Clematis 'Roko-Kolla' last year gained a coveted Royal Horticultural Society (RHS) Award of Garden Merit (AGM) after being judged with 67 other cultivars in a 3 year trial of Late Large Flowering Clematis which took place at the RHS gardens at Wisley, Surrey, south of London, UK. There were 9 clematis in the trial already with an AGM and only 5 new awards were issued so this is quite an achievement. The RHS has established C. 'Roko-Kolla' as a late large flowering clematis worthy of growing in our gardens.
It has a lovely clear white star shaped flattish flower with a tinge of greenish yellow down the centre of the tepals with a lovely yellow central boss. This attractive appearance is reflected in its name as 'Kolla' means 'yellow' in Estonian. There are usually 6 pointed sepals, but may only be 4 or 5, and the flower is 15-20 cm (6-8 inches) across. The plant is not overly tall for a late flowering clematis but certainly big enough for most small gardens as it will grow to cover the area of a fence panel so up to 2 meters (6 feet). In the picture of it growing at Wisley in the RHS Trial Grounds you can see it growing alongside the taller red flowering 'Remembrance' to the left, C. 'Rouge Cardinal' to the right (2 plants) with blue C. 'Semu' behind (2 plants) and to the front are the shorter blue C. 'Rhapsody' on the left and pink C. 'Rosamunde' to the right.
I first saw this clematis in Estonia in 1992 when the International Clematis Society visited Tallinn and a number of gardens in Estonia including the gardens and farm of the breeders Uno and Aili Kivistik. This time was shortly after the Baltic States had regained their independence and the outside world had very little knowledge of the Clematis breeding that had been taking place in Estonia and Russia. The Estonians were so welcoming to us and they wanted to show us all the clematis that they grew in their gardens. We were all bowled over by how many new clematis that we were discovering and C. 'Roko-Kolla' was one of these.
C. 'Roko-Kolla' was bred by the late Uno Kivistik and his wife, Aili, in 1982 and introduced in 1983 so it's been around for some time. One of its parents is C. 'Serebrianyi Rucheek', the other is unknown. C. 'Serebrianyi Rucheek' was bred by Maria Sharonova of the USSR and one of its parents was C. 'Ernest Markham'. So C. 'Roko-Kolla' is of good stock!
In Estonia all clematis get cut down to the ground every year by the weather so C. 'Roko-Kolla' was bred to provide a good display by growing up from the ground every year. As you can see it does flower all the way up and does not tend to have a bare base. Therefore I would suggest that you hard prune the plant over winter every year.
The tepals do mark and discolour a little in bad weather and they can turn brown before dropping off but the picture to the right, taken after some heavy rain, shows that the tepals do drop off reasonably cleanly and that the lovely golden seed heads are left on display.
I grew C. 'Roko-Kolla' in my garden for a number of years with the deep red C. 'Rüütel', another variety bred by Uno and Aili Kivistik, and the combination was attractive as the white C. 'Roko-Kolla' lightened the deep red of C. 'Rüütel. My garden is heavy clay and I grew these clematis in a mainly shaded position where they were in competition from nearby plants so they eventually gave up the struggle. At Wisley the soil is sandier and C. 'Roko-Kolla' was growing in full sun which did seem to suit the plant more.
In 2014 at the Wisley Trial Ground, in the second year of the trial, we met Chris Saunders, the head judge of the trial, who was impressed with C. 'Roko-Kolla' and I got the feeling then that it could receive an AGM. It is good that Uno and Ali Kivistik have at last received some formal recognition of the excellent plants that they have bred and introduced to our gardens.