelled delicious, without pausing to consider that they are living creatures: that a seed, often smaller than a grain of sand, contains within it all that elaboration of roots, stems, leaves, flowers, fruit - it is full of the mystery of life, just as we are ourselves, life in a vast variety of exquisite shapes that we can handle and nurture. There is a profound satisfaction in spreading the roots of a plant in earth, ensuring that the earth is of a kind that will suit it, feeding and watering it, watching it respond. When I was still able to do all that, I found it such an absorbing occupation that it quite took one out of oneself, always a most refreshing and beneficial experience. Even now, when it is beyond me to dig or heave compost on to a wheelbarrow or carry a full watering can, I still try to do a tiny bit of planting now and then - perhaps a couple of violas or a little group of crocuses - just to taste again the great pleasure of it.

And, of course, I can still choose plants and decide where they will go. I can't afford help in the garden more than a few hours once a week, which means that it's becoming rather shaggy, but I have been lucky in the help I have found: a sequence of three women, all of them knowledgable and vigorous, who garden not just as a job, but also because they love it. Each in her different way has contributed more to the garden than I have, so friendships have grown here as well as plants, which has added to the pleasure.

Of course, there are people whose temperaments and abilities steer them towards occupations other than gardening, and I am prepared to grant (though reluctantly) that they, too, are human beings. But they seem to me unlucky human beings, because they, too, will grow old, and growing old involves losing many happinesses. Therefore any kind of happiness that is a hardy perennial, capable of withstanding the frosts of winter, is a very good thing to have. And in my experience, anyway, gardening has proved to be one of those immensely valuable things.