How to Grow 'Black Knight' Delphinium

A Dark-Flowered Perennial

'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 gain delphiniums so much attention. The 'Black Knight' cultivar of delphinium sports semi-double, dark purple flowers. Grow 'Black Knight' when a mousy little wallflower of a plant just won't do. Plant this moderate-to-fast grower in the spring for best results.

Botanical Name Delphinium ('Black Knight' is a cultivar name)
Common Name Larkspur, 'Black Knight' delphinium, 'Black Knight' larkspur
Plant Type Herbaceous perennial
Mature Size 4 to 6 ft. tall, 1 ft. wide
Sun Exposure Full
Soil Type Moist, well-drained, fertile
Soil pH Neutral to acidic, alkaline 
Bloom Time Summer, fall
Flower Color Dark purple
Hardiness Zones 3 to 7 (USDA)
Native Area  North America 
Toxicity  Toxic to animals 

'Black Knight' Delphinium Care

As tall, floppy plants, Black Knight delphiniums require 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. So it is better to focus on giving them optimal drainage in winter rather than mulching them. If you do apply mulch, keep it away from the crown.

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 years out of them. In warmer climates, they may be treated as annuals.

'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 like neutral soil; if yours is acidic, amend with wood ash or lime to add alkalinity.

Water

'Black Knight' delphinium has average water needs. 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. It performs much better where summertime heat waves are more the exception than the norm. In hot, humid climates, these perennials may live only two or three years.

Fertilizer

Delphiniums need more fertilizer than you may be used to giving your 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.



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.

Growing 'Black Knight' Delphinium in Containers

Delphinium is a good option for container planting, partly because it doesn't live a long time, and most gardeners' pots tend to see a lot of turnover anyway. In view of the "thriller, filler, spiller" planting strategy, delphinium is clearly a thriller: Tts flower stalk shoots straight up for several feet, making it not only a vertical feature but also a dramatic centerpiece for the pot.

Landscape Uses for 'Black Knight' Delphinium

Along with hollyhocks (Alcea rosea) and foxgloves (Digitalis spp.), delphiniums are classic cottage garden plants for use in the back row of a mixed flower bed. 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).