Magnificent sedums

Sefum Red Cauli
Sefum Red Cauli
It barely needs saying that the promotion of excellence is the raison d'?tre of the Royal Horticultural Society. The most obvious symbol of this is the RHS Award of Garden Merit, which is granted to plants after a series of painstaking trials designed to identify the most outstanding performers.

Magnificent sedum: S. 'Cloud Walker' has a good chance of winning a coveted RHS Award of Garden Merit in this year's trials.

The award is highly valued by gardeners and by the horticultural trade, and selected varieties usually become widely available. The trials, which are open to the public, also sort out any duplicated names, and samples are taken for future reference. Plants can be submitted free and competition is fierce.

This year saw the conclusion of a three-year trial of taller sedums, which attracted 82 entries from 14 nurseries, including one from Holland and one from the US. The results will be announced next month.

Three plants of each sedum were laid out in rectangular beds on the south-facing slopes of the Portsmouth Field at RHS Wisley during spring 2004. Every two weeks the Floral Committee, an unpaid group of 20 or so eminent horticulturists - gardeners, nurserymen, botanists etc - assessed foliage, habit, length of flowering, character, and attractiveness to bees and butterflies.

These taller sedums were worth trialling because so many new varieties have appeared. Forty years ago, most gardeners grew forms of Sedum spectabile, the ice plant. These rather squat plants have pale-green leaves and produce wide, flat heads of sugar-pink flowers in autumn.

The best hybrid, 'Herbstfreude' (syn. 'Autumn Joy'), was introduced into Britain in 1952 and has held an AGM since 1993. It is so good that it will probably keep its AGM status, together with Sedum spectabile. Others are likely to lose out to new, improved varieties.

'Mr Goodbud', bred in America, is set to receive an AGM for its wide clusters of purple-pink flowers. These unfurl from enormous grey-pink buds (hence the name). Once the flowers open, the head is 20cm (8in) wide and the foliage has unusual jade-green overtones.

In recent years, dark-leafed varieties have taken centre stage. They flower earlier, usually producing rich-pink flowers in July and August, and their foliage darkens dramatically as the temperature rises.

One of the most eye-catching is the tall, red Sedum telephium subsp. maximum 'Atro-purpureum'. This airy plant reaches for the sky with outstretched arms, quite unlike any other sedum on the trial ground. It has stiff stems that divide into slender, upward-facing branches, topped by small, rounded clusters of maroon flowers.
'Atropurpureum' may lose its AGM, because it isn't as pretty as some of the newcomers, but two of its offspring almost certainly will get one, the widely available 'Purple Emperor' and 'Matrona'. 'Purple Emperor', often quoted as the finest dark variety of all, was spotted by Graham Gough of Marchants Plants in East Sussex roughly 12 years ago as a seedling in his parents' garden.

Its purplish-black foliage has a satin sheen not seen in other varieties and each leaf has a neatly crimped edge. The heads of mid-pink flowers fade to chocolate-brown in winter and the whole plant has great poise and balance.

Sedum telephium 'Matrona' is huge. It was discovered in Germany in 1986 and if you want to grow a smoky pink and grey sedum purely for clout and presence, this is the one. Its fleshy leaves, thick stems and pale, rose-pink fl owers are highly distinctive. Another smoky-leafed variety almost certain to win an award is 'Cloud Walker'. 'Black Jack', introduced by Notcutts in spring 2006, is a seedling from 'Matrona' and I love its soot-black foliage.

Two other new sedums are likely to gain an AGM, although they are not widely available yet.  'Xenox', a compact Dutch variety, is handsome at every stage of growth. Its pink-red flowers are enhanced by a hint of midnight-blue in the foliage Sedum heaven and buds.

The German variety 'Karfunkelstein' was the star of the trial. It is daintier, multi-branched and a dark reddish-black. It has pale-pink fl owers that emerge from rose-red buds. Some of the other sedums have shiny, bronze-red foliage, and the diminutive 'Red Cauli', found at Marchants Plants in 2000, is another probable AGM winner. It's a well balanced, shorter sedum with masses of radiating red stems topped with brilliant-red flowers. 'Marchant's Best Red' is another upright, bronze-red sedum with pink flowers, though not an award winner.

All sedums provide a longlasting presence combining handsome foliage, tight clusters of buds, starry flowers and strong winter seedheads. But in our changing climate, their biggest advantage is their ability to cope with dry summers. This year, during the hottest July on record, they formed a billowing patchwork of beetroot-red, light-green and pigeon-grey foliage on the trial field.

Where to buy

Marchants Hardy Plants, East Sussex (01323 811737) Phoenix Perennial Plants, Hampshire (01420 560695)

For more information about RHS trials, see www.rhs.org.uk/trials