The final results have yet to be concluded, but the ongoing RHS trial of hardy lavenders at Wisley highlights their value and versatility as garden plants. Tim Upson has been monitoring progress

Right: Lavendula 'Sawyers'
Photograph: Tim Sandall

The ongoing trial at RHS Garden Wisley has been testing 89 entries of lavenders over the past three years. Principally the trial has focused on the hardier lavenders, but it has also included many non-hardy species that are now becoming more widely available. The trials are still in progress but the judges have already suggested entries for the Award of Garden Merit (AGM), although these will not be confirmed until the end of the trial later this year.

Heavenly scent
Lavandula is a genus of about 32 or more species, and though most of the commonly cultivated species come from southern Europe and the Mediterranean, they are also found in the Canary Islands, North Africa, southwest Asia, the Arabian peninsula and India. The genus belongs to the family Lamiaceae (Labiatae) and, like so many others in this family, plants are aromatic due to the presence of glands containing essential oils.

The hardy lavenders tend to be small, woody shrubs although if you look at any of the non-hardy species, such as L. multifida, you will realise that it is a diverse genus. The flowers themselves are quite small, and are borne in dense spikes which massed together give a spectacular display. In some lavenders the calyx (which protects the flower) is often coloured and provides interest before the individual flowers bloom.

The origins of the name Lavandula probably come from the ancient use of this plant to perfume water for bathing, being derived from the Latin word lavare, meaning to be washed. Indeed, today's myriad of soaps, washing agents, potpourri mixes and many perfumes which are scented with lavender oil show that it is still especially popular and widely used.

Lavenders have long been valued as ornamental plants and an increasing interest in the genus has seen the recent introduction of many new cultivars. They make excellent flowering subjects for the general border and herb garden, and are ideal for low-growing informal hedges. Many lavenders also have attractive foliage, often grey to silver grey, providing year-round interest.

In cultivation the hardy lavenders are not demanding, given a dry and sunny situation. They will grow in most soils that are neither waterlogged nor extremely acidic, but are happiest on poor, free-draining soils, reminiscent of their wild habitats. Once established they do not need additional watering and feeding.

Lavenders must be pruned annually to keep plants attractive and flowering well. The traditional time to prune - when the plants are clipped with shears - is immediately after flowering. The flower stalks are removed, mimicking the commercial harvesting of lavenders. There are many plantings more than 30 years old treated in this way and flowering as well as ever. Alternatively, I have also found that plants respond well to clipping in early spring (February and March). Unfortunately, if you already have an old woody and ungainly plant it is too late, as lavenders do not respond well to restorative pruning.

With so many different lavenders now on sale at nurseries and garden centres it may be difficult to choose a fresh young replacement. Although the RHS trial at Wisley has yet to be completed, an update offers the perfect opportunity to assess the wide range of lavender species and cultivars currently available.

Tim Upson is Superintendent of the Cambridge University Botanic Garden and is researching the genus Lavandula

Results of Lavandula angustifolia

Results of Lavandula x intermedia

Results of Lavandula lanata and hybrids with L. angustifolia, also L. dentata

Results of Lavandula stoechas and L. viridis

Photograph and Key of Lavandula angustifolia

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