Sweet peas trial results 2022–2023

Objectives and purpose of the trial

This is the first trial of sweet peas (Lathyrus) to take place at the RHS since 2015. It is also the first trial that focused solely on Grandiflora sweet peas. This classification of sweet peas has smaller, simpler but more highly scented flowers, and put on a more floriferous display than the showier Spencer sweet peas. The RHS Trials team wanted to assess plants for the RHS Award of Garden Merit (AGM), demonstrate their cultivation and confirm correct naming and classification.
View of the sweet pea trial in full bloom at RHS Wisley
Rows of colourful sweet peas in the Trials Garden at RHS Wisley

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
  • Of 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:

  • General visual impact
  • Vigour and health of plants
  • Quality and quantity of blooms, distribution from top to bottom

  • Colour
  • Length of flowering season

Sweet pea AGM winners

This trial drew in garden visitors with its sweet scents and glorious blooms. It allowed the trial judges to grant 14 new AGMs and reconfirm five AGMs. Two AGMs were rescinded, and the list of existing AGMs now reflects the current availability of the best Grandiflora sweet peas.

We were able to assess thoroughly the comparison of autumn and spring sowing. This has the advantage of extending the season, much like successional sowing in other plants.

Phil Johnson, Sweet Pea Grower and Trial Chair

Why the AGM was awarded

New AGM winners with forum comments and hardiness ratings

Lathyrus odoratus ‘Alisa’ AGM (H3) 2023

  • Forum comment: Prolific. One of the best spring-sown entries. Good pale mauve flowers and good foliage
  • Final height: Autumn-sown up to 215cm, spring-sown up to 160cm

Lathyrus odoratus ‘Cedric Morris’ AGM (H3) 2023

  • Forum comment: Lots of flowers, brighter and earlier than ‘Quito’ and ‘Cupani’, habit also more compact. Different to others in trial with finer, grey-tinged foliage
  • Final height: Autumn-sown up to 200cm, spring-sown up to 145cm

Lathyrus odoratus ‘Chrissie’ AGM (H3) 2023

  • Forum comment: Amazing! Vigorous, bushy with a multitude of flowers, unique marbled colouring
  • Final height: Autumn-sown up to 220cm, spring-sown up to 180cm

Lathyrus odoratus ‘Fire and Ice’ AGM (H3) 2023

  • Forum comment: Exceptional. Neat and even, healthy plants. Tonnes of flowers. Colour varies in intensity creating interesting colour combinations
  • Final height: Autumn-sown up to 250cm, spring-sown up to 170cm

Lathyrus odoratus ‘Lady Grisel Hamilton’ AGM (H3) 2023

  • Forum comment: Lovely. Outstanding both years. Nice shape plants, neat, lots of flowers from top to bottom. Typical old-fashioned type with small flowers
  • Final height: Autumn-sown up to 220cm, spring-sown up to 195cm

Lathyrus odoratus ‘Marseilles’ (Riviera Series) AGM (H3) 2023

  • Forum comment: Vigorous, good shape plants. Well balanced. Lots of stems with five flowers. Great, rich flower colour
  • Final height after two years: Autumn-sown up to 235cm, spring-sown up to 200cm

Lathyrus odoratus ‘Miss Willmott’ AGM (H3) 2023

  • Forum comment: Lovely from the start. Neat plants. Lots of bright, clear coloured flowers, from top to bottom of the plants
  • Final height: Autumn-sown up to 190cm, spring-sown up to 135cm

Lathyrus odoratus ‘Mrs Collier’ AGM (H3) 2023

  • Forum comment: Vigorous. Prolific, flowering from top to bottom. Still lots of flower in July. Nice cream flowers. Neat foliage and good habit
  • Final height: Autumn-sown up to 200cm, spring-sown up to 175cm

Lathyrus odoratus ‘Prima Donna’ AGM (H3) 2023

  • Forum comment: Typical Old-fashioned sweet pea flower shape, in baby pink. Long flowering period
  • Final height: Autumn-sown up to 230cm, spring-sown to 165cm

Lathyrus × hammettii ‘Primrose’ AGM (H3) 2023

  • Forum comment: Lots of nicely formed, creamy yellow flowers. Good bushy plants, not too tall. Would fit into many gardens
  • Final height: Autumn-sown up to 170cm, spring-sown up to 145cm

Lathyrus odoratus ‘Queen Alexandra’ AGM (H3) 2023

  • Forum comment: Lovely plants, neat and tidy. Lots of flowers. Autumn- and spring-sown plants both very good. Flower colour intense
  • Final height: Autumn-sown up to 230cm, spring-sown up to 165cm

Lathyrus odoratus ‘Sweet Caroline’ AGM (H3) 2023

  • Forum comment: Fabulous. Plants even and healthy. Nice fine foliage. Prolific flowerer. Still looking good in July
  • Final height: Autumn-sown up to 250cm, spring-sown up to 150cm

Lathyrus × hammettii ‘Three Times As Sweet’ AGM (H3) 2023

  • Forum comment: Looks great. Prolific flowerer, well-spaced small flowers, producing flowers from top to bottom
  • Final height: Autumn-sown up to 215cm, spring-sown up to 165cm

Lathyrus odoratus ‘Valentine’ AGM (H3) 2023

  • Forum comment: Fabulous. Tall growing. Good foliage. Floriferous. Large, pure coloured flowers on long, strong stems. Many stems with five blooms. Spring-sown plants looked good early in the season
  • Final height: Autumn-sown up to 230cm, spring-sown up to 200cm
Reconfirmed AGMs with forum comments and hardiness ratings

Lathyrus odoratus ‘America’ AGM (H3) 1995

  • Forum comment: Good flake pattern. Neat, uniform plants. Lots of flower
  • Final height: Autumn-sown up to 210cm, spring-sown up to 130cm

Lathyrus odoratus ‘Cathy’ AGM (H3) 2002

  • Forum comment: Fantastic. One of the best. Strong growing with large flowers. Very floriferous, with lovely cream flowers
  • Final height: Autumn-sown up to 210cm, spring-sown up to 185cm

Lathyrus odoratus ‘High Scent’ AGM (H3) 2008

  • Forum comment: Dependable, benchmark for scent. Soft cream flowers
  • Final height: Autumn-sown up to 220cm, spring-sown up to 160cm

Lathyrus odoratus ‘King Edward VII’ AGM (H3) 1995

  • Forum comment: Outstanding. Healthy, vigorous plants. Good habit. Very floriferous. Good over both years
  • Final height: Autumn-sown up to 210cm, spring-sown up to 175cm

Lathyrus odoratus ‘Sicilian Pink’ AGM (H3) 1995

  • Forum comment: Consistently good. Neat plants with flowers from top to bottom. Pretty colour flowers
  • Final height: Autumn-sown up to 230cm, spring-sown up to 180cm
Retained AGMs with forum comments and hardiness ratings

Lathyrus odoratus ‘Bramdean’ AGM (H3) 2008

  • Forum comment: Did not perform well in the trial. Usually lots of strongly scented white flowers

Lathyrus odoratus ‘Janet Scott’ AGM (H3) 1995

  • Forum comment: Went over early, couldn’t assess. Hasn’t impressed either year but is known to be better outside of the trial
Forum judging the sweet peas trial at RHS Wisley in 2022
Judging of the sweet pea trial at RHS Wisley in 2023

AGMs rescinded

These selections were found to no longer be worthy of the AGM, and they will be removed from the AGM list. The forum recommended to rescind the RHS Award of Garden Merit from the following entries:
AGMs rescinded with forum comments

Lathyrus odoratus ‘Heathcliff’

  • Forum comment: Did not meet the plant stability criteria

Lathyrus odoratus ‘Matucana’

  • Forum comment: Did not meet the plant stability criteria. There are different stocks with different qualities available

Plants and location

What was planted?

This trial included 71 different selections of Grandiflora sweet peas. Grandiflora cultivars are highly scented with simple flowers and put on a great garden display. The colour range available is just as vast, from red, through purple and pink to white and cream, with bicolour and flaked or striped patterning too.

Where was it planted?

The trial took place in the Trials Garden at RHS Wisley, Surrey. Seeds of each trial entry were sown in November and in February. Sweet peas are annual plants, so fresh seed was supplied to sow each year. Here are some considerations for planting out the sweet peas:

  • Sow seeds undercover in autumn or spring; autumn-sown sweet peas flower earlier, while spring-sown sweet peas take longer to establish and flower later. Grow both if you have room.
  • Sweet peas will survive low temperatures down to -4°C, so they will be OK in a cold frame over winter
  • Sweet peas prefer a full sun site and need watering and feeding to produce lots of flowers when grown in the ground or in containers

  • Create a support for sweet peas to climb
  • Sweet peas produce tendrils that cling onto their supports, but they benefit from tying in with twine too
  • Keep on picking flowers from your plants, to keep them flowering for longer

Sweet pea facts

Sweet pea flowering peaks in June and July. Regularly picking the flowers extends the flowering season into late summer, as plants aren’t putting energy into forming seeds.

Cultivation

Planting process

Seed sowing was staggered each year to compare autumn-and spring-sown plants. Twelve seeds of each entry were sown in peat-free compost. Pots were placed on heated benches (20ºC) until germination.

Six plants of each entry were then potted into 2L pots with plant support cages. Once established, these pots were kept outside in cold frames over winter, and precautions were taken to stop mice taking the seeds. Seedlings were pinched out once they reached the top of the support cage, at about 10cm tall.

Soil analysis showed good nutrient levels in the Trials Garden beds. The beds were mulched with Apsley Farm Soil Improving Mulch as a weed suppressor and nutrient source. 

Autumn-sown entries were planted out in March followed by the spring-sown entries in April both years.

Plants were grown up wire cylinders, 45cm in diameter, spaced 1.2m between entries. The wire supports were formed of two cylinders of wire fencing with 15 × 15cm square holes, 120cm tall, placed one on top of the other, held together and secured into the ground using bamboo canes. The plants were left to branch naturally, and the tendrils were not removed. Shoots were tied into the support framework regularly using tape guns. Flowers were picked regularly before seed pods were able to develop.

How plant health was managed

Challenges

Sweet peas are prone to powdery mildew and aphids. Spring-sown entries were affected most by aphids, which transmitted viruses between plants. Symptoms included streaking and curling in the leaves and flowers of some entries. Towards the end of both years, the powdery mildew infection increased noticeably.

Weather

In February 2022, Storm Eunice damaged the cold frames housing the sweet peas and caused some labels to become dislodged.

Autumn-sown entries were planted out on 28 March 2022 and a -4.3°C night followed on 1 April 2022. Entries were individually covered with fleece over the weekend, and no plants were lost, however some foliage yellowing resulted.

By July 2022, the scorching temperatures caused many of the sweet peas to burn out early.

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:

June 2022

Lots of insect life, from beetles to bees, were spotted enjoying the sweet pea trial. Heavy leafcutter bees were able to access the flowers, and bumblebees were seen nectar-robbing from the side of the flowers. Birds were also regular visitors, helping to manage the aphid populations.

July 2022

There was much excitement about a mutant flower discovered on ‘Monaco’ (Riviera Series). This unusual flower form was identified as a reappearance of a mutation recorded in 1911 by William Bateson and Reginald Punnett, founders of experimental genetics in the UK. Although not of garden value, attempts were made to collect pollen to save the genetics. The mutation did not reappear in year two.

July 2023

We had a brilliant few days of staff and volunteers at RHS Wisley coming down to the Trials Garden to cut their own sweet peas. This helped us to keep on top of deadheading the trial, and helped us to spread the joy (and scent) of the sweet peas across the site.

Watch an aerial tour of the sweet pea trial at RHS Wisley

Did you know?

The RHS Herbarium team collect plant specimens from each RHS Plant Trial. See the collection of sweet pea specimens online >

Who was involved?

Judges of the trial

The sweet pea trial was assessed by a 10-person judging group during June and July, in 2022 and 2023. The forum was made up of gardeners, plant breeders, nursery owners, cut flower specialists and collection holders.

The sweet pea judges included: Phil Johnson (Sweet pea grower and Trial Chair), Keith Brewer (Sweet pea grower), Claire Brown (Cut flower grower), David Fitton (Sweet pea grower), Jim Marshall (Herbaceous Plant Committee Member), Jim McDonald (Sweet pea grower), Mary McDonald (Sweet pea grower), Alison Mulvaney (Commercial seed production expert), Roger Parsons (National Collection holder) and Graham Rice (Garden writer).

Suppliers to the trial

List of plants in the trial

View all the sweet peas trialled at RHS Wisley
  • Lathyrus × hammettii ‘Painted Porcelain’ (syn. ‘Spanish Dancer’) 
  • Lathyrus × hammettii ‘Adorabel’
  • Lathyrus × hammettii ‘Enigma’
  • Lathyrus × hammettii ‘Erewhon’
  • Lathyrus × hammettii ‘More Scent’
  • Lathyrus × hammettii ‘Primrose’
  • Lathyrus × hammettii ‘Three Times As Sweet’
  • Lathyrus odoratus ‘Albutt Blue’
  • Lathyrus odoratus ‘Alisa’
  • Lathyrus odoratus ‘Almost Black’
  • Lathyrus odoratus ‘America’
  • Lathyrus odoratus ‘Annie B Gilroy’
  • Lathyrus odoratus ‘Astrid’
  • Lathyrus odoratus ‘Black Knight’
  • Lathyrus odoratus ‘Bramdean‘
  • Lathyrus odoratus ‘Butterfly’
  • Lathyrus odoratus ‘Cannes’ (Riviera Series) 
  • Lathyrus odoratus ‘Cathy’
  • Lathyrus odoratus ‘Cedric Morris’
  • Lathyrus odoratus ‘Chrissie’
  • Lathyrus odoratus ‘Cream Eggs’
  • Lathyrus odoratus ‘Crown Princess of Prussia’
  • Lathyrus odoratus ‘Cupani’
  • Lathyrus odoratus ‘Dave R’
  • Lathyrus odoratus ‘Dr Robert Uvedale’
  • Lathyrus odoratus ‘Dragonfly’
  • Lathyrus odoratus ‘Emily‘
  • Lathyrus odoratus ‘Fire and Ice’
  • Lathyrus odoratus ‘Flora Norton’
  • Lathyrus odoratus ‘Genoa‘ (Riviera Series) 
  • Lathyrus odoratus ‘Heathcliff’
  • Lathyrus ‘High Scent’
  • Lathyrus odoratus ‘Indigo King’
  • Lathyrus odoratus ‘Jack Ellis’
  • Lathyrus odoratus ‘Janet Scott’
  • Lathyrus odoratus ‘Juliet’
  • Lathyrus odoratus ‘King Edward VII’
  • Lathyrus odoratus ‘Kingfisher’
  • Lathyrus odoratus ‘Lady Grisel Hamilton’
  • Lathyrus odoratus ‘Lady Turral’
  • Lathyrus odoratus ‘Little Red Riding Hood’
  • Lathyrus odoratus ‘Lord Nelson’
  • Lathyrus odoratus ‘Marseilles’ (Riviera Series) 
  • Lathyrus odoratus ‘Matucana’
  • Lathyrus odoratus ‘Memories’
  • Lathyrus odoratus ‘Miss Willmott’
  • Lathyrus odoratus ‘Monaco’ (Riviera Series) 
  • Lathyrus odoratus ‘Montpellier’ (Riviera Series) 
  • Lathyrus odoratus ‘Monty Don’
  • Lathyrus odoratus ‘Mrs Collier’
  • Lathyrus odoratus ‘My Navy’
  • Lathyrus odoratus ‘Nice’ (Riviera Series) 
  • Lathyrus odoratus ‘Nightingale’
  • Lathyrus odoratus ‘NZ Gardener’
  • Lathyrus odoratus ‘Our Heroes’
  • Lathyrus odoratus ‘Prima Donna’
  • Lathyrus odoratus ‘Prince Edward of York’
  • Lathyrus odoratus ‘Princess of Wales’
  • Lathyrus odoratus ‘Queen Alexandra’
  • Lathyrus odoratus ‘Queen of the Isles’
  • Lathyrus odoratus ‘Quito’
  • Lathyrus odoratus ‘Roosterville’
  • Lathyrus odoratus ‘Rosay’
  • Lathyrus odoratus ‘Scarlett’
  • Lathyrus odoratus ‘Senator’
  • Lathyrus odoratus ‘Sicilian Pink’
  • Lathyrus odoratus ‘Sweet Caroline’ (entered in 2022 as AB 1) 
  • Lathyrus odoratus ‘Valentine’
  • Lathyrus odoratus ‘Violet Wings’
  • Lathyrus odoratus wild type 
  • Lathyrus odoratus ‘Zorija Rose’
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