Sweet peas trial results 2022–2023
Objectives and purpose of the trial
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
Lathyrus odoratus 'Sweet Caroline'
sweet pea 'Sweet Caroline'
- 1.5–2.5 metres
- 0.5–1 metres
Lathyrus odoratus 'Lady Grisel Hamilton'
sweet pea 'Lady Grisel Hamilton'
- 1.5–2.5 metres
- 0.1–0.5 metres
Lathyrus odoratus 'Queen Alexandra'
sweet pea 'Queen Alexandra'
- 1–1.5 metres
- 0.1–0.5 metres
Lathyrus odoratus 'Sweet Caroline'
sweet pea 'Sweet Caroline'
- 1.5–2.5 metres
- 0.5–1 metres
Lathyrus odoratus 'Lady Grisel Hamilton'
sweet pea 'Lady Grisel Hamilton'
- 1.5–2.5 metres
- 0.1–0.5 metres
Lathyrus odoratus 'Queen Alexandra'
sweet pea 'Queen Alexandra'
- 1–1.5 metres
- 0.1–0.5 metres
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.
Why the AGM was awarded
Forum judging the sweet peas trial at RHS Wisley in 2022
Judging of the sweet pea trial at RHS Wisley in 2023
AGMs rescinded
Plants and location
What was planted?
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
Cultivation
Planting process
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.
How to grow sweet peas (Lathyrus)
Explore all the information you need to know to grow and care for sweet peas in your garden.
Discoveries
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.
Who was involved?
Judges of the trial
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
- English Sweet Peas (incorporating Johnson’s Sweet Peas)
- McDonald Sweet Peas – c/o the Trials Office
- Roger Parsons Sweet Peas
- Somerset Sweet Peas
- S Cook – c/o the Trials Office
List of plants in the trial
Handy tools and information
- Full article - Filter for AGM plants and refine by colour, height and season of interest
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Gardening advice
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Hardiness ratings
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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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