Inside the catmint trial
Under the auspices of the RHS, dedicated plant committees and trials teams grow different varieties of the same plants under controlled conditions, providing gardeners with unrivalled information. Judge Val Bourne reports from the Nepeta Trial
The RHS trials reveal valuable details of plants for all gardeners, says nepeta judge Val Bourne
ONE of the most useful—and least trumpeted—services provided by the RHS is its Plant Trials. Many varieties of the same plants are grown side-by-side in controlled conditions at RHS sites around the country and observed by expert members of the relevant Plant Committees, sometimes over several years, to see how they perform. Varieties are judged on a wide range of criteria and the best will be given the RHS Award of Garden Merit (AGM). Plants range from favourites such as catmint, the subject of the trial we report on today, to the new and less well known and the results are published, free, online, so all gardeners can look up to see which plants will work best for them.
The current Nepeta Trial, being held at RHS Wisley, showcases more than 120 varieties of catmint from a total of 250 species found in Europe, Africa and parts of Asia. It includes established stalwarts, such as ‘Six Hills Giant’, alongside brilliant new introductions, including ‘Summer Magic’ and ‘Hill Grounds’. Some varieties definitely fall into the catmint category, with clouds of tiny soft-blue flowers rising above aromatic grey-green foliage. Others have showy, darker blue tubular flowers and green leaves, which indicate a need for better soil and a tolerance of partial shade. A third variation on the theme, Nepeta nuda, offers branching heads of pale flowers on tall, very upright sultry stems. These are suited to prairie planting, offering structure and winter silhouette, although they’re not very widely grown—yet.
The panel’s co-chair, Libby Russell, landscape designer and plantswoman, admires these easily grown perennials because, she points out, ‘they do what it says on the ticket’. Reliability is key and Mrs Russell uses nepetas widely for their long-flowering season and easy-going personality. Many nepetas begin to perform in May and, after being sheared back to nothing in early July, they will oblige and do it all again.
Fi Reddaway, holder of the Plant Heritage collection since 2013, is a fellow trial judge, with a bee-keeping husband. She acquired a few purely to please their hives and then, as she puts it, ‘got carried away’—she now has 110 species and cultivars. The Wisley trial has allowed her to share her enthusiasm and knowledge with the other judges:
Graham Gough of Marchants Plants, Fergus Garret of Great Dixter and Steve Edney, holder of the Plant Heritage Collection of dark-leaved dahlias, among others. The Nepeta Trial at RHS Wisley, Surrey, continues. To find out which plants are being trialled and to read the results of previous events, visit www.rhs.org.uk
Nepeta nuda A tall Eurasian species named for its hairless foliage, this was admired by Mrs Reddaway for ‘its elegance and architectural form’. ‘Romany Dusk’, a willowy pale-pink variety with grey foliage and black stems, and ‘Purple Cat’, a smokier purple, are also favourites. In evening light, both take on a ghostly persona. During the trial, the widely admired ‘Anne’s Choice’ wooed
the panel with its dusky-pink flowers and open, airy habit. The very reliably hardy
Nepeta nuda, reputedly a plant for dry conditions, did splay in extreme drought and had to be irrigated on the trial field. After being cut back, it rose again and continued to provide a candelabra-shaped, airy vertical until late into the year.
N. racemosa ‘Amelia’ Most garden-worthy nepetas are resolutely blue. White-flowered forms brown badly as they fade and pink ones tend to die out in winter. This is the exception, with a roundel of grey foliage topped by a dainty array of pale-pink flowers for weeks. It can be sheared back after midsummer and will perform again in August and September.
N. ‘Six Hills Giant’ Most gardeners will know this catmint, which was discovered by Clarence Elliott (1881–1969), who used to own the Six Hills Nursery on the Great North Road at Stevenage, Hertfordshire. Elliott, whose real love was alpines, had a knack for spotting an eye-catching, easily propagated plant that had wide appeal and noticed
this taller nepeta close to a clump of Russian catmint in 1934. It’s sterile, so probably a hybrid, and, despite being ubiquitous, two distinct forms appeared in the trial. This suggests the original plant is, at best, confused, or even lost. In my opinion, Beth Chatto is selling the correct ‘Giant’, or at least the one I grew and knew for decades.
RHS trials have to sort out many a plant muddle so, in search of the truth, I went back to the Elliotts to acquire a ‘Six Hills Giant’ from Elliott’s grandson Martin, who still lives in Broadwell in Gloucestershire. It was more upright with bluer flowers, but the jury is still out. Resolving the issue will involve a visit to the RHS herbarium.
I had to abandon growing ‘Six Hills Giant’ because my cold, damp soil made it floppy and unattractive, but it does perform well for most, given a sunny position. Nepeta racemosa provides a more manageable sea of pale-blue froth. ‘Walker’s Low’ is less likely to flop, although it still reaches 2ft in height despite the name. ‘Junior Walker’ is much more compact and both are readily available. All make soft edgers under pink roses.
N. grandiflora This is a more upright catmint with larger flowers. ‘Bramdean’, found by Victoria Wakefield in her Hampshire garden, is a select form with sea-green stems and foliage that darkens in hot summers to a smouldering thundercloud. It is very hardy (even in my dampish garden) and the larger flowers are a deeper blue. A recent selection of N. grandiflora, ‘Summer Magic’, flowers for far longer than ‘Bramdean’, but with smaller lavender blue, although it has more radiating flowering stems.
N. ‘Dropmore’, a hardy Canadian hybrid, is one of several compact nepetas for rock gardens, screes and containers. N.x faassenii ‘Purrsian Blue’ is a neat, mound-forming catmint with periwinkle-blue flowers. ‘Kit Kat’, a truer Oxford blue, stays low to the ground.
N. ‘Weinheim Big Blue’ When it comes to darker, larger flowers, the trial favourite is this recent German introduction, which has outshone its less spectacular sibling ‘Weinheim Summer Blues’. Perhaps the most impressive flowers, held in rosy purple calices, belong to ‘Chettle Blue’. The recent American ‘Blue Dragon’, raised by Terra Nova Nurseries, proved lax at RHS Wisley and has run into other plants, never a good garden trait. ‘Souvenir d’ André Chaudron’, raised in Canada in 1848, disliked the dry conditions and high temperatures. The best ones I’ve ever seen were in deep soil at The Dell Garden in Bressingham, Norfolk. Originals were planted by Alan Bloom, although he acquired it from America under ‘Blue Beauty’ and stuck to the name for commercial reasons.
N. ‘Hill Grounds’ has amazed me both in my garden and during the trial. It’s named after the late Janet Cropley’s garden in Evenley in Northamptonshire, which also delivered the non-seeding Lychnis ‘Hill Grounds’, a plant I rely on. Her nepeta has the best foliage of any, in my opinion, with substantial sage-green crinkle-edged leaves that gently flounce over edges. The larger-than-average, deep-blue flowers appear in April, a full three weeks earlier than most.
Another important function of the RHS trials is to promote new plants and this one is already widely available. N. ‘Hill Grounds’ will hopefully be rewarded with the highest RHS accolade of all—an AGM.
“Anne’s Choice” wooed the panel with its dusky-pink flowers and open, airy habit