Q. Can you tell me how to grow and care for Delphinium?
Delphinium is also known as larkspur, a genus of the Ranunculaceae family with around 300 species under its umbrella. It was described as “lark’s heel” by Shakespeare, and knight’s spur by others (its monikers are many). And true to the classic notion of the beautiful and deadly, delphinium can be a killer if eaten in great quantities. Loved by many, the tall spires of deep blue make a grand statement and the broadening color choices now include indigo-blue, pale blue, pink, purple, white, red and multicolored varieties. A new nearly black cultivar has been recently been introduced.
These plants are identified with the gardens of New England because they grow best in climates where summers are relatively cool. It is not the easiest perennial to grow and should be viewed as a short lived perennial in the New York City area or treated as an annual. Many pests and diseases common to the garden - powdery mildew, root rot, blight, leaf spot, slugs, snails and aphids to name a few - afflict delphinium.
Delphiniums are lime lovers, requiring well-drained, fertile, humus-rich soil. They need to be planted in a garden spot that receives full sun or light shade. If starting from seeds, pre-soak for 24 to 48 hours for the best results. If you are introducing a potted plant from the nursery, don’t bury the crown; it may rot. Taller cultivars will need ample water, plentiful feeding and to be staked once they have reached a height of 2 feet. Take care to keep all delphinium evenly moist while in growth and to avoid spraying water on the leaves.
Feed a balanced fertilizer every two to three weeks. Cut back after flowering to small flowering side shoots. To increase the chance of rebloom in late summer, cut back to the base all stems that have flowered and feed.
Do not divide your plants in fall when they are dormant. Division should take place in early spring once growth has begun. Take care to allow a strong stem and plenty of roots in each division.
The best known delphinium are the Pacific hybrids named for characters from King Arthur’s court. Most grow to a height of four to six feet high though there are dwarf types, one to three feet high. The very tall spikes are packed with single or double flowers. These plants appreciate some light afternoon shade and struggle in a hot summer climate. They are unlikely to fare well in climate zones 7 and higher.
The Belladonna Group are 3 feet to 4 feet high and hardy in zones 3 to 8. They are bushy, seldom need support and actually like summer heat. And they are particularly nice for cutting.
The delphiniums that were on view in the NYBG's 2016 American Impressionism exhibit, were primarily Delphinium elatum which are fully hardy in zones 2 to 9. This is a group of showy hybrids, also known as English hybrids, that are considered the most impressive by many gardeners. Growing 2 1/2 to 3 feet tall, they produce closely packed spikes of regularly spaced, semi-double flowers, 3 inches to 4 inches wide. Somewhat confusingly, the exhibit also features plants known commonly as forking larkspur that are not in fact delphiniums and grow in two foot, bushy masses of flower.
The name larkspur is attached to other non-delphinium, garden plants that you may have encountered. Giant larkspur were once categorized as delphiniums but are today reclassified as Consolida regalis. These plants are easily grown from seed and produce flowers over the entire summer. They have fewer pest and disease problems and need less care, though deadheading is essential to prolong flowering. Like true delphiniums, they also prefer the cooler summers of the northeastern United States.
Courtesy of NYBG's Plant Information Service