Set out potted delphiniums in spring or early summer (RHS), in full sun and fertile, well-drained soil, sheltered from wind (NC State, Nebraska, RHS). From seed, start indoors 10 to 16 weeks before planting out and transplant after the last frost (Johnny's); expect flowers in about 18 months (RHS). Divide clumps in early spring (RHS). Put stakes in early, before growth reaches about a foot (RHS, Nebraska). Delphiniums like cool summers (NC State). All parts are poisonous to people and pets (ASPCA, NC State, Nebraska).
Delphiniums are tall perennials in the buttercup family, grown for spikes of blue, purple, pink or white flowers in early to midsummer (NC State, Nebraska, RHS). Most garden plants are hybrids of the bee delphinium (Delphinium elatum) (Nebraska, NC State). Nebraska notes that the annual larkspurs gardeners also grow are now usually placed in a separate genus, Consolida. Sizes vary widely: NC State lists short D. elatum types at 3 to 5 feet and tall ones over 6 feet, and Nebraska gives 1 to 4 feet for belladonna delphiniums. They are short-lived: Nebraska says most last 2 or 3 years, Johnny's says about 3 seasons, and NC State gives 3 to 5 years for D. elatum.
When to plant delphiniums
| Timing | What the sources say |
|---|---|
| Late winter (seed indoors) | Johnny's sows 10 to 16 weeks before planting out; germination takes 10 to 20 days at 65 to 70°F, and it advises against direct seeding. RHS sows in spring at about 55°F (13°C) and says seedlings appear in about 14 days. Use fresh seed; old seed can germinate erratically (RHS) |
| After last frost (seedlings) | Harden off and transplant after the last frost (Johnny's). RHS says plants from seed flower in about 18 months |
| Spring to early summer (potted plants) | RHS plants delphiniums in spring and early summer as they appear in garden centers, and says any time works if you water in dry spells and the soil is not frozen or waterlogged |
| Early spring (dividing) | RHS divides mature clumps in early spring every 2 to 5 years, into 2 to 4 pieces, discarding the woody center; divisions should flower the same year |
| Late spring (cuttings) | RHS roots pencil-thick basal cuttings 3 to 4 in long, taken before the shoots turn hollow, since hollow shoots tend to rot |
Our planting calendar counts back from your last frost date: 10 to 16 weeks before it is the window for starting seed indoors (Johnny's). Potted plants are the faster route to flowers this summer. In hot, humid areas, NC State says delphiniums are a challenge south of zone 7, so give them afternoon shade there. In our view, gardeners in the hot South may have better luck treating them as a cool-season show (GardenPal reasoning).
How to plant delphiniums
| Delphinium | |
|---|---|
| Light | Full sun; flowering is poor in shade (RHS). Afternoon shade in hot climates (NC State) |
| Soil | Fertile, humus-rich and well drained (Nebraska, NC State); not wet in winter (RHS). NC State says it tolerates alkaline soil; Johnny's prefers slightly alkaline |
| Spacing | 12 to 18 in as a perennial (Johnny's). RHS says clumps spread 18 in or more, up to 3 ft for Elatum types, and grow best uncrowded |
| Depth | Don't plant too deep: NC State warns crown rot follows deep planting or poor drainage |
| Shelter | A spot out of strong wind and heavy rain, or along a fence (NC State, RHS) |
Staking, feeding and rebloom
- Stake early. The flower spikes are hollow and brittle (Nebraska). RHS puts supports in before growth reaches 1 foot, and Nebraska sets stakes early so they don't damage roots later.
- Thin for bigger spikes. On mature clumps, RHS keeps the 5 to 7 strongest shoots in spring.
- Feed. Nebraska calls them heavy feeders and fertilizes in early spring and again later; NC State calls for frequent feeding. RHS feeds container plants but says plants in good garden soil shouldn't need it.
- Water and mulch in dry spells (Nebraska); RHS mulches with compost in spring.
- Cut back for a second show. Cut spent spikes down to the basal leaves for more flowers in late summer or fall (NC State, Nebraska, RHS); the second flush is smaller (Nebraska).
- Cut flowers last about 6 to 8 days in a vase (Nebraska).
Delphiniums in containers
RHS suggests the shorter Magic Fountains series, about 3 feet tall, for the front of borders and for containers. For permanent planting it uses a loam-based potting mix with added grit for drainage. In-ground plants shouldn't need feeding, but RHS feeds container plants because they have less access to nutrients. Given how much delphiniums dislike wet winter soil (RHS), in our view a pot also needs a free-draining hole and should never sit in a saucer of water (GardenPal reasoning).
Fall and winter
- Tidy up after frost. RHS cuts back old leaves once they die down after the first frosts.
- Hardiness: NC State lists zones 3 to 9; Johnny's gives zones 3 to 7. Good drainage matters most in winter (RHS).
- Plan replacements. Because plants live only a few years (Nebraska, Johnny's), start new plants or divide every few years (RHS).
Pests and problems
Slugs and snails are the main pest, especially on young spring growth (RHS, NC State). NC State also lists aphids, leaf miners, stem borers and mites, plus powdery mildew, botrytis blight, leaf spots and crown rot; plants in full sun resist powdery mildew better. RHS says mildew is worse on stressed plants in shade or dry soil. Nebraska notes the 'Mid-century' hybrids resist powdery mildew better.
Delphiniums around the vegetable garden
Keep delphiniums in the flower border, away from vegetable and herb beds and from where children and pets play, since every part is poisonous (Nebraska, ASPCA). Bumblebees visit the single-flowered types (RHS). Their tall spikes suit the back of a sunny border. For other tall cottage-garden flowers, see when to plant hollyhocks, when to plant foxgloves, when to plant peonies and when to plant snapdragons.
Frequently asked questions
When should I plant delphiniums?
Potted plants in spring or early summer (RHS). Seedlings go out after the last frost, from seed started 10 to 16 weeks earlier (Johnny's).
Do delphiniums bloom the first year from seed?
Usually not. RHS says plants from seed flower in about 18 months.
Do delphiniums need to be staked?
Tall types do. Their spikes are hollow and brittle (Nebraska); put supports in before growth reaches about 1 foot (RHS).
Will delphiniums bloom again?
Often. Cut spent spikes back to the base for a smaller second flush in late summer or fall (NC State, Nebraska, RHS).
Are delphiniums poisonous to dogs and cats?
Yes. The ASPCA lists larkspur (Delphinium) as toxic to dogs, cats and horses, and NC State rates it a high poison risk for people too.
How long do delphiniums live?
Most last 2 or 3 years (Nebraska); NC State gives 3 to 5 years for D. elatum.
Enter your frost dates to time indoor delphinium sowing and a spring transplant.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- NC State Extension Gardener Plant Toolbox, "Delphinium"
- NC State Extension Gardener Plant Toolbox, "Delphinium elatum"
- Nebraska Extension in Lancaster County (Sarah Browning), "Delphiniums: a beautiful garden flower"
- Royal Horticultural Society, "How to grow perennial delphiniums"
- Johnny's Selected Seeds, "Delphinium: key growing information"
- ASPCA Animal Poison Control, "Larkspur"
- USDA Agricultural Research Service, Poisonous Plant Research, "Larkspur (Delphinium spp.)"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.