Delphiniums are well-loved by so many with their gorgeous, blue flowers that tower over the garden. With a few tricks, you can grow them year after year with two sets of blooms each summer.
I’ll share my best tips for growth, care, and propagation by cuttings and starting from seed.
Want two sets of blooms in one growing season? This shows you how.
Tips and Tricks For Growing Delphiniums
Years ago I accidentally became a very successful delphinium grower, first by breaking the rules, and then by learning how to propagate more plants so I could fill my garden with them on a low budget.
I hope you’ll find this information helpful, and please tell me any tips you want to share as well.
Delphiniums are native throughout the Northern Hemisphere (and mountainous areas of tropical Africa) with approximately 300 species.
- Gardening zones: 3 to 7
- Best Growing Conditions: Sunshine, loamy soil, wind protection (but I have been successful in full shade with clay soil as well)
- Extra Blooms Tip: Cut back spent flowers for second blooms in the growing season
- Seed Starting Tip: Pre-soak the seeds in a damp towel for 36 hours for best germination rate
- Spring Propagation Tip: Take basal root cuttings from existing plants to start new plants
- Fall Propagation Tip: You can also plant the seeds directly outside in the fall
There’s more details on all of these tips below.
My Love Affair With Delphiniums
I started gardening because of delphiniums. I saw some in a friend’s garden—tall, shimmering blues and purples—and I knew I had to have some. I always had a love of nature and gardens, but it was that moment that my life as a gardener began.
I purchased a few delphiniums at a local nursery but the problem was, the tags said they needed full sun and rich, alkaline soil. The only place I could plant them was in a side shade garden with hard clay soil. But I wanted them, so I planted them anyway. That’s one of the benefits of being young and determined—sometimes ignoring the “rules” brings results you would never otherwise discover.
As you can tell from what I’ve said, despite the conditions, my delphiniums flourished. And may I say, they didn’t just flourish, they exceeded all expectations, reaching heights of 8-10 feet tall. The clay soil stayed quite moist (which helped) but there was just 2 hours a day of dappled sunshine maximum. So don’t let your own less-than-optimal growing conditions deter you: they are fairly adaptable plants.
Growing Delphiniums From Seed
Delphiniums are notorious for being difficult to grow from seed. I ran some experiments to test germination rates. Here’s the results:
1. Straight out of the packet | 20% germination rate after 16 days
2. Chilled overnight in the freezer (with some insulation) | 50% germination rate after 16 days
3. Pre-soaked between damp towels for 36 hours. | 90% germination rate after 11 days (same on day 16)
Clearly, the seeds start successfully if they have been pre-soaked in a damp towels for a few days before planting. I started these ones indoors in moist, soil-less potting mix.
How To Start Perennial Seeds Outdoors In The Fall
This tip came from writer Doug Green in The Thrifty Gardener (book) by Marjorie Harris
- Place a bottomless 6″ (or larger) pot in the ground, leaving 1″ above the surface.
- Fill the pot with garden soil.
- Pour a kettle of boiling water over the soil to kill off any existing seeds in the soil.
- When the soil is cool, plant the seeds to depth indicated on seed package.
- Mark the spot with plant labels.
- The seeds will germinate in the spring. Transplant as desired.
A Faster Way To Get More Delphiniums: Take Basal Root Cuttings
- Taking basal root cuttings involves nabbing some roots in the spring when the new shoots are just a few inches tall.
- You’ll know your timing is good if the plant stalks are not hollow yet (they become hollow later in the growing season).
- I often miss my chance for this (spring fever!) but if you want to try it, there are detailed instructions online with photos—an easier way to understand it than having me explain it without having any photos to show it.
Another reasons my delphiniums probably flourished is because my shady side garden was protected from direct winds. The plant will still grow in windy conditions but often, when the flower heads are really full, they get so heavy that it’s easy for a gust of wind to snap the stalks.
Some delphinium hybrids are really top-heavy with narrow stalks. If you can, look for plants with nice, thick stalks. I’m really picky about colours – preferring the dark blues and almost-black purples, and it seems that those species tend to be stronger plants.
You’ll also want to provide stakes to help the delphiniums stand straight. I use tall bamboo stakes and some natural twine around the plants, just below the flower heads, without actually tying them up. This can really help prevent damage in a wind or rain storm.
Bloom and Bloom Again
If your growing season is long enough (I’ve grown them in zones 5 and 6), you can probably get two sets of blooms before winter. The trick is to cut off the spent flower stalks as soon as they are done. Just follow the flower stalk all the way to the ground and cut it off there. This gives the plant the time and energy to put toward another set of flowers. As I write this, I’ve got second blooms going at the end of October—right into first frost.
If your soil has been tested and runs on the acidic side, you can amend the soil with lime when planting delphiniums. I personally only ever added homemade compost when planting mine—and that was fine for my particular hard clay soil.
No Need To Mulch
Despite many years of garden folklore advice, delphiniums do not require mulching around the base. Just as trees suffer from having mulch piled up against their trunks, delphiniums will be subject to rot if covered this way. You can, of course, always mulch the surrounding soil, as this often works nicely to keep moisture in the soil and suppress weeds.
After a few years the root balls become quite large and the centre portion may die off. I like to divide my delphiniums in the spring. With the old stalks cut down the ground and a few new shoots coming up, I dig up the entire plant and divide it into 2 or 3 new plants, cutting away any dead middle area. Add some compost to a the new planting holes, water generously, and the plants should still flower in the new season.
Delphiniums As Cut Flowers
I grow hundreds and hundreds of flowering plants but I can rarely bring myself to cut any flowers to have indoors. I just love them how they are outside. This said, delphiniums do work as a cut flower. Cut them just as the flower buds are opening. The neatest trick I’ve heard is to fill their hollow stems with water and immediately submerge them in a vase. This will keep them looking fresh and beautiful as the blooms unfold for a good long time.