From a plant's perspective, delphiniums are stately but vulnerable perennials. Their hollow flower stalks, while magnificent, are susceptible to breaking in strong winds and rain. Therefore, a primary need is for sturdy neighboring plants that offer subtle physical support without overcrowding their root zone. Furthermore, delphiniums are highly attractive to sap-sucking pests like aphids and are prone to powdery mildew. Companion plants that attract beneficial predatory insects or improve air circulation around the delphinium's base are not just neighbors; they are vital allies in maintaining health and vigor.
To address the need for support, plants with a strong, bushy, or spiky form are ideal. The deep taproot of the common foxglove (Digitalis purpurea) means it does not compete heavily with the delphinium's own root system, while its towering flower spikes create a stunning vertical repetition. Similarly, the airy, cloud-like umbels of Joe Pye weed (Eutrochium purpureum) or the strong, upright stems of veronicas (Veronica spicata) provide a contrasting form that helps break the wind, reducing the risk of damage to the delphinium's stalks. These plants act as a living staking system.
A delphinium's soft, juicy growth is a magnet for aphids. To combat this, planting companions that serve as insectaries is a key defensive strategy. The tiny, nectar-rich flowers of lady's mantle (Alchemilla mollis) are exceptionally attractive to parasitic wasps, which are voracious predators of aphids. The flat, open flower heads of yarrow (Achillea millefolium) and Shasta daisies (Leucanthemum x superbum) provide perfect landing pads for ladybugs and lacewings, whose larvae consume vast quantities of pests. These companions provide an ecosystem service, recruiting a protective army for the delphinium.
Delphiniums prefer consistently moist, cool roots, a condition that can be difficult to maintain. Low-growing, ground-covering companions help immensely by shading the soil, reducing moisture loss through evaporation, and suppressing weed competition. The dense foliage of catmint (Nepeta) sprawls beautifully around the delphinium's base, keeping roots cool and emitting a scent that may deter certain pests. Geraniums (hardy cranesbill), such as Geranium 'Rozanne', perform the same function while offering a long season of complementary blue or purple flowers that enhance the delphinium's cool color palette.
Finally, contrast is key. The delphinium's deeply lobed, palmate leaves provide a textural contrast to plants with different foliage forms. The broad, heart-shaped leaves of lady's mantle (Alchemilla mollis) catch and hold rainwater, which can be a minor water source, and its lime-green flowers complement every delphinium hue. The spiky, architectural foliage of iris provides a striking textural contrast and thrives in similar sunny conditions with well-drained soil, ensuring the two plants are compatible in their cultural needs without directly competing for resources.
