Ahead of the rest: Alliums make for useful exclamation marks among herbaceous plants

The enormous heads of Allium schubertii, at least 25cm across, are still standing in the garden, though they flowered in June. This is one of the great advantages of this particular allium: the seedheads last a phenomenally long time. The soft greyish-mauve sphere gradually bleaches to the colour of corn, but is just as eye-catching at this stage as it is earlier on.

The head is made up of perhaps a hundred little flowers, six-pointed stars, narrow-petalled, with a tripartite green button in the centre. You don't see them as individuals though. The mass is what matters, the whole construction like a geodesic dome. In terms of form, it's one of the best of the huge tribe of alliums, though the stems are perhaps not quite tall enough for the size of the head. They are strong and do not break, but tend to be pulled forward by the weight of the globe on top.

In the garden, it likes a hot, dry situation, but even there it will scarcely increase. The bulbs rot away altogether if they are too wet. We've got fast-draining soil in the new garden and here, at least, they reappear each year. On the clay on which I gardened before, they tended to dwindle. I've got them leaning over a grey-leaved spread of Dianthus 'Hidcote' and a new flush of giant chives. They are good companions, too, for lime-green alchemilla, which will disguise the allium's dying leaves, or you could grow them behind a low mound of purple sage, which would provide equally good foliage.

They rarely get above 45cm/18in tall, so you need to plant them fairly far forward in a border and remember that,f

in the wild, they come from rocky slopes in Iran and central Anatolia. They need well-drained soil. They are frost hardy, not fully hardy, but good drainage helps get them through the cold. I like them planted as singles, rather than in groups. The globe, standing alone, is more impressive than when jammed up with too many others.

I've space for three more A. schubertii, and they are top of my list for ordering this autumn. Avon Bulbs sell them at £8.50 for three, more expensive than Allium cristophii, which has similar star-burst heads. But A. cristophii (£2.50 for 3 with Avon), though excellent in its way, and flowering at much the same time, just isn't quite as good.

Most alliums are built on roughly the same lines: a strong, uncluttered stem with a blob on top. The differences have to do with the proportion of one to the other. A. cristophii has slightly too much blob for the height of its stem, but has sufficient presence to overcome the problem. A. caeruleum has 60cm of stem, which leads you to expect something rather splendid at the top. In fact, the flower is only about 3cm across, but the colour is a brilliant, clear-sky blue. If you plant enough of them, the effect is excellent, particularly when they poke through a low sea of something greyish, perhaps a prostrate form of Artemisia stelleriana. Unlike A. schubertii, A. caeruleum definitely needs to be planted in a big drift.

The alliums are an enormous group, with more than 200 sorts in cultivation. Most are hardy and easy to grow. The tallest alliums, such as purple A. giganteum, grow on stems more than a metre high, the smallest will fit into a sink garden. The outline of an allium is strong and well defined which makes them useful exclamation marks among mounds of herbaceous plants such as the magenta Geranium psilostemon or grasses such as Stipa tenuissima. Most are in the pink-mauve-purple part of the paintbox, apart from blue A. caeruleum and yellow A. moly 'Jeannine'. There is also an increasingly large range of white-flowered varieties, such as 'Mount Everest', which has heads the size of grapefruit on glossy green stems about 90cm/36in tall.

June is the month for the monster alliums. You can tell them by their names: 'Gladiator', 'Globemaster', 'Mars', 'Mount Everest', all of them more than a metre tall. In a garden, this height can be useful. At Hidcote, the National Trust garden in Gloucestershire, tall Allium giganteum is used down the back of a border of double peonies. The tall alliums also make good companions for the supreme foliage plant Melianthus major, perhaps with agapanthus to pick up the baton later. Try them with catmint and the white phlox 'Miss Lingard'. They also look lovely rising from a hazy spread of sky-blue nigella, jostling the rich, violet flowers of Geranium magnificum, or interplanted with silybum, a plant like a variegated thistle. Remember, though, that when these tall alliums' flowers are at their best, their foliage will already be dying back and looking scrappy. You need to arrange them so they float, like extra-terrestrials, over a sea of borrowed foliage, which will disguise their own quietly rotting leaves.

Several of the tall alliums, such as A. giganteum and 'Purple Sensation' are excellent planted out in a cutting garden. Here you need alliums with plenty of stem but not too big a head. Mixed, perhaps, with dark red gladioli, or the bronze-purple foliage of Cotinus 'Grace', alliums will last several weeks in a vase. In tubs and pots outside, shorter types are more suitable than the giants. Combine them with the grey foliage of helichrysum to cover the alliums' own leaves. Bear in mind, though, that the flowering stem of an allium rises up alongside the bulb, not from its centre. In too shallow a pot they may tip over at odd angles, especially if they catch the wind. A discreet stake may be necessary.

During the summer, I've been prowling around the flower garden, noting where there are gaps and where more alliums would provide a useful link between seasons. As well as more A. schubertii, I'm ordering more of the allium called 'Firmament' (£9.80 for 3 from Avon). It's tall (60-75cm/24-30in), a useful trait in mixed plantings, and it has wonderfully dark-purple flowers. I noticed this summer – it flowers in June – what a good job it was doing behind a dark-leaved geranium on the bank and think it will be equally good with the blue-flowered Geranium 'Brookside', whose foliage will provide good camouflage for the dying leaves.

Planting depth depends on the size of the bulb. The bigger they are, the deeper they should go, but if you intend to grow alliums among herbaceous plants, it is a distinct advantage to plant deep, at least 12-15cms down. You will then be less likely to spear the bulbs on your fork during an autumn clean-up, when you may well have forgotten where you put them.

Avon Bulbs, Burnt House Farm, Mid Lambrook, South Petherton, Somerset TA13 5HE, 01460 242177, avonbulbs.co.uk