Onion, sets in spring trial results 2025

Objectives and purpose of the trial

The trial focused on assessing a range of onion (Allium cepa) cultivars grown in spring from sets. At RHS Wisley, Surrey, the RHS Trials team compared, recorded and displayed the qualities of both new and established cultivars for garden use, with the strongest performers considered for the RHS Recommended: Award of Garden Merit. Eight onion sets proved their continued value for gardeners by retaining their existing award, including ‘Centurion’, ‘Jetset’, ‘Hercules’, ‘Setton’, ‘Sturon’, ‘Stuttgarter’, ‘Red Baron’ and ‘Rumba’. Building on earlier spring‑set onion trials held in 2002 and 2013, this work aimed to evaluate performance under current conditions, confirm consistent performers and demonstrate effective, reliable methods for growing the crop.
Close up of Allium cepa ‘Rumba’ in the trial beds at RHS Wisley
Harvested Allium cepa ‘Red Baron’ with its rich colour

What is an RHS Recommended: Award of Garden Merit?

The RHS Recommended: Award of Garden Merit helps gardeners choose plants that are reliable and perform well in real gardens. Plants that receive the RHS Recommended: AGM have been thoroughly tested and shown to be:

  • Easy to grow and reliable in normal garden conditions
  • Readily available to buy in garden centres or online
  • Healthy, robust plants with strong growth and appearance
  • True to type, maintaining consistent shape, colour and form
  • Reasonably resistant to common pests and diseases

Judging criteria

The following factors were all taken into consideration as part of the judging of the trial:

  • Yield
  • Size and shape
  • Colour

  • Skin quality
  • Lack of bolting
  • Storage until March

Onion set – RHS Recommended: AGM winners

Although small in scale, this spring onion set trial brought together a well‑balanced mix of cultivars, providing a clear opportunity to assess their performance under trial conditions. Despite challenging establishment and periods of unusually hot weather, the judges recorded sufficient consistency and reliability to confidently reconfirm eight existing RHS Recommended: Awards of Garden Merit, with no additions and none withdrawn. While the list of award holders remains unchanged, the trial generated valuable insights into cultivar performance and provides a strong foundation for future testing aimed at refining the best onion sets for home gardeners.
Allium cepa (Cepa Group) 'Stuttgarter'
onion 'Stuttgarter'
  • 0.1–0.5 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> (Cepa Group) 'Stuttgarter'
Allium cepa 'Hercules'
onion 'Hercules'
  • 0.5–1 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> 'Hercules'
Allium cepa (Cepa Group) 'Sturon'
onion 'Sturon'
  • 0.1–0.5 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> (Cepa Group) 'Sturon'
Allium cepa (Cepa Group) 'Stuttgarter'
onion 'Stuttgarter'
  • 0.1–0.5 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> (Cepa Group) 'Stuttgarter'
Allium cepa 'Hercules'
onion 'Hercules'
  • 0.5–1 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> 'Hercules'
Allium cepa (Cepa Group) 'Sturon'
onion 'Sturon'
  • 0.1–0.5 metres
  • 0.1–0.5 metres
<em>Allium</em> <em>cepa</em> (Cepa Group) 'Sturon'
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The fact ‘Red Baron’ showed that improved performance justifies the RHS Recommended: Award of Garden Merit in itself.

Edward Cooper, RHS Trials Team Leader

Why the RHS Recommended: AGM was awarded

Reconfirmed awards with forum comments

Allium cepa ‘Centurion’ 1993

  • Previous trial forum comment: Even crop of flattened globe-shaped bulbs with straw-coloured skins of good thickness. No bolters. Not heat-treated. Stores well
  • Yield: 28 bulbs from 45 sets

Allium cepa ‘Jetset’ 1993

  • Previous forum comment: Bolt resistant, attractive yellow round shaped bulbs with smooth skin, produce a uniform crop, quick to mature and good for storage
  • Yield: 23 bulbs from 45 sets 

Allium cepa ‘Hercules’ 2002

  • Previous trial forum comment: Uniform bulb, reasonable size, no bolters, not heat-treated. Stores well
  • Yield: 20 bulbs from 45 sets

Allium cepa ‘Setton’ 2002

  • Previous trial forum comment: Even crop, good size and quality. Dark brown skin. No bolters. Fair storage quality
  • Yield: 3 bulbs from 45 sets

Allium cepa ‘Sturon’ 2002

  • Previous trial forum comment: Uniform crop. Globe-shaped, yellow-brown skin. No bolters. Not heat-treated. Reasonable storage
  • Yield: 19 bulbs from 45 sets

Allium cepa ‘Stuttgarter’ 2002

  • Previous forum comment: High yield, well shaped, deep bulb, good skin
  • Yeild: 13 bulbs from 45 sets

Allium cepa ‘Red Baron’ 2005

  • Previous trial forum comment: Some variability in size. Flattened globe-shaped bulbs with dark-red skin. Heat-treated. Reasonable storage 
  • Yield: 31 bulbs from 45 sets

Allium cepa ‘Rumba’ 2013

  • Previous trial forum comment: Uniform crop, large bulb size, globe-shaped, with brown skin. No bolters. Heat-treated. Stores well 
  • Yield: 14 bulbs from 45 sets

Please note: Dates shown after plant names indicate the year the plant received its award.

Harvested Allium cepa ‘Setton’ with clean, even bulbs
Variable yield from Allium cepa ‘Pink Panther’ after lifting

RHS Recommended: AGMs rescinded

For this onion set trial, no existing RHS Recommdended: Award of Garden Merit selections lost their award.

Plants and location

What was planted?

Bringing together an inviting mix of 18 onion set cultivars from companies across the UK, this compact trial showcased the diversity and potential of the bulb onion (Allium cepa), one of the kitchen’s most adaptable vegetables. Although botanically a biennial, it is almost always grown as an annual.

Onions vary widely in size, flavour and colour. Yellow types develop pale golden skins, greenish-white flesh and a robust taste. Red varieties are milder, with glossy purple skins and red-tinged flesh, while white onions offer papery skins and a sweet, gentle flavour.

Throughout the trial, every entry was photographed to capture plant habit and detailed bulb characteristics. The RHS Trials team monitored each key growth stage, from the first emerging shoots through to harvesting, ensuring a clear picture of how each cultivar performed.

Where was it planted?

The onion sets were planted with care in the beds of the new Edible Trial Garden at RHS Wisley, creating the foundation for a season of steady growth, detailed observation and valuable comparison between cultivars. Here are some considerations for planting out onion sets:

  • Onions need a sunny, sheltered site with fertile, well-drained soil and should be kept weed-free
  • Sow sets 20cm apart in rows 30cm apart, covering with fleece against onion fly
  • Water until mid-summer when the bulbs swell as watering later may reduce the storage quality

  • Harvest when the foliage starts to turn yellow and topple over
  • Place the lifted bulbs on a rack in full sun outdoors or a well-ventilated greenhouse for about two weeks to ripen

Onion facts

Onions are usually grown from small immature bulbs called sets, which are planted in spring and give a faster, more reliable crop than growing from seed.

Cultivation

Planting process

After arriving at RHS Wisley in January 2025, the small onion bulbs were planted into beds in the Edible Trials Garden in March 2025. Each set was positioned with its tip just visible above a mulch layer. The trial used three rows of 15 plants, spaced 20cm apart with 30cm between rows. The beds were initially covered with fleece to deter birds and irrigation was applied as required to support early growth.

The intention had been to plant through a surface layer of compost or mulch, although no specific depth had been defined. Subsequent checks showed considerable variation in mulch thickness, in places reaching up to 10cm. The beds were maintained using a no-dig approach and regular watering. The Curatorial team noted that establishment, however, was inconsistent, and replacement sets were added in early May 2025 where possible.

A large proportion of plants ultimately failed to establish during the hot summer conditions, resulting in patchy and uneven yields. The remaining crop was harvested in late August 2025 and dried in the propagation unit. Although disappointing, the trial provided several practical insights and highlighted opportunities to refine methods and improve consistency in future work. Cultivation approaches will be reviewed on a trial-by-trial basis to build on these findings.

How plant health was managed

Challenges

Since germination was poor, the RHS Trials team asked the RHS Plant Health team to check and review the trial entries. They confirmed there was no evidence of disease, reassuring the team that the failures were likely driven by environmental factors rather than pathology. During forum visits in 2025, the issue was discussed and compared with the successful no-dig method used at RHS Rosemoor in 2024. This provided a useful contrast and practical ideas to explore in future trials.

Weather

Weather conditions added significant stress during the establishment of the trial. May rainfall reached only 55% of the long-term average, with June also drier than usual. RHS Wisley recorded a peak daily maximum of 33°C, with three heatwaves recorded across June and July. These challenging conditions, combined with variable mulch depth and the no-dig approach, helped explain the poor establishment and inconsistent yields. The insights gained from this trial will be used to develop more resilient trial protocols and better support sets through similar weather extremes in future seasons.

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Harvested Allium cepa ‘Red Baron’ onions, dried and stored in trays, with yields showing noticeable variation across the trial crop
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Onion sets arriving at RHS Wisley, packed in mesh bags and labelled for the trials
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The prepared trial bed at in early spring 2025, cleared, levelled, and ready for planting
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Harvested Allium cepa ‘Red Baron’ onions, dried and stored in trays, with yields showing noticeable variation across the trial crop
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Onion sets arriving at RHS Wisley, packed in mesh bags and labelled for the trials
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The prepared trial bed at in early spring 2025, cleared, levelled, and ready for planting
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Onions in the ground
How to grow onions

Explore all the information you need to know to grow and care for onions in your garden

How to grow onions
Did you know?

Onions can be stored for months after harvest if they are properly dried, meaning you can enjoy your homegrown crop right through winter and into spring.

Who was involved?

Judges of the trial

The trial was evaluated by seven invited specialists with expertise in vegetable growing, showing and judging, plant nurseries and retail, horticultural journalism, supported by members of the RHS Fruit, Vegetable and Herb Expert Group, including the Vegetable Forum, as well as invited guests from Which? magazine. Meetings were held in March, April and October, giving the assessors the opportunity to review the crop at key stages of its development, from early growth through mid-season performance to the final harvest.

The onion set trial judges comprised: Vicki Cooke (RHS Wisley Forum Chair and RHS Fruit, Vegetable and Herb Expert Group member), Keith Hine (National Vegetable Society judge), Richard White (former Trial Garden Manager, Tozer Seeds), Sarah Wain (Formerly West Dean Garden), Elizabeth Mooney (Horticulturist Edibles, RHS Wisley), Lady Lenzie (Edible Garden Designer) and Jim Arbury (RHS Fruit and Trials specialist). Guests: Adele Dyer and Sarah Wisson (Which? magazine researchers).  

Suppliers to the trial

List of plants in the trial

View all the onion sets trialled at RHS Wisley
  • Allium cepa ‘Centurion’
  • Allium cepa ‘Corrado’ F1  
  • Allium cepa ‘Crusado’
  • Allium cepa ‘Hercules’ 
  • Allium cepa ‘Jetset’ 
  • Allium cepa ‘Keravel Pink’
  • Allium cepa ‘Red Baron’
  • Allium cepa ‘Red Karmen’
  • Allium cepa ‘Romy’
  • Allium cepa ‘Rumba’ 
  • Allium cepa ‘Setton’
  • Allium cepa ‘Sturon’
  • Allium cepa ‘Stuttgarter Giant’
  • Allium cepa ‘Stuttgarter’
  • Allium cepa ‘Pink Rosanne’
  • Allium cepa ‘Pink Panther’
  • Allium cepa ‘STUR BC 20’
  • Allium cepa ‘Snowball’
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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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