How to Grow and Care for 'Black Knight' Delphinium

'Black Knight' delphinium plant with semi-double, dark blue-purple flowers closeup

The Spruce / Phoebe Cheong

Delphiniums are hard not to notice in the landscape. They are members of the buttercup family, and, if you take a good look at the deeply dissected, maple-like leaves, you will see the family resemblance. But it is the impressive height and tightly packed spikes of oddly shaped flowers that earn delphiniums so much attention. The 'Black Knight' cultivar sports semi-double, dark purple flowers on stems that can be as much as 6 feet tall. 'Black Knight' is a plant to grow when a mousy little wallflower of a plant just won't do.

Like other delphiniums, 'Black Knight' is usually planted as a potted nursery specimen in spring. It is a fairly fast growing but a relatively short-lived perennial, usually beginning to decline within three or four years. It is relatively easy to grow from seeds, often flowering in the first season if planted indoors in February or March.

All parts of the delphinium contain alkaloids that are toxic to humans and pets, causing severe digestive symptoms if eaten.

Common Name 'Black Knight' delphinium, candle larkspur
Botanical Name Delphinium elatum 'Black Knight'
Plant Type Herbaceous perennial
Mature Size 4–6 ft. tall, 1–2 ft. wide
Sun Exposure Full
Soil Type Moist, well-drained, fertile
Soil pH Neutral to alkaline  (7.0 to 8.5)
Bloom Time Summer, fall
Flower Color Dark purple
Hardiness Zones 3–7 (USDA)
Native Area  Europe, Asia 
Toxicity  Toxic to humans, animals 

'Black Knight' Delphinium Care

Hybrid delphiniums such as 'Black Knight' can be relatively easy to grow if conditions are just right, but maddeningly difficult for many gardeners. These are plants that demand somewhat alkaline, very well-draining soil, a full-sun location, and a relatively cool, moist climate but one that is not excessively humid. There is a reason this plant is so popular in the British Isles, as this is one of the few places that has ideal conditions for delphiniums. In North America, it is those regions with English-like conditions that are best suited to delphiniums.

Delphiniums need to be planted rather shallowly so that the crown is just barely buried, or even slightly exposed. As tall, floppy plants with hollow stems, 'Black Knight' delphinium almost always requires staking. If unsupported, their stalks snap easily in high winds. For this reason, it's helpful to plant them next to a fence or wall, where they get some protection from the wind.

Some gardeners mulch delphinium for winter protection, but be aware that crown rot can be a problem for these plants. The risk of losing them in winter from poor drainage is greater than the risk posed by the cold. While a good layer of dry mulch is helpful in regions with cold winters but little snow, mulch should not be applied until the ground is fully frozen, and should be removed promptly in the spring.

Unfortunately, no matter how much care you give them, delphiniums are not long-lived perennials. Even under the best of conditions, do not expect more than four or five years out of them. In warmer climates, they are often treated as annuals, planted afresh each spring.

'Black Knight' delphinium plant with semi-double blue-purple flowers with white centers closeup
The Spruce / Phoebe Cheong
'Black Knight' delphinium plant with blue-purple flowers on tall stems surrounded by buds
The Spruce / Phoebe Cheong

Light

Plant 'Black Knight' delphinium in an area that receives a daily dose of six hours or more of direct sunlight.

Soil

Grow delphiniums in a humusy soil that drains well. To ensure fertility, prepare the planting hole by mixing in compost. Delphiniums will tolerate neutral soil but prefer rather alkaline conditions (pH up to 8.0 or even higher). Acidic soils can need to be amended with wood ash or agricultural lime to raise the pH.

Water

'Black Knight' delphinium has average water needs—roughly 1 inch per week from rainfall and/or irrigation. Water young plants deeply to help develop strong roots. Keep the soil moist but not wet.

Temperature and Humidity

'Black Knight' delphiniums are part of the Pacific Series of cultivars, which are optimized for areas with cool to moderately warm summer climates, USDA zones 3 to 7. It performs much better where summertime heat is the exception rather than the norm. In hot, humid climates, these perennials may live only a year or two and are sometimes treated as annuals. Generally speaking, delphinium dislikes high humidity, even though they need regular water.

Fertilizer

Delphiniums need more fertilizer than most perennials. Apply a balanced fertilizer every two to three weeks from the time the plants emerge from the earth to the time they stop flowering. Either granular or water-soluble balanced fertilizer will work, but avoid foliar feeding.

Types of Delphinium

Delphinium is a very large genus including many perennial and annual species (the annuals are generally known as larkspurs), but most of the showy perennial types are cultivars of Delphinium elatum, a species originally native to Northern Europe and Siberia. There are dozens of different cultivars of D. elatum in various shades of purple, blue, pink, and white. In addition to 'Black Knight', some other popular varieties include:

  • 'Magic Mountain Lilac Pink', is 2- to 3-foot plant with lavender-pink flowers. Other small varieties in the magic mountain series include 'Sky Blue,' 'White', 'Lavender', and 'Blueberry Pie.
  • 'Princess Caroline' has bright salmon-pink flowers on 2- 3-foot stems.
  • 'Spindrift' has unusual pink, blue, and white flowers on 5- to 6-foot plants.
  • 'Cobalt Dreams' is a 4- to 6-foot plant with neon blue flowers with white centers
  • 'Black-eyed Angel' has pure white flowers with black centers, on plants that are 3 to 5 feet tall.

Pruning

'Black Knight' and most other delphiniums do well with a fair bit of trimming during the growing season. When the flower stalks become 3 inches tall in spring, thin them to ensure the remaining stalks are robust. For young plants, leave about two to four of the best stalks; for older plants, leave five to seven. During the bloom, deadhead the primary flowering stalks, leaving the smaller side-blooming flowers that are thriving. After the bloom, cut the flower stalks (not the whole plant) to the ground; in most areas, this will produce a second, smaller bloom late in the season.

Propagating 'Black Knight' Delphinium

Perennial delphiniums can be vegetatively propagated in a number of ways, including basal root cuttings or divisions performed in spring, crown cuttings taken in summer, or by rooting stem cuttings—but success can be unpredictable with each of these methods. Because these are short-lived perennials, the plants are steadily declining, and the various type of vegetative cuttings may not succeed with older plants. Of the vegetative options, the most reliable method is to take basal cuttings of the plant crown in spring:

  1. As new shoots begin appearing in spring, use a sharp knife to cut a shoot off the side of the plant, just below the surface of the soil, so that the shoot incorporates a healthy section of roots.
  2. Remove the bottom foliage from the cutting, but leave a few leaves to nourish the roots.
  3. Plant the cutting into a small pot filled with a mixture of compost and perlite. Water thoroughly, then place the planted cutting in a plastic bag.
  4. Place the enclosed cutting in a greenhouse or on a sunny windowsill, until the roots become well established and new growth starts. Periodically check the soil and water lightly if the potting mix becomes dry.
  5. Once new growth is well underway, the new plant is ready to transplant into the garden.

Growing 'Black Knight' Delphinium From Seed

Because vegetive methods of propagation are not always reliable, delphiniums are most often propagated by seeds. These plants mature fairly quickly when started from seeds, and if started indoors in February or March, the plants usually will reach flowering maturity that same year.

Sow purchased or collected seeds in flats filled with a mixture of standard potting mix and sand. Scatter the seeds over the flats, then cover them with a thin layer of peat moss. Cover the flat with a clear plastic cover and place it in a bright location at 60 to 68 degrees Fahrenheit. Germination usually takes 6 to 8 days but will take longer if the seeds are sown outdoors in cooler conditions. Keep the soil moist but not soggy.

At the point where true leaves begin developing on the seedlings, the plastic cover can be removed and the seedlings can be transplanted into individual small pots filled with standard potting mix. When soil temperatures are thoroughly warmed and all danger of frost has passed, the seedlings can be transplanted into the garden, after a week or so of gradual hardening off. In most cases, seeds started indoors in late winter will flower by late summer or fall.

Potting and Repotting 'Black Knight' Delphinium

Delphinium is a good option for container planting, partly because these short-lived plants are often treated as annuals anyway. In the framework of the "thriller, filler, spiller" planting strategy for mixed container gardening, delphinium is clearly a thriller: Its flower stalk shoots straight up for several feet, making it not only a vertical feature but also a dramatic centerpiece for the pot.

Use a large, deep pot with a support stake driven deep to the bottom of the pot. Because delphiniums need somewhat alkaline soil, it's best to mix a handful of agricultural lime with a peat-based potting mix.

Potted delphiniums can sometimes be successfully overwintered if you move the pots to a sheltered location after cutting the stems down to soil level. Another option is to bury the pots or heap them with compost to protect the roots for the winter. But more often than not, you'll need to treat potted delphiniums as annuals and plant them anew each spring.

Overwintering

Delphiniums can be quite prone to frost heaving in regions with very cold winter temperatures and low snowfall. In such regions where there is little snowfall to insulate the soil, it's important that the crowns be mulched with a thick layer of straw or another dry mulch, applied after the grown is frozen. Don't use standard compost or leaf mulch, as this can introduce root rot. And in the spring, this protective mulch should be removed promptly before it can become soggy and saturate the crown of the plant.

Common Pests & Plant Diseases

Garden delphiniums are considered one of the more temperamental perennials, susceptible to quite a number of serious pest and diseases. Aphids, leaf miners, stem borers, and mites can all be problems, treatable with horticultural oil or chemical pesticides. Slugs and snails can also devastate foliage.

These plants are also very prone to a variety of fungal and fungal diseases, including powdery mildew, leaf spots, and botrytis blight. These are often treatable with fungicides, and preventable by growing them in full sun, ensuring good air circulation, and watering at the base to avoid wet foliage. More serious are the crown and root rot diseases that often kill plants that are grown in poor-draining soil, or if the plant crowns are too deep.

How to Get 'Black Knight' Delphinium to Bloom

There can be a number of reasons why your delphiniums might not bloom in early to midsummer as you are expecting:

  • Plants are too young. Seeds that are direct-sown in the garden may not reach maturity in their first year. If you wait another year, you'll likely be rewarded with the expected June and July blossoms.
  • Plants are under fertilized. These plants need regular feeding every two or three weeks in order to support robust blossoming.
  • Plants are too old. Delphiniums in the elatum group rarely live more than 4 or 5 years, and they may begin to decline in their third year. If you notice that the center of your delphinium is beginning to die out, you may be able to take basal cuttings from the surrounding shoots, replant them, and discard the woody center of the plant.
  • Plants have not been deadheaded. Removing the early season blooms—and even cutting the plants down to near ground level—can help stimulate a second, smaller flush of flowers in late summer and fall.
  • Soil is too acidic. To properly take up nutrients, D. elatum needs a neutral to alkaline soil pH, and if forced to grow in acidic soil, blooming may be much reduced. If a soil test indicates the need to raise pH, a soil amendment such as agricultural limestone may be called for.

Common Problems With 'Black Knight' Delphinium

In addition to the many common disease and pest issues (see above), gardeners sometimes grow weary of the need to stake up these plants to avoid stem collapse, and also their short-lived nature that requires you to replant delphiniums every few years. There is really no solution to these complaints—either the spectacular blossoms are worth the high maintenance you will face, or you'll need to look for a different plant.

FAQ
  • How can I use this plant in the landscape?

    Along with hollyhocks (Alcea rosea) and foxgloves (Digitalis spp.), delphiniums are classic cottage garden plants for use in the back row of a mixed border garden. Used by themselves and planted in a mass, they can form a border or soften the look of a fence. Delphinium blooms make excellent cut flowers, and many gardeners plant them to enjoy their striking flowers indoors (after cutting, that is).

  • What are the other delphiniums groups?

    In additon to the D. alatum group, which includes 'Black Knight' and the other Pacfic Giant cultivars, there are two other very popular groups:


    • Delphinium belledonna group include hybrid crosses between D. elatum,  D. cheilanthum, and D. grandiflorum. They grow 3 to 4 feet tall and are hardy in zones 3 to 7.
    • Delphinium grandiflorum group, also known as Chinese or Siberian delphiniums, include more compact, 1- to 2-foot plants that have better heat tolerance than other groups. They are hardy in zones 4 to 7.



  • Are there any similarly spectacular plants that are easier to grow?

    The tall stature and deep purple flowers of 'Black Knights' can be approximated with monkshood, baptisia, and some varieties of lupine. All of these plants are considerably easier to grow than delphinium for most gardeners.