Anna Pavord: Grow rosemary and sage and you've got yourself aromatherapy for free - plus the perfect ingredients for a savoury tart

I'm puzzled by rosemary. Not because it's difficult to grow. All it needs is sun and half-decent drainage (grit helps). The puzzle arises from the colour of the flowers. The ones I like best are a deep, intense blue. Over the years, I've taken masses of cuttings from Italian plants that have those kind of flowers.

But when the cuttings have grown and start to flower in the garden, the flowers come out the same greyish-blue as all the other rosemarys we've got. It must have something to do with the soil, I think. They all grow on a south-facing bank in full sun. They are all fine, vigorous plants. But I'm still hankering after that intense blue in the flowers.

Top marks for survival go to a rosemary that, 20 years ago, I bought for a pot on a small, square bit of roof garden in London. The owners were rarely there and nothing ever got watered. For nine months, the flat was entirely empty, and when I finally cleared out the roof garden, the rosemary was the sole survivor. I brought it down to Dorset for some well-deserved R&R.

It's now enormous and cascades down the slope here in a pleasantly relaxed way. Of all the rosemarys (there are nine of them) it's the closest to the kitchen, so gets regularly raided for cooking. But you'd never know. It's easy to nip off a couple of the young, leafy side shoots without leaving an ugly gap.

Why do I need nine? Well of course I don't. But when you've raised a cutting into a thumping little plant, it seems a pity not to use it. And in that quest for the right blue, I've raised a lot of cuttings. If I squint, I could almost pretend that 'Sissinghurst Blue' has deeper flowers than the rest. But it's the least successful plant in terms of form. It dies back more readily than the other rosemarys. It doesn't seem to shrug off cold wind, as the others do. And the foliage is too fine to be effective, in the way that other rosemarys are.

Most of them are planted with colonies of species tulips, fritillaries and crocus in between. The whole of this front part of the bank is covered in gravel, which suits the bulbs as well as the rosemarys. Because the flower garden behind is rather hectic, it seems to me an advantage to have a calm foreground, with one key plant repeated at intervals. And the more rosemarys you have, the more chance there is of brushing up against them in the garden and releasing the smell, which I honestly believe does you good. It is a calming smell – aromatherapy for free. And a few twigs on a barbecue gets you to the Mediterranean without the air miles.

Rosemary needs very little attention. Cut off straggly branches after flowering, if you want to keep your bush within bounds. Ours are left alone; I just cut out any branches that have died during winter. If you want to propagate them, take cuttings of side shoots – roughly 10cm long – in July or August. Strip off the lower leaves and sink the cuttings round the edge of a pot of compost. The following spring, separate out the rooted cuttings and pot them on, each in its own container, before eventually planting them out. If you have the kind of heavy, damp soil in which lavender rots during winter, use rosemary instead. It is much more forgiving. 'Severn Sea' will tumble horizontally down steps and slopes. 'Miss Jessopp's Upright' will grow in a more vertical fashion. All are excellent in pots.

The Romans brought us rosemary, as well as chervil, chives, fennel, parsley, rue, thyme. And sage. The useful thing about rosemary and sage is that both can be built into a planting scheme for a flower garden. As well as aromatic, they are handsome, permanent and evergreen. You could grow rosemary with the tall spires of asphodel and a scattering of Californian poppies. Or try a clipped upright bush of rosemary with clumps of grey-leaved dianthus, a small iris such as Iris graminea and the scented-leaf pelargonium 'Candy Dancer'.

The ordinary kind of sage has grey, felted leaves, but there is a purple-leaved selection (Salvia officinalis 'Purpurascens') which is a great ally in planting schemes of dark purple and bronze. Try it alongside tulips, with alliums to follow, and a fringe of giant chives. Both the grey and the purple-leaved kinds look best if you clip them over in spring to encourage a fresh flush of leaf. You lose the flowers, but that doesn't much matter. The foliage is more important. The variety called 'Tricolor' has leaves that are rather too hectic, grey-green variegated with white, pink and purple. This is generally less robust than the others, with a height and spread around 40cm. Ordinary sage and 'Purpurascens' get to 60cm.

Sage is a key ingredient in one of my favourite savoury tarts. Start with a pre-cooked pastry case, about 30cm across (or two smaller ones). Melt some butter and in it fry an onion, about 500g celery and at least 15 sage leaves, all the ingredients finely chopped, until the onion and celery are properly limp. Add some grated zest and the juice of half a lemon. Spread the mixture in the tart case and crumble about 250g of Stilton over the top. Beat up three eggs with a big (284ml) carton of double cream and pour it over the top. Bake it for about half an hour at 180C/gas mark 4. The tarts freeze beautifully.

This recipe came originally from Sarah Raven's Food for Friends and Family cookbook (Bloomsbury, £30) and has become a staple, because both Stilton and celery are ingredients that you have hanging around in the fridge long after their first, fresh showing on the table. The precise amounts don't matter much, but without the sage, the tart would be bland and much less interesting.

In the garden, sage is easy to grow, better in sun than shade, though we have a vigorous bush growing in quite a damp, half-shaded spot. Like rosemary, it gets woody with age, but not so elegantly. There'll come a point when it fails to respond to your cutting it back. Yank it out and plant a new one.