Dahlia trial 2023–2024

Objectives and purpose of the trial

The purpose of this trial is to assess and compare brand new Dahlia cultivars in the ground and in pots. The trial will also evaluate how the plants perform in their second season after being overwintered in the ground. Those that perform best will be awarded the RHS Award of Garden Merit (AGM).
Plant portrait of Dahlia ‘Sarah Raven’
Dahlias on display in the Trial Garden at RHS Wisley

Plants and location

What was planted?

More than 80 Dahlia cultivars are being grown as part of the trial. This trial includes dahlias that are new since the last trial in 2015 and is the second part to our programme of assessing these plants, the first taking place from 2021–2022.

Where is it planted?

The dahlias were planted in the open ground and in terracotta pots as tubers in the Trials Garden at RHS Wisley, Surrey. Here are some considerations for planting out dahlias:

  • Dahlias need sunny conditions, and well drained soil. Dahlias dislike very dry or very waterlogged conditions
  • Dahlias grow from underground tubers, with the top growth dying back in winter and re-sprouting in spring. The tubers usually need to be stored in a frost-free place over winter. In milder parts of the UK they may survive winter outdoors, especially on lighter soils that don’t sit wet over winter
  • You can 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

Dahlia facts

Did you know there are more than 14 different Dahlia shapes – single, cactus, star, pompom and waterlily to name just a few.

Judges and criteria

Judges of the trial

The trial assessment forum comprises 10 invited experts, with backgrounds in plant nurseries, garden management, floral art and design and members of the RHS Herbaceous Plant Committee. The forum will gather at seven regular intervals between July and October of each season.

The trial results will be confirmed by the RHS Herbaceous Plant Committee, and the RHS Trials team hope to share the results in time for buying dahlias in summer 2025.

The 2023–2024 Dahlia forum includes: Robin Pearce (Chair, RHS Herbaceous Plant Committee), 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 Plant Committee), David Kent (National Dahlia Society Judge), Georgie Newbery (Common Farm Flowers), Josie Lewis (Head Gardener, Perch Hill), Sophie Jones (Dahlia specialist).

Judging criteria

The following factors are 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

The forum judging the Dahlia trial at RHS Wisley
Plant portrait of Dahlia ‘Rudie’s Romance’

Cultivation

Planting process

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.

During the growing season, each trial entry was fed fortnightly with a balanced feed using drip irrigation. As the 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 the winter months. The container entries were moved under cover for the winter.

How we’re managing plant health

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.

The most common question I’ve been asked so far is what to do with the tubers over winter. The answer depends on the visitor, what their soil conditions are like and whether they have room to store tubers over winter.

Henry Merrick, RHS Trials Horticulturalist
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Colourful blooms on show in the Trial Garden at RHS Wisley in August 2024
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Drone shot of the Dahlia trial in open ground during 2023
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Dahlia trial beds in full bloom in October 2023 at RHS Wisley
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Colourful blooms on show in the Trial Garden at RHS Wisley in August 2024
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Drone shot of the Dahlia trial in open ground during 2023
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Dahlia trial beds in full bloom in October 2023 at RHS Wisley
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How to grow dahlias

Discoveries

While carrying out the trial, a lot of time is spent observing the plants, and details are noticed that do not relate to the formal trial assessment. Here are some of the interesting findings from the trial:

September 2023

At the 2023 RHS Wisley Dahlia Show, Dahlia ‘Feline Yvonne’ won the votes of the public to be crowned Visitor’s Choice.

Watch judge Georgie Newberry visit the Dahlia trial at RHS Wisley

Suppliers to the trial

List of plants in the trial

View all of the dahlias on trial at RHS Wisley
  • Dahlia ‘Ferg’s Best White’
  • Dahlia ‘Hadrian’s Centurion’
  • Dahlia ‘Hadrian’s Summerwine’
  • Dahlia ‘Hawaiian Sunrise’
  • Dahlia ‘Lou Farman’
  • Dahlia ‘Tangerine Gem’
  • Dahlia ‘Abigail’
  • Dahlia ‘Fenna Baaij’
  • Dahlia ‘Richards Fortune’ 
  • Dahlia ‘Rosie Raven’
  • Dahlia ‘Ashpire Fancy’ 
  • Dahlia ‘Crellow Raven’s Apricot’ 
  • Dahlia ‘Crellow Raven’s Blood Orange’ 
  • Dahlia ‘Lemon Sherbet’ (Sweet Candy Series)
  • Dahlia ‘Liquid Desire’
  • Dahlia ‘Rhubarb and Custard’ (Sweet Candy Series)
  • Dahlia ‘Rudie’s Romance’
  • Dahlia ‘Sarah Raven’
  • Dahlia ‘Skyfall’
  • Dahlia ‘Strawberry Bon Bon’ (Sweet Candy Series)
  • Dahlia ‘Tiger Eye’ 
  • Dahlia ‘Onesta’
  • Dahlia ‘Hollyhill Letitia’ 
  • Dahlia ‘Ryecroft Caroline’s Beauty’ 
  • Dahlia ‘All Directions’
  • Dahlia ‘Aphrodite’
  • Dahlia ‘Bacardi’
  • Dahlia ‘Barbarry Embrace’ 
  • Dahlia ‘Barbarry Lace’
  • Dahlia ‘Belfloor’
  • Dahlia ‘Ben Oude Nijhuis’ 
  • Dahlia ‘Bettina Verbeek’
  • Dahlia ‘Espacio’
  • Dahlia ‘Eye Candy’ 
  • Dahlia ‘Feline Yvonne’ 
  • Dahlia ‘Great Hercules’ 
  • Dahlia ‘Great Silence’
  • Dahlia ‘Jennifer Mary Ellen’ 
  • Dahlia ‘Joel’s Favourite’
  • Dahlia ‘Labyrinth Twotone’ 
  • Dahlia ‘Melissa Anna Marijke’ 
  • Dahlia ‘Milena F’ (synonym Dahlia ‘Milena Fleur’)
  • Dahlia ‘Miss Alana Trota’
  • Dahlia ‘Miss Alisa Trota’
  • Dahlia ‘Miss Anne Francis’ 
  • Dahlia ‘Miss Trucella’
  • Dahlia ‘Molly Raven’
  • Dahlia ‘Night Silence’
  • Dahlia ‘Pacific Time’
  • Dahlia ‘Rainbow Silence’
  • Dahlia ‘Ryecroft Porcelain’
  • Dahlia ‘Sebastian’
  • Dahlia ‘Tracy Diane’
  • Dahlia ‘Westerton Gatehouse’
  • Dahlia ‘Blyton Red Ace’
  • Dahlia ‘Copper boy’
  • Dahlia ‘Downham Peggy’
  • Dahlia ‘Ryecroft Jill’
  • Dahlia ‘Kasasagi’
  • Dahlia ‘Allens Starfire’
  • Dahlia ‘Eastwood Jane’
  • Dahlia ‘Polventon Fireball’
  • Dahlia ‘Eastwood Essex’
  • Dahlia ‘Hollyhill Seeing Red’
  • Dahlia ‘Polventon Phyllis’
  • Dahlia ‘Fluffles’
  • Dahlia ‘Missis Dutch’
  • Dahlia ‘Octopus-Sparkle’
  • Dahlia ‘Pacific Jewel’
  • Dahlia ‘Pacific View’
  • Dahlia ‘Princess Nadine’
  • Dahlia ‘Cornwall Island’
  • Dahlia ‘Clearview Del Sol’
  • Dahlia ‘Barbarry d’Armour’
  • Dahlia ‘Sandberg Hilda’
  • Dahlia ‘Hollyhill Lemon Ice’
  • Dahlia ‘Mountain of Fire’
  • Dahlia ‘Hadrian’s Bubblegum’
  • Dahlia ‘Hadrian’s Glowing Embers’
  • Dahlia ‘Magneta Magic’
  • Dahlia ‘Pride One’
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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 [email protected].

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