How to prune delphiniums?

How to Prune Delphinium

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The genus Delphinium contains nearly 300 species of flowering plants found in cool, temperate environments around the world. Many cultivated varieties of Delphinium go by the common name larkspur, which refers to the unusual spur-shaped protrusion on the back of each violet-blue blossom. The 4- to 6-foot-tall flower stalks emerge in early summer and quickly fade, but the blooming period can be significantly prolonged if the plants are pruned immediately after the first bloom. Pruning Delphinium is a simple task, but precautions must be taken because all parts of the plant are toxic to humans.

Put on gloves before handling or working with Delphinium plants, which contain toxic alkaloids that cause severe skin irritation and gastrointestinal problems and convulsions or paralysis if ingested.

Grasp the spent flower stalk of the Delphinium to steady it. Reach into the center of the foliage and sever the spent flower stalk at the base, using bypass shears. Discard the flower stalk in a green waste bin rather than composting it.

Trim away yellow leaves as they appear during the growing season, using bypass shears. Snip the leaves away at the base. Discard the yellowed leaves in a green waste bin.

Continue removing the spent flowering stalks during the summer to prompt renewed flowering. Leave the last set of flowers in fall to dry out and produce seed, if desired. Cut off the stalk after the seeds have been gathered.

Cut away all the leaves within 2 inches of the ground in autumn to help the Delphinium overwinter with minimal damage.

Learn About Delphiniums

Common Delphinium Disease Problems

Aster Yellows: Leaf-like tissue forms where flower parts should be located. Petals turn green and become deformed. This virus-like condition is spread by leafhoppers. Burpee Recommends: Remove infected plants and control leafhoppers. Remove weeds in the area which serve as alternate hosts to the disease.

Bacterial Crown and Stem Rots: Leaves turn yellow and plants are stunted. Foul smelling ooze forms under wet conditions and stems may topple.Burpee Recommends: Remove and destroy infected plants.

Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.

Root Rots: A number of pathogens cause root rots of seedlings as well as mature roots. Burpee Recommends: Pull up and discard infected plants. Make sure your soil has excellent drainage.

Virus (Various causes): This causes conspicuous rings and line patterns appear on foliage and plants are stunted. Burpee Recommends: Remove and destroy infected plants. Never smoke in the garden as Tobacco Mosaic Virus can be transmitted from a smoker’s unwashed hands while handling plants

Delphinium Common Pests and Cultural Problems

Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.

Cyclamen Mite: These mites damage plants by sucking juice from stems and leaves. They multiply rapidity in hot, dry weather. They can only be seen using a magnifying glass. Plants will look distorted and stunted, and may not bloom. Flowers will be distorted, streaked and blotched. Leaves can become cupped, curled, dwarfed and thickened. Burpee Recommends: Discard plants that are severely infested. Avoid working with infested plants. Keep plants watered in dry weather. For heavy infestations consult your Cooperative Extension Service for insecticide recommendations.

Leafhoppers: Leafhoppers cause injury to leaves and stunt growth. They also spread disease. Burpee Recommends: Remove plant debris. Use insecticidal soaps. Consult your Cooperative Extension Service for other insecticide recommendations.

Leafminers: These insects bore just under the leaf surface causing irregular serpentine lines. The larvae are yellow cylindrical maggots and the adults are small black and yellow flies. They do not usually kill plants, but disfigure the foliage. Burpee Recommends: Remove affected foliage.

Stalk Borer: The larvae of this insect tunnel up and down inside the plant stem causing the plants to wilt. By the time the plant wilts it is too late to save it. The larva is 1.5 inches long, greyish brown with one dorsal stripe and two lateral stripes on each side. The lateral stripes on the front half are interrupted and the lower brown stripe extends forward onto the side of the head. The eggs hatch in May to early June, after the moth lays them the previous September or October. Burpee Recommends: Remove and destroy all plant debris and nearby weeds.

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How to Grow Delphiniums from Seed, a less expensive way to get more of this gorgeous perennial.

Delphiniums are a beautiful element of the classic English Cottage garden. They stand tall and stately (up to 6 feet) and are worth the fuss to keep them looking fabulous and healthy.

I show you how to grow delphiniums from seed in this post to get great germination rates.

I updated this post and at the end I will share how I started more seeds this year.

What type Delphiniums

This post I will focus on the Delphinium elatum, a hybrid that performs so much better than the Pacific hybrids. They are hardier and taller, while being much less mildew prone.

Where to buy Delphinium seeds

As far as buying the Delphinium elatum as plants the nearest to me supplier is over 3 hours away at Annies Annuals which is in Richmond, CA, and if you visit over on her site you will see she has only high praises for this particular type of Delphinium.

Instead of plants I chose to order the seeds from the breeder in New Zealand…Dowdesewell’s Delphiniums. I have had wonderful results. (they also have a list of growers that sell their Delphiniums as plants, you may have someone near you)

I have a friend growing Pacific Giants and I am giving them a try: Pacific Giants from Botanical Interests I will let you know my results.

The Pacific Giants have done very well. We have dry summers and very little humidity so mildew was not an issue. So feel free to grow these beauties.

Grow Delphiniums from Seeds

So do you want to know how I grow these beauties from seed?

(this is one way, I share another further down that eliminates the pre-germination part.)

It is very simple. When they first arrive I refrigerate or pre-chill the seeds for a week.

When I am ready to pre-germinate them I wet a coffee filter place the seeds on one half of the now wet coffee filter and fold over, insert into a large plastic bag (I used a zip loc bag) and I set it in an out of the way spot.

For me that was on top of the outside of the refrigerator.

(do not use a paper towel, the roots will penetrate it and you will have to cut them apart which is fiddly and messy),

I hold the bag up to a window and check it after about 5 days (Delphs germinate in about 7 to 10 days but you never can tell when you get an early bird).

From that day forward I check it daily and when I see a little white tail I know it is time to put the germinated seeds into pots.

I have some tubs with seed starting mix all ready to go… and I transfer the sprouted seeds to the tub.

Cover the seed oh so lightly with the mix or chick grit and press in. For more on how I start plants from seed

The not yet sprouted seeds get folded back up in the coffee filter and returned to the ziploc bag until they do germinate.

Pretty soon you should have a bunch of these..baby delphiniums..Yay!

Pot them up into individual pots as they grow taller and stronger.

Plant Your Delphiniums

Once planted in the garden you will need to protect them from slugs, for some reason they find Delphiniums a tasty delight.

This type Delphinium is a perennial and will return year after year.

They are a heavy feeders so side dress with compost throughout the summer. Staking will most likely be necessary too as the heavy blooms spiking towards the sky will topple over in a wind or stiff breeze.

In dry summer areas be sure to water regularly and deeply.

One day soon I would love to have a hedge of Delphiniums…most likely not practical but wouldn’t this be a show stopper!

via Dowdeswells Delphiniums

This beauty is next on my list. I don’t have any with a white bee…..yet!

Several shades of blue are in my collection but I am looking to add more..the pinks have caught my eye and one called Pagan Purples..and my oh my, the Lilac Ladies are dreamy! I wish I could order them all.

I tried sowing in containers in my greenhouse this past season without first pre-sprouting and it worked very well. My greenhouse has no supplementary heat, the delphiniums get the chill they need and start to sprout if sown at the optimum time. That would depend on your location.

I have not had success direct sowing in the ground yet. That may be because of the wild Springs we have. It can be freezing cold one day and warm and mild the next then back to a deep freeze.

Happy Delphinium Planting!

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Germination Experiment

Reader Feedback

I ran across your web site last fall when researching delphiniums. I have been collecting seeds from my delphinium plants for several years and have had zero success. Not even one seed ever sprouted. I decided that I needed to do a little research. I discovered your web page and read about your germination experiment. I decided to try it. Amazing! I think that every seed I planted must have sprouted! It was truly amazing! Thank you so much for passing on the information through your web site! You really are the Empress of Dirt! ~A.D.

I recently conducted an experiment to see which seed starting method yields the best germination rate. I used seeds from the same packet planted in seed starter mix under the grow lights. Just the seed preparation methods differed.

I tried these three things with the seeds

1. Straight out of the packet.
2. Chilled overnight in the freezer (with some insulation).
3. Pre-soaked between damp towels for 36 hours, stored in refrigerator.

Each set of seeds was then planted in seed starting mix as usual.

3 Delphinium Pruning Tips

The delphinium plant blooms in early June and requires pruning to promote a second flowering as well as more blooms.

Removing Faded Blooms

Removing faded blooms from a delphinium not only cleans up its appearance but also interrupts the reproductive cycle of the plant. The fading of a flower is the climax of a plant’s growth cycle when seeds are released to propagate the plant. If this process is interrupted, the plant begins its cycle again, thus encouraging repeat blooms. Remove spent blooms by pinching flower tips and gently pulling until released. Cut with pruning shears if they do not come away easily.

Cutting Back

Cutting old stalks back will give the delphinium renewed vigor and encourage a second flowering later in the summer. The remaining stalks must be removed at the crowns when the plants foliage begins to turn yellow. Ideally, for the delphinium to reach its full potential, only one flower spike should be allowed to develop in the first year, three in the second year and five in the third year. The delphinium is one of the few plants that will flower to “exhaustion” and this sequence will allow the plant attain its best.

Wintering

To weather the winter safely and to ensure a full bloom for the following year, the delphinium must be cut back hard. Stems should be pruned to within 1 or 2 inches of the ground.

It’s around this time of year when delphiniums begin to stand tall and proud as they bloom amongst our borders. With their vibrant colours and bold heights, these are certainly a flower we look forward to seeing every year.

We asked Chief Horticultural Advisor from RHS Wisley, Guy Barter, to share his top care tips.

Growing and planting…

  • There are two ways to begin your delphinium journey – through seed raising and through propagation by cutting.
  • Barter prefers cutting a shoot in spring as opposed to raising seeds every time as this is more likely to result in beautiful, tall flowers.
  • If you prefer to plant seeds, make sure they are fresh – Delphinium Societies are a great place to purchase.
  • For optimal results, it is best to grow them like cabbages and have a dedicated patch with very fertile (slightly alkaline) soil, lots of sun and water.
  • It is very important to keep delphiniums constantly moist and not to let the plant dry out – this could cause mildew to grow which can ruin the plant.

Dan KitwoodGetty Images

As they grow…

  • Support delphiniums with canes as they grow tall, such as these 120cm bamboo canes from Gardman.
  • They are very heavy feeders and require a lot of nutrients. At the start of the season, use mulch with manure to help keep them nourished.
  • Thinning is very important for the plant’s health. A newly planted one-year-old should have one spike, two-year-olds should have three spikes and an eight-year-old can have anywhere from 20 to 50 spikes.

Getty Images

Pests…

Slugs are a major problem for delphiniums. To find out how best to deter slugs, read our guide.

Barter recommends performing a garlic drench. You can either purchase this or make your own by boiling two pints of water and adding crushed garlic cloves. Boil for half an hour and then strain into a bottle. Keep the liquid in a cool place and add two teaspoons to a gallon of water and use to water the plants once a week.

To prevent powdery mildew from ailing the plants – a common problem in dry years – try planting them widely and thinning out the shoots as they emerge from the crowns early on.

What’s next…

Once your delphiniums have finished flowering, cut them to ground level and give them a sprinkling of fertiliser to help them grow healthily the following year. Continue this process for some years and they will blossom annually until they eventually get too tired.

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