Anyone now planning their colour scheme for summer is likely to have a geranium somewhere in a pot, hanging basket or border.

But for me among the most worthwhile of geraniums are the hardy ones, widely known as cranesbill, which provide fantastic ground cover, keeping weeds at bay and providing plenty of colour to boot.

You can find varieties for every setting, from underplanting roses to creating a prairie.

A trial by Gardening Which?, the Consumers' Association magazine, tested some 37 varieties over a two-year period to find out which performed best.

The plants were grown in full sun at four test sites across the UK, from Dorchester to Glasgow, thereby taking in regional temperature variations. The plants weren't given fertiliser, were only watered when becoming established and were cut back after flowering to encourage a second flush.

But many did well, come rain or shine, in north and south, with no mollycoddling.

Among the best bloomers were G. Dilys', the first dark pink flowers appearing on long stems in July and continuing until the end of October. Its spreading nature makes it a great choice for ground cover.

If you want impact at the front of the border go for G. sanguineum Glenluce' and G. sanguineum var. striatum, both of which have a neat compact habit, a profusion of flowers in June and July and are also at home in a rockery.

While the foliage of many hardy geraniums dies down during the cooler months, you can obtain varieties which are semi-evergreen, retaining their leaves in winter to add interest to what is an otherwise dull vista.

G. oxonianum A T Johnson', which produces small pink flowers, retained its foliage in winter in milder areas, but should be cut back after flowering to encourage fresh foliage and a second flush later in the season.

If you favour blue flowers and have a big gap to fill, go for G. Orion, which produces masses of striking lilac-blue flowers with a white eye in June and July and has a fast spreading habit, so will fill a space quickly.

Be careful where you plant it, though, or it may start to take over.

If you love that autumn period when the leaves change colour, G. wlassovianum won't disappoint, as its leaves turn rich shades of orange, red and purple.

New growth also provides interest in the spring, with yellow and red hues in the leaves. It's purple flowers in summer almost seem insignificant to its foliage at other times of the year.

For stunning, vibrant colour, go for G. Johnson's Blue', which looks fantastic planted en masse, producing a sea of violet-blue. They look great planted alongside white-flowering perennials.

Try some effective plant partnering - the acid yellow of Alchemilla mollis is a fantastic foil for G. Johnson's Blue' - or merge white and soft blue geranium varieties together for a cottage garden look.

Favourite white varieties include G. sanguineum Album' and G. phaeum Album', which is a semi-evergreen variety and one of the tallest tested.

Don't be afraid of cutting hardy geraniums back hard after flowering, leaves and all.

In a couple of weeks new leaves will grow and you may even get a second flush of blooms.

l Full results of the trial are featured in the March issue of Gardening Which? For details on how to receive Gardening Which? free for a month, telephone 01992 822 800 or visit which.co.uk