How to choose and grow the best clematis varieties

I f plants are like people, then clematis are comparable with the Hollywood movie stars of the Technicolor era, combining cinematic good looks with high-wattage charm. Used to cloak a wall, pergola, obelisk or arbour, to scramble through the branches of a companion plant, or as container specimens, clematis add great swathes of colour to gardens, while some varieties also offer scent and/or evergreen foliage.

In April, as the soil finally begins to warm up, is one of the best times to plant them. But with several thousand different varieties in cultivation, the problem for many gardeners has been the abundance of choice. Step forward the Royal Horticultural Society’s Award of Garden Merit (AGM), the equivalent of an Oscar for luminaries of the plant kingdom, given only to plants of exceptional quality and sturdy constitution that are vigorous, easily cultivated, disease-resistant and readily obtainable.

The big difference between an Oscar and an RHS AGM, however, is that the latter is not for life. Once-prized cultivars are ruthlessly demoted if they no longer make the grade or are superseded by better varieties, which is what happened this spring when the society unveiled its newly revised list of more than 7,000 AGM plants, its first comprehensive review in 10 years.

Of those clematis which had held the coveted AGM in 2011, 15 were deleted, and six new varieties were added. A total of 79 clematis now hold the award, a testimony to the great variety of different garden-worthy plants. Some of these such as the evergreen early-flowering Clematis cirrhosa ‘Freckles’ C. Wisley Cream’ and Clematis armandii ‘Apple Blossom’ have a new RHS hardiness rating of H4, making them suitable only for warmer gardens.

Tougher by far are the many varieties of C. alpina and C. macropetala. Hardy and floriferous, these will happily tolerate a shady north or east-facing spot as long as the soil is free-draining, while their small, bell-shaped flowers, which appear in April-May, have a discreet beauty a world away from that of the flashier, large-flowering types. Look out for the award-winning ‘Frances Rivis’, ‘Constance’ and ‘White Columbine’ amongst others.

Clematis montana is one of the clematis that Irish gardeners know best, with pink/white flowers that are often seen festooned through the branches of trees or draped along the roofs of garden sheds in late spring/ early summer. Of the various cultivars available, the RHS singles out five – C. ‘Mayleen’, C. ‘Elizabeth’, C. ‘Freda’, C. montana var. grandiflora and C. montana var. rubens ‘Tetrarose’. With the exception of the relatively well-behaved ‘Freda’ (cherry-pink flowers), the rest of these shade-tolerant clematis varieties will romp through any garden, so beware of planting in a confined space . Neither, with a hardiness rating of only H4, are they suitable for cold or exposed gardens.

By May, clematis belonging to what’s known as the early large flowered group will take centre stage. Gone from the RHS list are ‘Henryi’, ‘Josephine’, ‘Miss Bateman’, ‘Mrs Chalmondeley’ and ‘Royalty’ but plenty of other old favourites still make the grade including the candy-striped ‘Nelly Moser’, ‘Niobe’ (red) and ‘The President’ (purple). Compact and colourful, this group make wonderful wall or container-grown specimens.

Summer marks the peak of the clematis season, beginning with a wealth of large, late flowering cultivars (LLs) such as the ever-popular C. ‘Jackmanii’ (purple), ‘Comtesse de Bouchaud’ (pale pink), ‘Ernest Markham’ (crimson-red) and the freshly-AGM-anointed C. ‘Aotoroa’ (purple). These are joined by varieties of Clematis texensis (the pink-flowering ‘Princess Diana’ is the only AGM variety), C. tangutica and orientalis cultivars as well as the herbaceous integrifolia and diversifolia types. And then, in early autumn comes the spreading, sun-loving C. rehderiana, with its elegant clusters of small, creamy-yellow flowers that smell like cowslips.

But if I had to grow only one clematis, I would choose a variety from the hardy viticella group, probably the longest-flowering (July-September), most floriferous, vigorous, versatile and garden-worthy of all the categories. Many feature in the RHS’s updated list (see rhs.org.uk), with classics such as C. ‘Purpurea Plena Elegans’ (double, magenta) , C. ‘Etoile Violette’ (blue-purple), ‘Mme Julia Correvon (red) and ‘C. Venosa Violacea’ (purple-white) included alongside new additions such C. ‘Poldice’ (violet-blue, white centre) and C. ‘Walenburg’ (red-purple) .

Viticellas belong to Group 3 of the clematis genus and along with so many of the other plants in this category, they look wonderful growing through or up a vigorous companion plant to extend or add to its season of interest.

How to plant clematis
Before planting, soak the plant's rootball in a bucket of water. Dig a hole 45-60cm deep and wide (45-60cm away from the base of a wall, 60cm away from the base of a host tree). Fork over the base of the hole, then cover with a layer of well-rotted organic matter (compost, manure) followed by a thin layer of soil (clematis don't like manure touching their roots or stems). Mix homemade garden compost/John Innes compost and a couple of handfuls of slow-release fertiliser with the remaining loose soil, then take the soaked rootball and gently loosen some of the coiled roots before placing the plant in the hole. Excluding herbaceous and evergreen types, the traditonal advice is to plant deeply to avoid clematis wilt, ensuring the top of the rootball is roughly eight centimetres below the finished level of the soil. If it's a dry spot, consider placing an open-ended pipe (7.5cm diameter) in the hole, which will allow you to water the plant effectively until established. Backfill, water, and protect against slugs. Continue to water regularly until well established. If growing against a wall/support, plants will need to be tied to horizontal wires, or a trellis fixed on battens. All clematis should be hard-pruned at some point within the first year of planting. For Group 1, prune immediate after flowering while for Group 2 and Group 3 types, prune in February/March the following spring. Mulch established plants with manure in early spring, making sure that it doesn't rest against the stems. To ensure good flower production, add a handful of sulphate of potash to the surrounding soil in April and water in.

Pruning
For ease of cultivation, clematis are divided into three main groups. Group 1 comprises the earliest flowering types. These plants should only be pruned to tidy them up/restrict their size, immediately after flowering has finished. Group 2 comprises the early-large-flowered types that flower on last year's growth. Lightly prune in February and again after flowering to encourage a second flush. The final category, Group 3, encompasses the late large-flowered hybrids as well as the viticella, texensis and orientalis types that flower on new growth. Each stem should be hard pruned back to 45cm above ground and a pair of strong buds. The only other time to prune clematis is if they are struck with the fungal disease known as clematis wilt, which mainly affects the early-large-flowered types. If this happens, cut growth to the ground and dispose carefully of the pruned material.

DATES FOR YOUR DIARY
April 10th-24th: Powerscourt Tulip Festival. Events include walking tour with head gardener Michael Byrne (April 12th, 11.30am) and garden design with Tim Austen (April 13th, 12.30-3pm), see powerscourt.ie. April 13th-14th : Mount Venus Nursery 'Flowers In the Shade', 11am-6pm, see mountvenus.com.
April 14th : Start of week-long Tulip Fest, based around gardens in Birr, Co Offaly, pre-booking essential, see angelajupe.ie