How and when to sow delphinium seeds for vivid, towering displays

The Delphinium Society advises 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”. 

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).  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. 

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. 

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 head gardener 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.”