Primula (primrose and polyanthus) trial results 2021–2023
Objectives and purpose of the trial
Colourful primulas blooming in the trial beds at RHS Bridgewater
Close-up plant portrait of Primula vulgaris ‘Taigetos’ in the trial beds
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:
- Earliness to flower
- Longevity of flowering
- Weather tolerance of blooms
- Foliage colour and presentation
- General plant vigour
Primula AGM winners
Primula vulgaris Drumcliffe ('K74'PBR) (Pr/Prim)
primrose [Drumcliffe]
- 0.1–0.5 metres
- 0.1–0.5 metres
Primula vulgaris 'Lilacina Plena' (Pr/Prim/d)
- 0.1–0.5 metres
- 0.1–0.5 metres
Primula vulgaris (Pr/Prim)
primrose
- Up to 10cm
- 0–0.1 metre
Primula vulgaris Drumcliffe ('K74'PBR) (Pr/Prim)
primrose [Drumcliffe]
- 0.1–0.5 metres
- 0.1–0.5 metres
Primula vulgaris 'Lilacina Plena' (Pr/Prim/d)
- 0.1–0.5 metres
- 0.1–0.5 metres
Primula vulgaris (Pr/Prim)
primrose
- Up to 10cm
- 0–0.1 metre
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Having the straight cultivar Primula vulgaris in the trial was really interesting but the whole team agreed it was a brilliant and consistent plant and therefore worthy of an AGM.
Why the AGM was awarded
New AGM winners with forum comments and hardiness ratings
Reconfirmed AGMs with forum comments and hardiness ratings
Retained AGM with forum comments and hardiness ratings
Close-up plant portrait of delicate Primula ‘Guinevere’
Close up flourishing primulas in the trial beds
AGMs rescinded
AGMs rescinded with forum comments
Plants and location
What was planted?
As Primula is such a large genus (split into 38 sections of botanically similar species), the trial only included primrose and polyanthus forms, all from Primula section, formerly known as the Vernales section. These types of primulas are between 10 and 30cm tall and the same for their spread, apart from Primula vulgaris ‘Taigetos’ which reaches a height of 30cm and a spread of 50cm.
Many Primula cultivars can cross-pollinate readily, or are sterile and will not seed. To obtain true plants, they were all selected from divisions and not seed-raised.
Where was it planted?
Young Primula cultivars were planted out in one of the trial beds of the Old Frameyard at RHS Bridgewater, Greater Manchester – this was the first time this trial bed had been used since the site was built. Here are some considerations for planting out primulas:
- Plant primulas in spring or autumn
- Primroses originally came from hedgerows or woodland and prefer partial shade
- They grow in most soils, if not too wet or dry, but less well in very alkaline soils
- Polyanthus forms are often used as annuals in containers and baskets
- Use multi-purpose peat-free compost with added grit in containers
Garden proven primulas are very much underused today in the garden border and containers. Try using these AGM cultivars over the early, larger flowered selections sold in February and March.
Cultivation
Planting process
Soil analysis was repeated in March of each of the following two years. In 2022, the pH was again 6.6 and sulphate of ammonia was applied at a rate of 20g/m2, a potassium-rich fertiliser at 40g/m2, and a magnesium-rich fertiliser at 20g/m2 on 21 June. In 2023, the pH was slightly lower at 6.5, but the fertiliser recommendations (potassium-rich fertiliser at 50g/m2 and magnesium-rich fertiliser at 20g/m2) were not followed as the trial was due to finish imminently.
Most of the plants arrived in February and March 2021 in 9cm pots, with the Primula BELARINA plants arrving in 1L pots. These were held for at least four weeks so that the roots could be checked thoroughly for the presence of golden root mealybug (Chryseococcus arecae). The mealybug has a wide host range and once present in the garden would be impossible to eradicate.
The trial was planted out into the bed in the Old Frameyard during the first week of May 2021. For each entry, a plant was positioned at each corner of a 40cm square, with a 40cm space between entries. The bed was large enough to accomodate four rows, each of 20 squares.
The last three Primula entries, which arrived in April 2021, were planted at the beginning of June 2021. To suppress weeds and minimise moisture loss, the soil around the plants was covered with strulch, a wheat straw mulch. Irrigation was provided as needed, particularly during the heatwaves.
How plant health was managed
Challenges
Vine weevils (Otiorhynchus sulcatus) were a particular problem for the primulas on trial and nematodes were applied on 12 May, 14 September and 2 October 2022 to keep them at bay. In addition, nematodes were also applied on 15 September to combat leatherjackets, as it was felt that they were eating some roots.
Forum members had suspected the presence of Ramularia at the first assessment meeting in 2021, but investigations by the RHS Plant Health team confirmed that the problem was bacterial leaf spot, which is usually caused by bacterial canker (Pseudomonas syringae) on Primula. Infected leaves were removed and destroyed.
Weather
After planting in early 2021, the weather turned very warm, producing temperatures of above 35°C, and was very dry for several weeks. The autumn was mild and wet before gradually cooling to frosts over winter. In 2022, the trial endured a temperature difference of over 45 degrees. The summer heatwaves produced nearly 40°C and mid-December temperatures dropped to -8.5 °C, following a very mild and wet November, which meant there was no gradual chilling. The weather then turned back to very wet before becoming very cold for ten days in January 2023 and again in February. March produced a brief spell of snow before turning unseasonably mild.
How to grow primroses and polyanthus
Explore all the information you need to know to grow and care for primroses and polyanthus in your garden
Discoveries
2021–2023
For one entry, Primula ‘Francisca’, most forum members thought that there was a lack of good stock available commercially and so further investigations were undertaken and summarised to all members before an email vote was taken as to whether the AGM should be reconfirmed, retained or rescinded.
Who was involved?
Judges of the trial
At the final meeting on 2 May 2023, forum members noted trial issues caused by poor bed performance and decided to consider broader experience with the plants. Awards were reconfirmed for strong performers or retained if the bed was at fault rather than the plants.
The Primula judges included: Simon Crawford (Chair and plant breeder), Sue Beesley (Nurserywoman), John Fielding (Plant breeder, writer and photographer ), David Foreman (Vice Chair of RHS Herbaceous Plant Expert Group), Vicky Fox (Co-owner of Plantagogo), James Hall (Woodland Team Leader, RHS Bridgewater), Joanne Ryan (Living Collections Central Team, National Trust), Janice Shipp (Freelance garden writer) and Sue Wood (Local authority Horticulturist).
Suppliers to the trial
- Binny Plants
- Kerley & Co.
- Ardfearn Nursery
- Westcountry Nurseries
- Walnut Tree Garden Nursery – no longer trading
- Farmyard Nurseries
- Beeches Nursery
- Broadleigh Gardens – no longer trading
- FitzGerald Nurseries
- Abriachan Nurseries
List of plants in the trial
View all the primulas trialled at RHS Bridgewater
Handy tools and information
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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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