Dahlia trial results 2023–2024

Objectives and purpose of the trial

The aim of the trial was to assess and compare newly introduced Dahlia cultivars, both in open ground and in containers at RHS Wisley, Surrey. The trial also examined the plants performance during their second growing season after being overwintered. Cultivars that demonstrated outstanding qualities were granted the RHS Award of Garden Merit (AGM).
Dahlias on display in the Trial Garden in late summer 2024
A spectacular showcase of dahlias in full bloom at RHS Wisley

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

This dazzling display of dahlias featured a wide selection of new varieties, trialled both in open ground and containers, all of which endured a particularly cold and wet winter. Among those grown in the ground, 22 cultivars were granted an AGM. In contrast, none of the dahlias trialled in pots met the criteria required for an award. This trial serves to update the current AGM list, offering an up-to-date guide to the most reliable and high-performing dahlias available for home gardeners.

The range of dahlias on the trial was incredible. There was so much variety in shape and colour. Dahlias owe their current popularity to being so versatile: they’re good for pots, garden borders, cutting, pollinators and have edible flowers.

Josie Lewis, Head Gardener, Sarah Raven

Why the AGM was awarded

New AGM winners with forum comments and hardiness ratings

Dahlia ‘Adam’s Choice’ AGM (H4) 2024

  • Forum comment: Interesting antique red colour blooms with proportionate flower heads which fades beautifully. Early flowering. Good even habit with long dark stems on green healthy foliage. Beautiful bright autumnal colour
  • Average height: 90cm

Dahlia ‘All Directions’ AGM (H4) 2024

  • Forum comment: Striking bright red flower with solid green centre. Colour pops all season long. Bushy, upright habit, with long dark stems. Blooms sit nicely above foliage and blends beautifully with healthy foliage. Flowers all season long
  • Average height: 90cm

Dahlia ‘Ashpire Fancy’ AGM (H4) 2024 

  • Forum comment: Cheerful, fun dahlia that stands out in lovely autumn light. Unique blooms and variation of colours. Flowers and buds held well above stems; late blooms hold beautiful form. Performed well over the season. Very useful Dahlia, good all-rounder
  • Average height: 120cm

Dahlia ‘Bettina Verbeek’ AGM (H4) 2024 – subject to availability

  • Forum comment: Jazzy, strong, convincing colour, perfect in an autumn border. Early flowering, prolific blooms, glossy healthy foliage with flowers held well above foliage. Consistent and proportionate. Stunning cut flower. Very popular with the public
  • Average height: 70cm

Dahlia ‘Copperboy’ AGM (H4) 2024

  • Forum comment: Gorgeous rustic rich red. Prolific and consistent. Great contrast of colour with foliage. Beautiful intentional fade. Known to lean beautifully from pots. Good for floristry. Unique
  • Average height: 100cm

Dahlia ‘Cornwall Island’ AGM (H4) 2024

  • Forum comment: Floriferous blooms, healthy dark foliage. Strong, neon-like vibrant flower colour. Stems delicate but strong – good cutting dahlia. Demonstrated favourable disease resistant qualities. Strong growth habit. Standout performer
  • Average height: 100cm

Dahlia ‘Espacio’ AGM (H4) 2024

  • Forum comment: Sophisticated dahlia. Soft, elegant lilac colour with lime coloured centre. Holds colour and form all season. Prolific flowering dahlia that faces up well above foliage – excellent for floristry. Slim but strong stems that bend and breathe. Holds colour and form
  • Average height: 100cm

Dahlia ‘Eye Candy’ AGM (H4) 2024 – subject to naming

  • Forum comment: Strong, uniform, healthy plants. Pretty pink lilac blooms – versatile colour for floristry. Lovely green eye that adds zing. Multiple buds on stems. Good dahlia for a new grower
  • Average height: 60cm

Dahlia ‘Feline Yvonne’ AGM (H4) 2024

  • Forum comment: Great combination of all elements; stature, form, foliage colour. Striking large flowers; nice contrast from soft coloured flower to slim purple stems. Good cut flower; faces upwards; ideal in bouquets
  • Average height: 100cm

Dahlia ‘Fenna Baaij’ AGM (H4) 2024

  • Forum comment: Beautiful and floriferous anemone dahlia with strong tall stems ideal for cutting. Gorgeous lilac, yellow and pink colourings. Dramatic and interesting centre creates interest. Upright habit, flowers held well above foliage, generous percentage of blooms to foliage
  • Average height: 70cm

Dahlia ‘Great Silence’ AGM (H4) 2024

  • Forum comment: Prolific flowers and buds. Warm salmony pink colour with lemon lift. Long stems  ideal for floristry. Beautifully tall dahlia, consistent blooms with great form. Would display well in herbaceous border. 100% consistent
  • Average height: 40cm

Dahlia ‘Lemon Sherbet’ (Sweet Candy Series) AGM (H4) 2024

  • Forum comment: Joyful. Great border and floristry dahlia. Would shout in a border. Yellow flower centre softens the lemony colour of the flowers. Attractive dark foliage, floriferous flowers held on long stems well above foliage. Excellent collerette
  • Average height: 90cm

Dahlia ‘Liquid Desire’ AGM (H4) 2024

  • Forum comment: Eye catching. Vibrant, interesting flowers with dark strong foliage that sits well above foliage. Shows great colour across all stages of bloom. Great size for ground or pots. Excellent multi-purpose dahlia
  • Average height: 70cm

Dahlia ‘Lou Farman’ AGM (H4) 2024

  • Forum comment: Wow-factor dahlia! Gorgeous individual flowers that fade beautifully. Long, slim stems, great for cutting. Flowers face upwards and are held well above the foliage. Prolific flowers - great value from end of July
  • Average height: 90cm

Dahlia ‘Meteorite’ AGM (H4) 2024

  • Forum comment: Brings drama and commitment. Cheery, bright neon yellow. Abundance of flowers and buds complements the dark stems, buds and foliage. Striking colour combo. Performed brilliantly
  • Average height: 75cm

Dahlia ‘Molly Raven’ AGM (H4) 2024

  • Forum comment: Trendy, gothic, sombre flower colour. Tremendous combination with dark lush foliage. Thick dark stems and foliage, upright dense habit with flowers present at base of stems. Well balanced. Great value for money. Stunning
  • Average height: 120cm

Dahlia ‘Onesta’ AGM (H4) 2024

  • Forum comment: Classic Waterlily dahlia. Beautiful pink bloom with a hint of yellow. Good for new growers; perfect size as a cut flower and very useful in floral art. Fades nicely with a long flowering season
  • Average height: 100cm

Dahlia ‘Rainbow Silence’ AGM (H4) 2024

  • Forum comment: Flower power. Flowers held well above foliage with strong stems. Tall dahlia; perfect for the back of a border. Purple colour in buds complements the pink and orange blooms. Strong performer and weather resistant. Consistently strong performer over the two seasons. Show off dahlia
  • Average height: 150cm

Dahlia ‘Ryecroft Jill’ AGM (H4) 2024 – subject to naming

  • Forum comment: Glorious pink colour with a yellow centre and white petal tips. It glows. Large flowerheads, good proportion of flower to lush green foliage, attractive airy habit
  • Average height: 120cm

Dahlia ‘Sarah Raven’ AGM (H4) 2024

  • Forum comment: Stunning. Open habit allows it to breathe and move beautifully. Ideal for delicate planting. Dark stems compliments foliage well and ideal for cutting! Gorgeous deep red and orange blend, unlike any other dahlia. Long flowering season. Unique – nothing similar on the market
  • Average height: 90cm

Dahlia ‘Skyfall’ AGM (H4) 2024

  • Forum comment: Eye catching flower, beautifully formed with striking dark centre. Floriferous flowers that retain colour with age. Long dark stems that stand well above foliage. Long flowering season. Versatile dahlia for the garden
  • Average height: 90cm

Dahlia ‘Tangerine Gem’ AGM (H4) 2024

  • Forum comment: Star performer. Dark foliage complements the bright zingy orange blooms beautifully. Full and healthy habit, good proportions and colour contrast, long flowering season. Just stunning
  • Average height: 75cm
Judges gather in August 2024 to assess the dahlias
Judging team reunite in September 2024 to evaluate the trial

AGMs rescinded

For this dahlia trial, no existing RHS Award of Garden Merit (AGM) selections lost their AGM.

Plants and location

What was planted?

A total of 76 dahlia entries, including varieties introduced since the last assessment in 2015, were grown in the ground and four entries were grown in pots over the two-year trial period. In 2024, an additional 11 dahlias were introduced into the ground and three in pots as more space became available in the trial beds. The dahlias were supplied by leading dahlia growers and breeders from the UK and the Netherlands. This marked the second phase of the dahlia review, following the initial trial conducted from 2022–2023.

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

Dahlia facts

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.

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.

Discoveries

While carrying out the trial, a lot of time was spent observing the plants, and details were noticed that did not relate to the formal trial assessment. Here are some of the interesting findings from the trial:
Dahlia People's Choice

August–September 2024 | People’s Choice

Visitors to RHS Wisley were given the opportunity to vote for their favourite dahlias on display in the RHS People’s Choice Award poll, which ran between August and September 2024.

See the winners
Dahlias in pots

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.

Dahlia Show display

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.

Watch judge Georgie Newberry visit the Dahlia trial at RHS Wisley

Did you know?

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

Who was involved?

Judges of the trial

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

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

View all the dahlias trialled 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’
RHS Award of Garden Merit logo
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