Gadenin日本語Get the appWhat a good spike looks like
A delphinium flowers in early to midsummer, ahead of most of the tall border. Each stem ends in a dense vertical spike of florets that opens from the bottom upward over ten days to a fortnight, so the spike is at its best when roughly the lower two-thirds are open and the tip is still in tight bud. On an established, properly thinned clump that spike is a foot to eighteen inches (30-45 cm) of solid flower on a stem four to six feet (1.2-1.8 m) tall.
The tell-tale of a well-grown spike is that the florets touch each other all the way up, with no bare stem showing between them. Gaps mean the plant ran short of water or food part-way through building the spike, and you can often read the week it happened from where the gap sits.
A first-year plant flowers late and modestly, and a clump divided that same spring may not flower properly at all. Neither is a fault. Judge a delphinium in its second full season, not its first.
Cut spikes for the house when about a third of the florets are open; they carry on opening in water. Cutting also counts as deadheading, so it costs the plant nothing you were not going to take anyway.
The four things that cost you flowers
Work through these in order, because the first one accounts for more disappointing delphiniums than the other three combined. A plant that grows strong healthy foliage and still gives you short, thin, half-empty spikes has almost never got a disease problem; it has a resource problem, and the resource is usually being split too many ways.
The second most common cause is light. Delphiniums want a genuinely open, sunny position with at least six hours of direct sun, sheltered from wind but not overhung. In part shade they grow taller and softer, lean toward the light, flower a week or two later, and produce noticeably fewer florets on a thinner spike.
The third is water at the wrong moment. The critical window is the few weeks while the spike is extending and the buds are forming. Drought then is what produces a short spike or one with bald patches, and no amount of watering afterwards fills those gaps in.
The fourth is age. A clump left undivided for four or five years builds a woody, congested crown with the vigor only at the outer edges. It sends up plenty of shoots and cannot support any of them properly. Splitting it restores the plant completely.
| Symptom | Most likely cause | What to do |
|---|---|---|
| Many thin stems, all with short sparse spikes | Too many shoots left in spring | Thin to five or six of the stoutest when they are four to six inches (10-15 cm) tall |
| Tall soft leafy growth, few flowers, leaning | Too much shade or too much nitrogen | Move to a spot with six hours of sun; feed balanced, not high-nitrogen |
| Bald gaps part-way up an otherwise good spike | The soil dried out while the spike was extending | Deep soak weekly from the moment stems start to lengthen; mulch two inches (5 cm) |
| Plenty of shoots but every spike is feeble; clump is old | Congested woody crown | Lift and divide in spring, keeping only the vigorous outer pieces |
| Shoots come up and then stop, or vanish overnight | Slug damage to the growing point | Barrier and torchlight patrols from the moment shoots break ground |
| Buds form but the stem snaps before they open | Staked late, or not staked individually | One cane per spike, set at a foot (30 cm) of growth, tied every twelve inches (30 cm) |
Getting the second flowering in autumn
The single best trick with delphiniums is that a plant cut back promptly after its first flush will flower again before the season ends. It is not a marginal thing that sometimes happens; on a healthy plant that is watered and fed it is reliable, and the autumn spikes arrive when the border has very little else standing.
The timing is what matters. Cut as soon as the top florets of a spike are going over, before any seed pods swell. If you leave the spike until it has set seed, the plant has already committed its reserves and the regrowth will be weak or will not come at all.
Cut the spent stem back to a good leaf low down, about six to nine inches (15-23 cm) above the soil, rather than to the ground. Leaving that low foliage in place keeps the crown fed while the new buds break. Then water thoroughly and give a light balanced feed to fuel what follows.
Treat the new shoots as a second spring. Thin them if a forest comes up, cane the ones you keep, and stay on top of slugs, which are often more numerous in late summer than they were earlier in the year.
- Cut as the top florets fade Do not wait for the whole spike to brown. The moment the tip is going over and no pods have swollen, that stem's work is done and the sooner it is off the stronger the regrowth.
- Take the stem down to a low leaf Cut cleanly just above a healthy leaf six to nine inches (15-23 cm) from the ground. Wear gloves; the sap is an irritant and every part of the plant is poisonous.
- Water and feed straight away Soak the root area and apply a balanced general fertilizer at the label rate. The plant is being asked to build a second set of stems from scratch and needs the material to do it.
- Thin and stake the regrowth When the new shoots are a few inches tall, reduce them to four or five and put a cane to each one you intend to flower. Autumn spikes are shorter but they blow over just the same.
Record the date you cut each clump back and the date the second spikes opened. Do that for two seasons and you will know precisely how many weeks your garden needs between cut-back and repeat flowering, which tells you the last date in the season that cutting back is still worth doing.
Blind shoots, and the case for taking buds off
A blind shoot is one that grows to full height and simply never forms a spike, ending in a tuft of leaves instead. On delphiniums this is usually a shoot that was damaged at the growing point when it was small, most often by a slug that grazed it without destroying it, or by a late frost catching the tip. There is no cure once it has happened; cut the shoot out at the base so the plant is not carrying it.
The other reason for a missing spike is deliberate. On a plant in its first year from seed or from a cutting, taking off the flower buds as they appear puts everything into root and crown instead. You lose one season of flowers and gain a plant that comes back the following spring with the strength to carry real spikes.
Do the same with a clump you have just divided. A division that flowers in its first summer establishes badly, because it is trying to build a crown, new roots and a spike at once with very little root behind it. Pinch out the bud and let it grow.
Beyond the first year, the only routine bud removal worth doing is on the weakest of the stems you decided to keep. If one of your five is clearly behind the others as the buds form, cut it out rather than let it take water and food to produce a spike you will not want to look at.