Delphinium, from seed trial results 2023–2025

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

A total of 45 Delphinium entries were grown from seed, including Delphinium grandiflorum, Delphinium elatum hybrids and selections from the Belladonna Group, all available commercially. Plants were grown on to reach their full potential by the third year.

• Delphinium elatum hybrids are tall perennials, reaching up to 2m, typically producing unbranched flower spikes in mid-summer
• Delphinium grandiflorum are shorter‑lived perennials, often grown as annuals, producing multiple stems with loose clusters of large flowers
• Delphinium (Belladonna Group) are herbaceous perennials, with a bushy, clump-forming habit and branching spires of single or semi-double flowers

All seed entries were sourced and submitted by breeders and growers from the UK, Japan and New Zealand.

Throughout the trial, each entry was photographed to record plant habit, consistency and key characteristics. The RHS Trials team monitored plants at every stage of development, from the first emergence through to flowering, enabling performance and garden value to be assessed over time.

Overall plant health was good, with fewer losses than expected, particularly over winter. A number of recurring challenges were observed across the trial.

Plant losses and root conditions

Losses in the Delphinium elatum trial beds were limited and typically linked to crown rot. Affected plants often wilted during the growing season and had little viable root, making them easy to pull out. Contributing factors likely included slug damage and wind rock, both of which can weaken the crown. One Delphinium elatum trial bed was also found to retain more moisture than others, which may have increased the risk of loss.

Challenges

Powdery mildew was the most significant issue, appearing as plants began to produce flower spikes. It affected foliage, buds and flowering performance.

• Delphinium grandiflorum and Belladonna Group entries were the most susceptible, with some plants failing to flower properly
• Delphinium elatum hybrids were less severely affected, although symptoms were more visible on darker flowered cultivars

In 2024, plants were not treated initially, allowing susceptibility to be assessed. Affected growth was removed during deadheading in June and regrowth emerged clean. A later treatment applied in July using an environmentally friendly mildew control product that works through physical action and does not remain in the environment for long improved flowering in Delphinium grandiflorum. By the third year, early spring treatment was used to support performance.

A separate issue, black blotch, was identified on Delphinium ‘Blue Donna’ in the first year. Cutting plants back to ground level resulted in healthy regrowth. Other viral symptoms, seen as yellow streaking, were noted but rare and affected plants were removed promptly to prevent spread.

Slugs, which commonly damage young Delphinium shoots, were not a significant problem in the trial. The open site conditions limited their impact and winter-applied grit provided an effective barrier.

Weather and environmental factors