Delphiniums, with their stately, tall spires of vibrant blooms, are a favored addition to many gardens, offering dramatic vertical interest and a burst of color. These flowers, often recognized for their true blue hues, also appear in shades of purple, pink, and white. The name “delphinium” comes from the Greek word “delphis,” referring to the dolphin-like shape of the flower buds before they open. Many varieties are known by the common name “larkspur,” which highlights the flower’s resemblance to a lark’s spur or claw. Gardeners value delphiniums for their ability to create a classic cottage garden aesthetic and attract pollinators like bees and butterflies.
Growing Delphinium
Site Selection and Planting
Select a spot that offers shelter from strong winds, as their tall, hollow stems can easily break. Space plants about 2 to 3 feet apart to allow for proper air circulation and growth, which also helps prevent fungal diseases. When planting, dig a hole twice the diameter of the plant’s container and ensure the top of the root ball is level with the soil surface.
Light Requirements
Delphiniums perform best with ample sunlight, receiving six to eight hours of direct sun daily. In regions with hot summers, afternoon shade can protect plants from heat stress and extend blooming. A location with morning sun and light afternoon shade provides an ideal balance.
Soil Needs
Well-draining soil is a requirement for delphiniums, as they are susceptible to root and crown rot in waterlogged conditions. The soil should be rich in organic matter, such as compost or aged manure, to provide nutrients and improve structure. A neutral to slightly alkaline pH, ranging from 6.0 to 7.5, is most suitable. Amending heavy clay soils with sand and organic matter enhances drainage.
Watering
Consistent moisture is important for delphiniums, particularly during active growth and blooming. They prefer moist soil but should never be waterlogged, as this can lead to root rot. Water deeply at the base of the plant to keep foliage dry and reduce fungal disease risk. During dry spells, delphiniums may require more frequent watering.
Fertilizing
Delphiniums are heavy feeders and benefit from regular nutrient supplementation to support vigorous growth and abundant flowering. Incorporate well-aged compost or a balanced, slow-release fertilizer into the soil at planting. During the growing season, apply a balanced liquid fertilizer every two to three weeks, starting in early spring. Avoid high-nitrogen fertilizers, as this can promote lush foliage at the expense of blooms.
Ongoing Care and Maintenance
Staking
Staking is needed for delphiniums due to their tall, heavy flower spikes and hollow stems, which can easily break from strong winds or heavy rain. Insert sturdy supports early in the growing season, when plants are about 12 inches high, so growing stems can conceal the stakes. Various methods include placing several stakes around the plant and looping garden twine to create a supportive cage, or tying individual stems to stakes with soft twine in a figure-eight pattern.
Pruning for Repeat Blooms
Deadheading spent delphinium flowers can encourage a second flush of blooms later in the season. Cut the main flower stalk back to the ground or to small, flowering side shoots after the initial bloom fades. This redirects the plant’s energy from seed production into new flowers. A final cut to the entire plant can be made in late autumn after all blooming is complete to prepare it for winter and reduce fungal disease development.
Troubleshooting Common Issues
Pests and Diseases
Delphiniums can be affected by pests, including slugs, snails, and aphids. Slugs and snails create irregular holes in leaves and can be managed by handpicking or using barriers. Aphids are tiny insects that cluster on the underside of leaves, causing distortion; insecticidal soaps or neem oil can help control them. Common diseases include powdery mildew, appearing as white, powdery patches on leaves, and crown rot, characterized by yellowing leaves and wilting. Proper plant spacing, good air circulation, and avoiding overhead watering help prevent these issues.
Environmental Stress
Delphiniums prefer cool, mild summers and do not fare well in hot, dry conditions, which can lead to wilting or scorching. Providing afternoon shade in warmer climates helps mitigate heat stress. Insufficient water can cause wilting, requiring consistent, deep watering during dry periods. Conversely, overwatering or poorly draining soil leads to waterlogging, which can cause crown and root rot.