how to plant, grow & care for delphiniums
complete growing guide
Delphiniums put on a magnificent show each summer, like rockets or fireworks that have left their blazing trail standing in the air. They are the most vigorous and meaty verticals the summer garden can provide; eremurus and foxgloves are somehow more delicate, making less of a solid block, but the forest-like density of delphinium colour is an eye-catching and unmissable contribution to the garden.
In tones of deep indigo, clear blue, vibrant purple and pure white, delphiniums can make a lot of noise. To be sure it’s a full concerto, plant in threes and create blocks of colour, rather than dotting them about a border, that way you’ll get their full impact, and have some for the vase too.
Explore our full range of delphiniums, which are ready to plant out in spring or autumn.
details
- Common name: Larkspur
- Latin name: Delphinium
- Type: Hardy Perennial
- Height: 1.5 (5ft) to 2m (6½ft)
- TLC rating: Moderately easy
- Aspect: Full Sun, Sheltered
- Planting position: Back of border
- Suitable for pots: No
- Good for pollinators: Yes
- Good for cut flowers: Yes
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how to grow delphiniums
where to grow delphiniums
Soil type: Delphiniums love well-drained soil that’s rich and fertile. This means adding plenty of grit on heavy soils to help with soil drainage, as well as adding well-rotted organic matter to the planting holes.
Aspect & position: A sunny, well-sheltered position is best for delphiniums.
when to plant delphiniums
Sow delphinium seeds undercover in March-April. Or sow undercover in August, overwintering in a cold frame, to plant out the following spring. Plants grown from seed will flower the following year.
Plant young delphinium plants in the garden in April-June, or September-October.
how to plant delphiniums
sowing delphinium seeds
Plants grown from seed will flower the following year, but it’s a cost-effective way to get lots of plants for the garden.
Sow undercover in a seed tray filled with peat-free compost and water lightly (do this first to avoid displacing the seeds). Sow the seeds thinly, then lightly cover with vermiculite. Don’t forget to label them.
Once the seedlings have three pairs of true leaves, prick them out each into their own individual 9cm (4in) pot with peat-free potting compost with added grit. Again, don’t forget to label them. Let them grow on until they are ready to plant out in the garden in May-June.
Before planting out, harden off your plants by allowing them to acclimatise to the outdoors bit by bit over a couple of weeks.
planting out delphiniums
Choose a site in full sun or dappled shade. Delphiniums need well-drained soil so add plenty of grit to heavy soils to help with drainage. If delphiniums sit wet in their dormant winter season, they can die, so make sure you prevent this with good drainage.
They also need to be in soil that is rich in nutrients, so add organic matter to the planting hole. Add a slow-release fertiliser, such as blood, fish and bone, a scattering of bone meal or organic chicken manure pellets. After that, you can use a seaweed fertiliser for feeding.
Plant 60cm (2ft) apart – they are best in groups of two or three, but they need good air circulation around them to prevent powdery mildew. Once they’re growing, thin out shoots emerging from crowns early in the season to increase air circulation and help prevent mildew forming.
Delphinium shoots are very tempting to slugs. The plants break through the ground in March and if you don't watch out, slugs can have every one. That's the moment for employing maximum protection, whether that’s nemaslug or another form or preventative.
Delphiniums will need sturdy staking to keep their hollow stems upright. See more on this in the staking advice below.
how to care for delphiniums
watering
Water regularly during the summer.
fertilising
Delphiniums are hungry feeders, so feed a couple of times through the summer, then again in autumn before the plants die down. Use a slow-release fertiliser only once when planting, then feed with seaweed fertiliser to give them a boost.
staking
Staking is vital for delphiniums; any strong wind or heavy rain will flatten them and break their hollow stems.
You can sink three bamboo canes around each plant and use string to create a ‘cage’ support. This looks fine in a full border where you don't see the base of the plants. Alternatively, you can buy a plant support ring or a plant support grid – placed at about 1m (3ft) high, the grid allows the delphiniums to grow through and up, and becomes almost invisible as they mature.
deadheading
Delphiniums will flower in June and July. After this first flush and when the flowers have faded and are looking a bit tatty, cut the flowering spikes right down to ground leaving the foliage in place. It feels harsh, but if you cut back you’ll get a second flush of flowers in August and September.
propagating
Delphiniums can be grown from seed, or propagated by basal cuttings.
Take basal cuttings in spring when the plant is in active growth. Cut a shoot below the soil level (the stem above the soil level is hollow and won’t root) and pinch off the side leaves.
Insert a couple of cuttings into a 9cm (3½in) pot filled with gritty compost and leave on a heated propagator bench to root. When the roots peak through the holes at the bottom of the pot, pot on into individual pots.
overwintering
Delphiniums are hardy perennials so can survive the winter and return the following year. They can be damaged by waterlogged soil – if delphiniums sit wet in their dormant winter season, they can die. Make sure the soil is free-draining and doesn’t become compacted and soggy.
seasonal checklist
spring
- Sow delphinium seeds undercover in early spring for flowering plants the following year.
- Plant out young delphinium plants in late spring and early summer.
summer
- Stake delphiniums while they are still young to give them support as they grow.
- Water and feed delphiniums regularly.
- After flowering in June and July, cut flowering spikes down to ground for a second flush of flowers in August and September.
autumn
- Plant young delphinium plants to establish in time for next summer.
- Feed the delphiniums as they’re dying back for stronger plants the following year.
winter
- Plan which delphiniums you’d like to sow and grow in the new season.
pests, diseases & common issues
slugs
Delphinium shoots are very tempting to slugs. The plants break through the ground in March and if you don't watch out, slugs can have every one. That's the moment for employing maximum protection, whether that’s nemaslug or another form or preventative.
One tip I picked up at delphinium specialist Blackmore & Langdon is the use of a garlic drench to deter slugs. You can make your own with two crushed garlic cloves added to two pints of water. Boil for half an hour and then strain off the liquid and bottle. Plastic milk cartons are good. Keep the liquid in a cool place. Once a week add two teaspoons of drench to one gallon of water and water the plants, drenching a good circle of soil around them. Do this during the delphiniums’ early growing season of March and April.
powdery mildew
In a dry year, delphiniums can suffer from powdery mildew. It’s a fungal disease that shows up as a powdery coating on the foliage.
For prevention, plant your plants widely and thin out the shoots emerging from the crowns early in the year to allow for air circulation. You can also remove any affected leaves and try an organic solution such as homemade comfrey tonic to keep mildew at bay.
black blotch
Delphinium black blotch is a bacterial disease that can be a problem in wet, humid summers. You’ll notice black spots on the leaf surface (the leaf might turn brown) and a lack of vigour and stunted growth. If a delphinium is affected, lift it and dispose of it, and avoid replanting delphiniums in the same spot.
do slugs eat delphiniums?
Slugs love delphinium shoots so make sure you protect the plants early in the season.
why are my delphiniums dying?
Delphiniums that have sat cold and wet through winter may die, so ensure good drainage and use a mulch over winter. A slug attack when the plant is young might also impact vigour and cause the plant to fail. You should also check for black blotch.
why are my delphiniums drooping?
Ensure the plants are well watered during droughts. Also make sure they are staked effectively – the hollow stems combine with heavy flowers can mean they can bend and even snap.
what keeps eating my delphiniums?
This is likely to be caterpillars (which don’t cause a problem for delphiniums) or slugs.
frequently asked questions
are delphiniums poisonous?
Yes, delphiniums are very toxic to dogs, other pets and humans.
when do delphiniums flower?
Flowering time for delphiniums is June and July, with a second flush of flowers in August and September if cut back properly.
when should you cut back delphiniums?
Delphiniums will flower in June and July. After this first flowering, cut the flowering spikes right down to ground and you’ll get a second flush of flowers in August and September.
do delphiniums self seed?
Delphiniums do self seed but are unlikely to come true to the parent plant.
are delphiniums perennial?
Yes, given proper care, delphiniums will return year after year.
how do you stake delphiniums?
Staking and support is essential for delphiniums as they are tall, heavy plants with hollow stems. You can sink three bamboo canes around each plant and use string to create a ‘cage’ support. This looks fine in a full border where you don't see the base of the plants. Alternatively, you can buy a plant support ring or a plant support grid – placed at about 1m (3ft) high, the grid allows the delphiniums to grow through and up, and becomes almost invisible as they grow.
how tall do delphiniums grow?
Delphiniums can grow taller than 2m (6½ft) and bring beautiful height to borders.
how do you dry delphinium petals?
The easiest way is to harvest fresh delphinium stems and then hang them upside down somewhere cool, dry and dark such as a shed or cellar. The petals should dry in a couple of weeks and can be gently pulled from the stems.
how to cut & arrange delphiniums
Delphiniums make great cut flowers in summer and will last up to 7 days in a vase.
Pick when most flowers are open. If you’ve been brave enough to pick them, you want the flowers to last as long as possible, so use one of these methods.
For tall spires, insert a cane into the hollow stem to avoid it bending and breaking. Alternatively, use water – sink the stem into the vase, then put your thumb over the end of each stem and lift it out. Then lower back into a vase full of water.
The final tip is to keep your arrangement away from fruit bowls as they are sensitive to ethylene gas which is omitted when fruit ripens.
Get more inspiration for displaying your flowers with our flower arranging videos:
