Rosa (red roses) trial results 2021–2024
Objectives and purpose of the trial
What is an RHS Award of Garden Merit?
The RHS Award of Garden Merit (AGM) is intended to help gardeners choose plants that are likely to perform well, and is only awarded to plants that are:
- Excellent for ordinary use in appropriate conditions
- Available
- Good constitution
- Essentially stable in form and colour
- Reasonably resistant to pests and diseases
Judging criteria
The following factors were all taken into consideration as part of the judging of the trial:
- General impression (plant, foliage, vigour, novelty)
- Disease resistance (fungal diseases, pest tolerance)
- Fragrance
- Flower (blooms, buds, colour, abundance of flowering, recurrent bloom, post-flowering, novelty)
Red roses AGM winners
Why the AGM was awarded
AGMs rescinded
Plants and Location
What was planted?
Where was it planted?
Bare-root rose plants were carefully planted out in dedicated trial beds in the Trials Garden at RHS Wisley, Surrey. Here are some considerations for planting out roses:
- Roses tolerate a wide range of conditions, but usually prefer an open site in full sun
- Most roses thrive in soil that is moderately fertile, moist, well-drained and humus-rich
- Plant bare root roses in late autum to early spring
- Container-grown roses can be planted all year round
Cultivation
Planting process
In March 2021, three plants of each entry were planted 60cm apart in triangles across four trial beds. Drip lines were installed and the beds were mulched with strulch to act as a weed suppressor. Well-rotted horse manure and soil-improving mulch were used as mulches in the following years. The trial was fed with an organic fertiliser each year.
The plants were watered regularly in the first year, as the plants established. The watering regime was reduced over the following years, and in the final year watering only occurred in very dry spells.
To help stave off any problems, in the first year, a rose tonic was sprayed during the growing season. Signs of interveinal yellowing suggested magnesium deficiency, which is common at RHS Wisley due to the composition of the subsoil. A way to remedy this was to apply Epsom salts and kieserite. A natural spray was also used to treat aphid infestations in the first year when plants were small.
Gaps in the trial were filled over winter 2021/2022, and the younger plants were marked with a cane. Suckers were recorded and removed.
How plant health was managed
Challenges
Rust and rose black spot were the most notable diseases to affect the trial, leading to the partial defoliation of many trial entries in late summer. Some selections of roses were recorded as not ‘self-cleaning’ and hanging on to old diseased leaves. Fallen leaves were collected, and any leaves remaining on the plants over winter were picked off to slow the spread. Powdery mildew was spotted affecting leaves and some flower buds.
Dead stems with splitting bark that had broken at the base were noticed on Rosa RED FINESSE (‘Korvillade’) (F), Rosa THE TIMES ROSE (‘Korpeahn’) (F) and Rosa PRECIOUS RUBY (‘Noa16131’) (F) in 2024. Rose crown gall (Rhizobium radiobacter) was diagnosed by the RHS Plant Health team due to the swelling at the base of the stems. All three entries were removed and burned, the soil around the plants was removed and replaced with topsoil. Caliente mustard was sown in the autumn to bio-fumigate the beds before the next trial.
Grey-blue larvae were found in some of the dead stems that were cut back over winter. Some wilting flower buds were seen to have small holes in the stem below the flowers. Holes were noticed on the outside of the stems and in the centre of stems when pruned. Stem boring sawfly (Ardis brunniventris) was identified as the cause. The sawfly caused some stem dieback and loss of flowers, but the trial was not overly affected.
The trial saw various other insects, including caterpillars on flower buds and leaves, rose sawflies on leaves with characteristic egg-laying scars on stems, aphids on flower buds and leaves, rose chafers, leaf hoppers, and spiders.
Pruning and deadheading
In autumn 2021, wind rock (wind damage that can affect newly planted or establishing plants or those with weak or shallow roots) was seen in some entries, particularly those located at the ends of the rows, and these entries were pruned back by half in October. Canes were used to support the worst affected entries. Planting the graft lower into the soil would have increased the sturdiness of the trial entries, as the subsoil in the garden is sandy.
Each winter of the trial, pruning was carried out from January to February. Dead, damaged and diseased material was pruned out from every entry. Cuts were made above buds without aiming for a particular slant. The style and method of pruning were discussed and reassessed each year, to consider all the judges’ different strategies.
In the first year, roses were pruned to 40cm for shrub roses, 30cm for floribunda roses, and 20cm for hybrid tea roses. In the second and third years, one-third of growth was removed. Shrub roses were shaped with central stems for even flowering, and outer stems on hybrid teas and floribundas were pruned. Vigorous stems were trimmed for a good eye-level display. Low-growing roses were lightly pruned, with unhealthy branches cut back hard to promote recovery.
During 2022 and 2023, it was decided that one plant of each of the Kordes Rosen entries was to be pruned back harder, to 20cm from the base, as suggested by the growers. This pruning varied in results for different roses in the trial, the most vigorous entries ended the season with three similar sized plants, while for the less vigorous rose entries, it was clear which were the hardest pruned at the end of the year as they remained a lot shorter. For Rosa CAYENNE (‘Koroutofko’) (S), hard pruning helped to stabilise the plant.
Deadheading was carried out regularly during the flowering season.
Weather
In the early stages of the trial, April 2021 saw the UK’s coldest April since 1922. Summer 2022 in comparison was dry, with a record high temperature of 38.6°C registered on 19 July. The autumn saw high rainfall and mild temperatures, with no frosts recorded in November.December 2022 was the coldest start to a meteorological winter since 2010, and plants had not been subject to gradual chilling. The lowest temperature of -8.1°C occurred on 15 December. Clear skies overnight led to intense radiation frost, which caused most of the damage across RHS Wisley.
The start of 2023 saw a second cold wave in January that lasted for 10 days. Lots of blackened and dead material was removed. The winter of 2023 was fairly mild and wet. The wet weather continued into the spring of 2024 and this was a good year for rose flowering in June.
![Pink rose](/getmedia/b2aa1d0b-e64b-442d-9900-dc1ace675204/rosa-web-use-940x627.jpg)
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Discoveries
2021–2024
Although the trial focused on red-flowered roses, the pink-flowered Rosa CHARISMA (‘Noa16071’) AGM was recommended for an AGM due to its exceptional performance. It stood out with its fragrant blooms, red buds that fade gracefully with age, profuse flowering in clusters, and consistent, well-shaped plants with striking red-tinted, grey-green foliage.
Who was involved?
Judges of the trial
The red roses judges included: Kerry Austen (Chair, Rochfords International Rose Trials), Ian Kennedy (British Association of Rose Breeders), Emily Lamey (Horticulturist), Louisa Mansfield (Future Gardeners), Michael Marriott (Rose expert and rose garden designer), Daniel Myhill (National Collection holder), Charles Quest-Ritson (Author of RHS Encyclopedia of Roses), Mark Rowe (Horticulturist), Janice Shipp (Garden writer), Marilyn Stevens (Roses UK), Adam Stoter (Royal Parks) and Rod White (RHS Woody Expert Group).
Suppliers to the trial
- C & K Jones
- Cants of Colchester – no longer trading
- Curbishleys Roses
- David Austin Roses
- Dickson Roses
- Joseph Rochford Gardens Ltd
- Kordes Rosen
List of plants in the trial
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Get in touch
For more information on our RHS Plant Trials or to access older trial reports, please get in touch with our team via email at trials@rhs.org.uk.
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