How and when to sow delphinium seeds for vivid towering displays

Calling all cottage-core enthusiasts: if delphiniums aren’t already part of your blooming borders, they should be.

They are King Charles’ favourite flower, and rightly so. These majestic giants can stretch to heights of 2m (6ft 5in), with more than 400 cultivars to choose from. They offer architectural contrast to lower lying cottage garden-style essentials like hardy geraniums and lavender.

Growing delphiniums from seed can be a cost-effective way to introduce them to your garden. But do you know when to sow delphinium seeds?

Getting started at the right time of year is essential if you don’t want to wait a whole year for them to flower.

How to sow delphinium seeds

Knowing when to sow delphinium seeds is one thing, but knowing how to execute it properly is another. It’s not quite as simple as “sow and go”; delphinium seeds require a little more TLC to establish themselves.

The Delphinium Society has shared its best practice for sowing delphinium seeds:

1. Sowing (day one)

Sow seed thinly on the surface of moist compost in a pot or a tray. Add a thin layer of compost or vermiculite to cover the seeds.

Ensure the compost is thoroughly wet by placing the pot/tray in a bowl of water for a few minutes.

Remove the pot/tray from water and cover with foil, plastic or glass and keep out of direct sunlight and under 20°C (68°F).

Stand the pot/tray where the temperature is 15-20°C (59-68°F); higher temperatures can cause germination to fail, lower temperatures can result in slow germination. There is usually no need for further watering until germination. 

2. Germination (days 14-28)

Check the pots/trays daily for signs of germination. Once you have five or more seedlings, remove the cover.

Move into good light but shade from direct sunlight, which can scorch seedlings. Keep the surface of the compost moist so the other seeds can germinate.

If the compost is frequently dry then cover the pot/tray with a polythene bag to reduce evaporation. Once germination is almost complete, open the cover to increase ventilation and harden growth. 

3. Pricking out (three-to-four weeks after germination)

Wait for the first true delphinium leaves to appear before pricking out.

True leaves are those that grow after the first pair of “cotyledon” leaves, which hatched from the seed; true leaves will look slightly different to these, often resembling the leaves of the mature plant. 

Seedlings can now be transplanted into individual pots of general compost, keeping them moist but not wet. Keep pots in full light but protect them from direct sunlight for the first few days.

Stand pots on the bench of a cool greenhouse or cool windowsill until they establish in the compost. They will then grow rapidly if kept outside, but protect them from heavy rain or frost. 

4. Planting out (mid-May to mid-June)

Seedlings should now have three or more adult leaves and their roots will fill the pot. Plant in well prepared, fertile soil approximately 60cm (2ft) apart.

If you want to check flower colour before moving them into their final position, keep them closer together, around 30cm (1ft) apart. 

Delphiniums need plenty of weed-free space and a sunny location to thrive. A spoonful of dried blood, fish and bonemeal (high-nitrogen) fertiliser mixed in the planting hole will help boost growth.

Protect plants from slugs and snails by putting coarse sand around the stems. Keep the soil moist by watering when dry, or adding a mulch between plants.

Seedlings grow rapidly and should produce a sturdy flower stem that thickens as it grows.If the stem is small and spindly then pinch it out to force new growth from the base.

Support the flower stems with canes and twine and wait for the flowers to open. 

When to sow delphinium seeds

The advice is that you can technically sow delphinium seeds at any time of year, but that “seeds sown late winter or early spring can usually flower within six months of sowing.

"For best results sow seeds indoors or in a greenhouse from mid-January to late March.”

Luke Bartle, head gardener at Trentham Gardens, a visitor attraction in the heart of the Trentham Estate in Staffordshire, told us early springtime is best for sowing delphinium seeds – “from late February to March.

"This should give your delphiniums the best chance for planting out and flowering the same year.”

Delphinium specialist grower Blackmore & Langdon agrees, saying February is ideal to start sowing, even though it’s possible to sow both earlier and later.

Keeping a strict sowing schedule will be worth it. Guy Barter, chief horticulturist at the Royal Horticultural Society, told us you’re “aiming to plant out in mid-summer, then the plants will establish well and might even flower in September”.

Keeping your delphinium seeds fresh

Don't waste your time on old seeds

If you find an old packet of delphinium seeds in the shed and decide to sow them, don’t be too surprised if your efforts turn out to be wasted. “It is known that delphiniums lose viability after a year, so sow the entire packet within a year,” says head gardener Bartle. “This is because they go off quickly – some seeds deteriorate quicker than others."

Seed viability depends on the condition of the seed when first stored, how long it is stored and what seed is being kept.

Sowing delphinium seeds can be hit and miss, with success depending on variety. Not all types re-seed, and they are not guaranteed to be identical to parent plant.

“Sowing delphiniums seeds have the best start with cold stratification (exposing the seeds to cold temperatures to break dormancy – copying nature)," says Bartle.

"Some people pop their seeds in the fridge to get this going for about a week before sowing.”

This is what the folks at the Delphinium Society suggest, too: “Keep seeds fresh by putting them in an airtight container in the fridge until you are ready to sow them.”

This should give your seeds a helping hand when it comes to germination.

If you’re curious to test this out, you could try a batch that you refrigerate and a batch you don’t, and compare results.

Ways to grow delphiniums

You can propagate or divide

If you’d like to try growing more delphiniums, sowing seeds from scratch isn’t the only approach.

You can propagate plants from basal cuttings (fresh shoots at the bottom of the plant) or by dividing larger clumps of existing flowers.

“Young shoots are taken in spring for cuttings,” explains the RHS’s Barter. 

“They can be quite tricky though, so I am not so sure they are easier [than sowing delphiniums from seed]. We root ours in pure perlite.

"Now that seed-raised delphiniums are so much better than they used to be, I feel seed is the way to go for most gardeners, but nothing matches the selected clonal forms offered by top delphinium growers. These have to be propagated by cuttings.”

Meanwhile Bartle suggests cutting a shoot from just below the soil, cutting away some foliage, leaving one or two leaves:

“Get it in the potting compost with good drainage (delphiniums don’t like wet ground). Keep in a greenhouse or sunny windowsill to root well before pricking out and potting on.”

For preference, Bartle likes to divide large mature clumps of delphiniums, and has never grown the flower from seed:

“Do this in spring and keep it watered – new plants should then flower the summer of the same year. You’ll have near enough guaranteed success of getting the same plant.”

“Be careful to ward off slugs,” adds the RHS’s Barter. “They are partial to delphiniums and a seedling can disappear overnight.”  Find out how to keep slugs out of the garden.