THE temporary gardens at the Royal Horticultural Society summer shows can be an inspiration. They are also evidence of waxing and waning plant fashions. Lupins may rule supreme at one year's Chelsea Flower Show; cow parsley the next; and alliums the year after. It is easy to fall for the current star, and make hasty post-show purchases.

This year, at Chelsea, I was left swooning over the stately spires of verbascums, and I suspect that there will be plenty at the Hampton Court Flower Show, too. Their use at two events, six weeks apart, would testify to the long flowering season of many of the showy hybrids, which can only be a positive thing, back in our own more permanent gardens.

The way the garden designers use the strong verticals of the flower spikes as punctuation marks in a froth of more billowy plants would translate, too. But it is worth understanding these plants a little more before we make a purchase.

There are about 300 species of verbascum, commonly known as mulleins, of which only a few are enjoyed in gardens. They are diverse in appearance, but mostly biennial, forming a rosette of leaves in their first year, and sending up flowering spires in the second. Most need a hot, well-drained spot. If the British native, common, or great mullein, Verbascum Thapsus, has gate-crashed your plot, you can be fairly sure that its more refined relatives will be happy, too.

Verbascum phoeniceum is a good place to start. It has flowers that range from white through pink and lavender to maroon and purple, on one-metre tall plants. If, as old-fashioned gardeners are wont to say, it "likes" you, Verbascum phoeniceum will self-sow in your garden, and, if you are careful when weeding, you need never be without it, even though the parent plants are not reliably perennial.

If you grow it with Verbascum olympicum, a species from Turkey, which makes huge rosettes of white-felted leaves, and then displays lemon-yellow flowers on spikes up to two metres high over a long period in the second year, the two will hybridise. The offspring will be intermediate in size, and will have lilac and tan flowers. They will also be sterile.

Most of the garden hybrids wooing us at the shows with their antique pastel or rusty shades were originally produced by crossing V. phoeniceum with yellow species. They are sterile, and, as such, will not self-seed in your garden. What is more, they are themselves short-lived. You will need to take root cuttings to make new plants.

As well as V. phoeniceum, I value Verbascum chaixii in my garden. It has small soft yellow flowers, with a little tuft of purple stamens. V. chaixii "Album" is the white form. Both are more reliably perennial than V. phoeniceum, but share the attribute of gently self-seeding around.

If you find yourself falling for a hybrid verbascum such as "Cotswold Queen", "Helen Johnson", or "Merlin", then remember that, without some work on your part, it may be a short love affair.