Gladiolus (gladioli) trial results 2024–2025

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

Challenges

During the first year of the trial, gladiolus thrips had a significant impact. Damage included streaking and mottling of florets, discolouration of foliage and, in severe cases, flower buds on the spikes failing to open.

In response, the beds were treated three times between August and September 2024 with a soapy solution, followed by a final spray aimed at controlling adult insects, larvae and eggs. Despite repeated applications, this approach proved ineffective and was not considered a reliable or preventative control method.

In the second year of the trial, the focus shifted from attempted eradication to management, recognising that thrips is a persistent pest. In 2025, an Integrated Pest Management (IPM) strategy was introduced to reduce damage. Key actions included:

• Mid-April: Direct sowing of alyssum (Lobularia maritima) across the gladiolus beds
• Early May: First application of a Steinernema feltiae nematode drench
• Early June: Second application of a Steinernema feltiae nematode drench

This approach aimed to encourage a more balanced ecosystem by providing alternative feeding sites for thrips. Early observations in June and July suggested a reduction in thrip damage, with flowering spikes opening more fully and showing less distortion.

However, in June and July 2025, three closely spaced heatwaves affected the trial. Despite regular hand watering, signs of heavy thrip infestation re-emerged by late July. Two additional biological controls were considered as part of the IPM approach:

• Amblyseius cucumeris, a predatory mite that feeds on thrips
• Orius laevigatus, whose nymphs primarily consume thrip larvae

These were not introduced, as it was felt they would be ineffective so late in the season. By this stage, most flower spikes had already passed their peak due to the prolonged hot and dry conditions.

Alongside the Trials Garden, the Gladiolus Trials Forum also observed three cultivars planted within a mixed border in the RHS Wisley Garden. In this setting, the gladiolus performed well and showed minimal thrip damage. This comparison suggested that while thrips presented a significant challenge in the trial environment, they may be less problematic in garden settings where gladiolus are grown within mixed herbaceous planting.

Weather and watering

Spring and summer 2025 were notably warmer and drier than average at RHS Wisley, creating challenging growing conditions for the trial.

Rainfall: Spring rainfall totalled 45.9mm, making it the fourth driest spring on record at RHS Wisley. May received just 55% of the 1990 to 2019 average rainfall, while June was also drier than usual. Although July rainfall reached 92% of the long-term mean, much of the South East was already experiencing drought conditions by this point.

Temperature: Three heatwaves were recorded across June and July. In June, temperatures exceeded 27°C for three consecutive days, with a maximum of 32.4°C recorded on 30 June. Peak temperatures reached 33°C in July. The Gladiolus beds were not fitted with drip irrigation, so watering was carried out manually twice a week throughout the growing season.