Larkspur (Delphinium)

Delphiniums bring unrivaled height and color to summer gardens with their showy spikes of vibrant flowers. The most common colors are shades of blue or purple, but there are others available. They bloom with single, semi-double or double flowers, also called florets.

Delphiniums can be high-maintenance, needing just the right conditions. They prefer cool, mild summers with low humidity. Plant them in a spot with adequate moisture and well-draining soil. Thankfully, new strains of hybrids are being developed that aren’t as fussy.

I. Appearance and Characteristics 

Delphinium is a genus of about 300 species of annual and perennial flowering plants in the family Ranunculaceae, native throughout the Northern Hemisphere and also on the high mountains of tropical Africa. The genus was erected by Carl Linnaeus.

All members of the genus Delphinium are toxic to humans and livestock. The common name larkspur is shared between perennial Delphinium species and annual species of the genus Consolida. Molecular data show that Consolida, as well as another segregate genus, Aconitella, are both embedded in Delphinium.

The genus name Delphinium derives from the Ancient Greek word δελφίνιον (delphínion) which means “dolphin”, a name used in De Materia Medica for some kind of larkspur. Pedanius Dioscorides said the plant got its name because of its dolphin-shaped flowers.

The leaves are deeply lobed with three to seven toothed, pointed lobes in a palmate shape. The main flowering stem is erect, and varies greatly in size between the species, from 10 centimetres in some alpine species, up to 2 m tall in the larger meadowland species.

In June and July (Northern Hemisphere), the plant is topped with a raceme of many flowers, varying in colour from purple and blue, to red, yellow, or white. The flowers are bilaterally symmetrical and have many stamens. In most species each flower consists of five petal-like sepals which grow together to form a hollow pocket with a spur at the end, which gives the plant its name, usually more or less dark blue. Within the sepals are four true petals, small, inconspicuous, and commonly coloured similarly to the sepals. The uppermost sepal is spurred, and encloses the nectar-secreting spurs of the two upper petals.

The seeds are small and often shiny black. The plants flower from late spring to late summer, and are pollinated by butterflies and bumble bees. Despite the toxicity, Delphinium species are used as food plants by the larvae of some Lepidoptera species, including the dot moth and small angle shades.

All parts of these plants are considered toxic to humans, especially the younger parts, causing severe digestive discomfort if ingested, and skin irritation. Larkspur, especially tall larkspur, is a significant cause of cattle poisoning on rangelands in the western United States. Larkspur is more common in high-elevation areas, and many ranchers delay moving cattle onto such ranges until late summer when the toxicity of the plants is reduced. Death is through cardiotoxic and neuromuscular blocking effects, and can occur within a few hours of ingestion. All parts of the plant contain various diterpenoid alkaloids, typified by methyllycaconitine, and are very poisonous.

II. How to Grow and Care

Sunlight

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.

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.

Watering

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.

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.

Fertilizing

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.

Pruning

Removing mature flower spikes encourages flowering on developing side shoots. Find the axial 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.

Propagation

Division

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:

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

Cuttings 

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:

• Choose a shoot two to three inches long and use your gloved hands or a spade to dig down into soil around the shoot..
• Use a sharp knife to cut the shoot below soil level, making sure the cut portion is solid and not hollow at the bottom.
• Lightly clean to remove soil.
• Trim off excess foliage leaving just one or two leaves.
• Plant the cutting immediately or keep it moist and out of sunlight.
• 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.
• 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.
• Transplant new plants into the garden after the danger of frost has passed.

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.

Potting and Repotting 

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.

• Fill the container with a loose mix of potting soil and compost or mix in fertilizer according to label directions.
• Prepare a wide planting hole in the center of the pot.
• Place the plant in the hole at the same soil level as in its original pot and back fill with potting soil or compost.
• Water thoroughly at soil level but do not wet the foliage.
• 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.

Pests and Diseases

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.2 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.

Common Problems

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.

III. Uses and Benefits 

The juice of the flowers, particularly D. consolida, mixed with alum, gives a blue ink.