Dahlia trial results 2023–2024
Objectives and purpose of the trial
What is an RHS Award of Garden Merit?
The AGM is intended to help gardeners choose plants that are likely to perform in their gardens, 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:
- Floriferousness and quality of bloom
- Length of flowering season
- Pest and disease resilience
- Foliage quality
- Stem strength
Dahlia AGM winners
Why the AGM was awarded
AGMs rescinded
Plants and location
What was planted?
Where was it planted?
The dahlias were planted as tubers in both open ground and terracotta pots within the Trials Garden at RHS Wisley. Here are some considerations for planting out dahlias:
- Dahlias need sunny conditions, and well drained soil. Dahlias dislike very dry or very waterlogged conditions
- Buy plants as potted plants in summer, or as dormant tubers or rooted cuttings earlier in the year
- Dahlias require regular watering in hot, dry weather in the ground and in containers. Feeding will also boost flowering
- Tall-growing dahlias might need staking to support them. Shorter plants should grow without the need for support
- Deadheading dahlias keeps them flowering for longer, and the flowers are great as cut flowers
Cultivation
Planting process
First year (2023)
Plants for the trial arrived as a mixture of tubers or 2L pots. The trial was planted out in the Trial Garden at the start of June 2023 by the RHS Curatorial team. Three dahlias per entry were planted in a triangle formation, 75cm apart with 1m between entries, in the open ground. The dahlias in terracotta pots (43cm diameter) were planted with one plant per pot in all purpose peat-free compost. As the dahlia entries grew in size, support was provided by bamboo stakes and twine.
In October 2023, after being hit by any early season frost, the stems of the dahlias were cut down to 4in above ground and strulched (covered in a straw-based mulch) in readiness for overwintering. The container entries were moved under cover for the winter.
Second year (2024)
In the first year, 15 entries were identified as having a virus. These were removed in May 2024 and replaced with new dahlia plants. Over the winter, the Propagation team also took cuttings from tubers as replacement stock was not available from Dutch suppliers. These cuttings were replanted in the garden in May 2024.
During both growing seasons, each trial entry in the ground and containers was fed fortnightly with a balanced feed using drip irrigation.
How plant health was managed
Challenges
In the first year of the trial, the forum noticed that some dahlias were showing a mixture of mottling, ring spot and oak leaf pattern on the foliage and that it was spreading across plants into other entries within the trial.
On inspection, the RHS Plant Health Team confirmed the presence of cucumber mosaic virus, mosaic virus and tomato spotted wilt virus.
Although it is widely recognised that dahlia’s are susceptible to a number of different viruses and can quite happily perform well while having a virus present, the forum agreed those that were showing poor health would be removed in an effort to minimise the spread to other entries in the trial.
In the second half of 2024, due to the warm and wet summer period, some dahlia entries were hit by an infestation of spider mite. To reduce the spread, the infected leaves were removed and destroyed and the remainder of the plant sprayed with a preventative soapy solution before being planted.
Although powdery mildew is a well-known fungal disease that affects dahlias and has been present in previous dahlia trials, fortunately, this dahlia trial was not affected.
Weather
During the winter months of the trial, RHS Wisley experienced some very cool (-5°C for a prolonged week in early December 2023), wet (higher than average rainfall) and overcast spring conditions. This cold and wet winter period affected the overwintering success of the dahlias and as a result, the number of dahlias that returned in spring 2024 was impacted.
Out of 76 dahlias, 35 ended the 2023 season with three healthy plants that returned in spring 2024. Another 12 dahlias returned with two or more healthy plants, and six more came back with at least one plant. Only six dahlias didn’t return after winter – some of these had been affected by a virus the year before.

How to grow dahlias
Explore all the information you need to know to grow and care for dahlias in your garden
Discoveries
2023–2024 | Dahlias in pots
A number of dahlias were demonstrated growing in pots. In the first year (2023), the dahlias were grown in small terracotta pots, using peat free compost and hand watered, but the dahlias did not flower and bush out in the pots as was hoped. In the second year (2024), the dahlias were moved to larger terracotta pots with drip irrigation. However, there was no dramatic improvement in the performance of the dahlias in pots.
2023–2024 | Staking
Two different staking methods were trialled during the course of the two-year trial. In 2023, five canes per entry were used in a square formation with a central cane. In 2024, three canes were used per entry – nine in total. It was found that the second year method offered the best support to individual plants and allowed for easy adjustments as the dahlias grew and wrapped around the canes.
September 2023 | Dahlia Show
At the 2023 RHS Wisley Dahlia Show, Dahlia ‘Feline Yvonne’ won the votes of the public to be crowned Visitor’s Choice.
Who was involved?
Judges of the trial
The 2023–2024 Dahlia judges included: Robin Pearce (Chair, RHS Herbaceous Plant Expert Group), Tom Brown (Head Gardener, West Dean Gardens), Paul Dalby (National Dahlia Society Judge), Emma Crawforth (Horticultural Editor, BBC Gardeners’ World magazine), Anne Barnard (Rose Cottage Plants), Ben Pope (Horticulturalist, The Working Garden, RHS Herbaceous Expert Group), David Kent (National Dahlia Society Judge), Georgie Newbery (Common Farm Flowers), Josie Lewis (Head Gardener, Perch Hill), Sophie Jones (Dahlia specialist).
Suppliers to the trial
List of plants in the trial
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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