Delphiniums are a cottage garden and herbaceous border staple, bringing height and colour to displays and mixing well with roses, peonies and other vertical flowers such as lupins and verbascums. The flowers are loved by bees and look stunning in a vase.
Delphiniums are in the buttercup family (Ranunculaceae) and have been grown in the UK for centuries. The word delphinium comes from the Greek delphinos, meaning dolphin, probably because of the shape of the opening flowers. Their common name, larkspur, dates back to Tudor times.
Most delphiniums are hardy perennials, but annual and biennial varieties are also available. The Elatum Group of delphiniums are the most commonly grown and are the tallest type, with spikes of single or double flowers reaching up to 2m. Belladonna delphiniums are shorter, with a looser, more branching habit and single flowers. 'Pacific Hybrids' were developed in the 1950s and 1960s, and are short-lived perennials or biennials. They look similar to 'Elatum' delphiniums but are shorter. The 'Magic Fountain' series of delphiniums are short and more compact, suitable for smaller gardens.
Delphiniums need care to keep them looking good. They need protecting from slugs in spring, staking, moist soil and plenty of feeding. If you cut them down to the ground after flowering, you may be rewarded with a second flush of flowers in September.
How to grow delphiniums
Grow delphiniums in moist but well-drained soil in full sun. Delphiniums struggle in wet winter soil, so aid drainage by adding grit to the planting hole if you have heavy soil. Protect emerging shoots from slugs in spring and stake as soon as they start to grow. Feed weekly with a high potash fertiliser. After flowering, cut stems back to encourage a second flush of blooms. Mulch in autumn with well-rotted manure or leaf mould.
How to Grow Delphiniums From Seed
Delphiniums grow easily from seed, however, not all types reseed and hybrids might not produce plants identical to the parent plant. Start seed in winter or early spring or provide cold stratification by placing them in the refrigerator for several weeks. Gather seed, small pots or a seed tray, loose potting medium, compost, plastic covering, and a spray bottle. Follow these steps:
- Fill a seed tray or small three to four inch pots with damp, loose soil mix. Sprinkle seeds and cover with 1/8 inch of compost.
- Lightly spray with water and cover with a plastic dome or bag.
- Keep soil moist and temperature at 70 to 75 degrees Fahrenheit. Germination occurs in three to four weeks.
- Once seeds sprout, remove plastic covering and place the plants in a sunny window or under grow lights.
- When seedlings are sturdy enough to handle, transplant them into individual pots.
- Continue to repot into slightly larger pots as needed until all danger of frost has passed and it's safe to plant in the garden.
How to plant delphiniums
You can plant a delphinium at any of time of year, but spring or autumn is best as the soil will be warm and moist. Dig a planting hole and add compost or well-rotted manure in the bottom, for a nutritious boost. Delphiniums struggle in winter wet, so add grit to heavy soils to aid drainage. Plant your delphinium at the same depth it was growing in the pot, and back-fill with soil, firming in gently. Water in well.
How to care for delphiniums
The Delphinium group of plants offers plenty of versatility for the home gardener. Short or medium varieties add color to the front or middle of the garden with tall types working well as dramatic backdrops and screens. Flower spikes are hollow, so taller varieties need support. Delphiniums attract pollinators and are coveted as cut flowers. These plants are vulnerable to pests and disease so plan to devote some time to your delphiniums.
Light
Delphiniums put on their best show in plenty of sun, 6 to 8 hours daily. However, this is not a plant that does well in dry heat. A location that offers morning sun with light afternoon shade during hot weather can improve and extend flowering.
Soil
Well-draining loamy soil with a slightly acidic soil pH of 6.5 to 7.0 yields the best results. Adding compost at planting time boosts growth for these heavy feeders.
Water
Water regularly especially during hot weather. Keep soil moist but not waterlogged because these plants are vulnerable to crown rot. The planting bed should never dry out completely. Water at ground level, do not wet the foliage, to discourage fungal and leaf spot problems. A two to three inch mulch layer preserves moisture in the soil.
Temperature and Humidity
Long, cool springs and cooler summers with average temperatures between 70 and 80 degrees Fahrenheit provide ideal conditions. These plants dislike high humidity and heat, and humidity can encourage powdery mildew to develop. Most perennial types are fully cold hardy to -30 degrees Fahrenheit.
Fertilizer
Delphiniums need plenty of nutrients to produce their showy flower spikes. Work well-aged compost or a balanced fertilizer such as a 10-10-10 into the soil at planting time. A second application mid-season might prolong bloom and support a second bloom period. When leaves start to turn yellow, it's often a sign a nutrient boost is needed.
Types of Delphinium
Many popular and widely available delphiniums are cultivars of D. elatum or D, grandiflora with crosses between the two identified as D. x belladonna. Some are series of plants that share identical features but bloom in varied or combinations of colors.
D, elatum 'Magic Fountain' grows two and a half to three feet tall with flowers in white, white with a dark center, dark blue, sky blue and pink.
D. elatum 'Centurion' features three foot tall, erect stems with semi-double flowers in white, purple shades and blue shades with white eyes,
D. elatum 'Pacific Giant' is usually grown from seed resulting in combinations of bloom colors. This type reaches three to six feet in height.
D. grandiflorum ‘Summer Nights’ is considered a dwarf variety at 10 to 12 inches tall and features midnight blue flowers.
D. x belladonna 'Blue Donna' produces uniform bright blue flowers on multiple branching stems and grows two and a half to three feet tall.
Pruning
Removing mature flower spikes encourages flowering on developing side shoots. Find the axil where branches form off the main stem and use a sterile hand pruner to remove the center flower spike. Once the initial bloom period ends, cut back delphiniums to two inches above ground level to encourage a second late summer or autumn bloom. Cutting back is repeated either in autumn or early spring to encourage new seasonal growth.