How to Grow and Care for Delphinium

Delphinium describes a group of plants that produce some of the biggest, boldest blooms of summer. The genus includes more than 300 species with Delphinium elatum most easily recognized. This species grows to a towering five to six feet tall with flower spikes from pastels to shades of deep, rich blue, pink, purple and white. Some types feature a contrasting eye and others produce semi-double or double blooms. Large, two inch, cup or spur-shaped flowers cover an erect branching bloom spike with deeply lobed, fan-shaped leaves mounded at the base.

Delphiniums are short-lived, lasting just three to five years and grow best in regions with long, cool spring seasons. Bloom time starts from late spring to early summer and can continue all season depending on variety. They are toxic to people, pets and horses.

Delphinium Care

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.

Propagating Delphinium

Delphiniums grow and multiply rapidly and are propagated by division and basal cuttings. These methods are most successful in spring when new growth first appears. To propagate by division, you'll need a hand spade, watering can, and gloves, Here's how:

1. Water plants thoroughly 24 hours before to prevent transplant shock.
2. Choose a section from the outside portion of the clump.
3. Remove lengthy stems and excess foliage from your selection.
4. Starting at five to six inches out from the center use the hand spade to dig around the section.
5. Lift it out and gently shake off excess soil.
6. Choose a new location with direct sun and well-draining soil and work in compost or a balanced fertilizer.
7. Place each division into a separate hole keeping it at the same soil level as in the original location.
8. Backfill with garden soil and gently water the new plant at the base.

To propagate from basal cuttings you'll need a sharp sterile knife, small hand spade, three to four inch pots with drainage holes, well-draining potting medium, compost, and gloves. Here's how:

1. Choose a shoot two to three inches long and use your gloved hands or a spade to dig down into soil around the shoot..
2. Use a sharp knife to cut the shoot below soil level, making sure the cut portion is solid and not hollow at the bottom.
3. Lightly clean to remove soil.
4. Trim off excess foliage leaving just one or two leaves.
5. Plant the cutting immediately or keep it moist and out of sunlight.
6. Place it in a three to four inch pot with a combination of compost and perlite or a moist, loose, potting soil. The portion of the cutting growing below soil level should be buried at the same level in the pot as it was in ground. Alternatively the cutting can be placed in one to two inches of water until roots form.
7. Keep cuttings in a cool location, preferably at 50 to 55 degrees Fahrenheit. and avoid bottom heat. New growth appears in three to four weeks indicating roots have formed.
8. Transplant new plants into the garden after danger of frost has passed.

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:

1. 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.
2. Lightly spray with water and cover with a plastic dome or bag.
3. Keep soil moist and temperature at 70 to 75 degrees Fahrenheit. Germination occurs in three to four weeks.
4. Once seeds sprout, remove plastic covering and place the plants in a sunny window or under grow lights.
5. When seedlings are sturdy enough to handle, transplant them into individual pots.
6. 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.

Potting and Repotting Delphinium

Delphiniums spread rapidly and do not like crowding, so choose a container large enough to hold a plant at least twice the size. Choose a pot with plenty of drainage holes and avoid materials like clay or terra cotta that dry out quickly. Dwarf varieties work especially well, but tall varieties can be grown in pots if wind protection and/or staking is provided. Pots need frequent, regular watering and a balanced fertilizer every two to three weeks.

1. Fill the container with a loose mix of potting soil and compost or mix in fertilizer according to label directions.
2. Prepare a wide planting hole in the center of the pot.
3. Place the plant in the hole at the same soil level as in its original pot and back fill with potting soil or compost.
4. Water thoroughly at soil level but do not wet the foliage.
5. Place the pot where it will receive plenty of morning sun and filtered afternoon shade..

Your potted plant will double in size by its second year so spring division might be an annual chore. To repot a division, prepare a second large pot or location in the garden. Follow the steps under 'Propagation' and return the parent plant to its original pot.

Overwintering

Perennial delphiniums are frost hardy but can be damaged by cold wet soil. Mulching for winter protection discourages frost damage. Potted plants do not need to move indoors but should be protected from wind and excessive wet weather.

Common Pests

Delphiniums are susceptible to slugs, aphids, mites, cutworms, stalk borers and leaf miners. Signs include failure to bloom, deformed flowers, wilting, and blackened, distorted foliage. Inspect plants regularly for signs of trouble and take recommended steps to eradicate the specific pest.

Common Diseases

Powdery mildew is caused by high humidity and the presence of a soil-borne fungus. Crown rot is a fungal growth with no known control so affected plants and surrounding soil should be discarded. Delphiniums also are vulnerable to leaf spot, blight, stunting, root-knot nematodes and viruses. Strong, healthy plants are the best defense. Remove diseased plants. Avoid soggy soil and overhead watering, keep the planting bed weed-free, and plant disease-resistant varieties.

How to Get Delphinium to Bloom

Failure to bloom can result from inadequate growing conditions. Delphiniums need at least six hours of full sun daily and consistently-moist soil. Provide plenty of nutrients, but keep in mind that too much nitrogen can result in lush foliage at the expense of flower development. Extend the bloom period by removing the center bloom spike when flowers begin to drop. When the initial bloom period ends, cut the plant back to two inches to encourage a second bloom period.

Common Problems with Delphinium

Overcrowding

These plants spread rapidly and do not like to compete for space. Overcrowding inhibits air circulation which can lead to powdery mildew and the spread of other fungal and bacterial problems. Divide or thin new growth in early spring to alleviate this problem.

Yellow leaves

Delphinium foliage should be uniform blue-green in color. Yellowing foliage is caused by overwatering and also could be a sign the plant lacks nutrition. Grow delphiniums in a location or in pots with good drainage and feed them regularly.

Plants droop and fall over

This is a problem especially with tall varieties. Hollow stems with heavy flower spikes need support. Install stakes or grow delphiniums against a trellis or fence. Flimsy plants can also result from not enough sunlight.