The Delphinium Year: What to Do from the First Shoots to the Autumn Cut-Back

Late winter into early spring: get ready before anything shows

The most important work of the delphinium year is done while there is nothing to look at. As soon as the worst of the cold has passed, go round each clump and clear away every dead leaf, old stem and clod of loose mulch within a hand's width of the crown. That litter is where slugs spend the day, and leaving it means they only have to travel a few inches at night to reach the shoots.

While the ground is bare and you can see the crown, work an inch (2.5 cm) of garden compost into the soil around it, keeping the material off the crown itself. On heavy ground that lies wet, this is also the moment to fork in grit around the plant, because crown rot over winter is the other way delphiniums die.

Have the slug barrier ready before the shoots break, not after. Grit, a copper collar, whatever you use, it needs to be in position on the day the first tip shows. Damage on a delphinium is not something you respond to; by the time you see it the shoot is gone.

This is also when you check labels and note which clump is which, before the bed fills up and every plant looks the same.

Spring: emergence, thinning, cuttings and canes

Four jobs fall in a few weeks and the order matters. From the moment the crown moves, run the slug defense: barrier in place, ground clear, and torchlight checks on the first mild damp evenings. Keep it up until the stems are eight to ten inches (20-25 cm) tall, at which point the plant is out of danger for the spring.

When the shoots are three to four inches (8-10 cm) and still solid, take basal cuttings if you want more plants, cutting each shoot with a sliver of the crown attached. At four to six inches (10-15 cm), thin the clump: snap out the weakest shoots at the base until five or six of the stoutest remain on an established plant, four on a second-year one, two or three on a first-year one.

Feed once with a balanced general fertilizer as the retained shoots reach six inches (15 cm), then mulch two inches (5 cm) over the root area, pulled back from the crown. Do not use a high-nitrogen feed; it produces soft stems that flop and the growth mildew prefers.

At about a foot (30 cm) of growth, put a cane to every stem you intend to flower and make the first tie at eighteen inches (45 cm). Canes go in now because you can still push them past the fleshy roots without damage, and because the stem then grows against the support instead of being bent to it later.

1. At emergence: barrier and patrols Clear the crown, lay grit or a collar, and go out with a torch on the first mild damp nights. This window closes when the stems reach ten inches (25 cm) and it is the one that decides whether the plant survives.
2. At three to four inches (8-10 cm): take cuttings Cut the shoots you are about to remove anyway, each with a heel of crown, while they are still solid through the base. Hollow shoots rot instead of rooting.
3. At four to six inches (10-15 cm): thin the clump Reduce to five or six of the fattest stems, evenly spaced. This one decision sets the spike size, the standing ability and the mildew risk for the whole season.
4. At six inches (15 cm): feed and mulch One dressing of balanced fertilizer, then two inches (5 cm) of compost or bark over the root area, kept an inch or two clear of the crown.
5. At a foot (30 cm): cane every stem One cane per spike, set on the windward side, first tie at eighteen inches (45 cm) and another every twelve inches (30 cm) as the stem extends.

Early summer: extension, flowering and cutting back at once

While the spikes are extending, water is the variable that decides how good they are. Deep soakings once or twice a week that wet the top eight inches (20 cm) beat daily sprinkling. Drought during this few-week window is what produces short spikes and bald patches part-way up, and nothing you do afterwards fills those gaps. Water at the base in the morning, never over the foliage.

Keep adding ties as the stems lengthen and check the existing ones weekly, slackening any that have tightened. Stems thicken fast in the last weeks before flowering. A light second feed as the buds color is worthwhile; stop feeding once the florets start to open.

The spikes open from the bottom up over ten days to a fortnight. Cut for the house when about a third of the florets are open. Anything you cut is one less spike to deadhead later, so take what you want.

Then the pivotal moment: as the top florets of a spike go over, cut that stem down to a good leaf six to nine inches (15-23 cm) from the ground, before any seed pods swell. Do this promptly, spike by spike, rather than waiting for the whole clump. Water and give a light balanced feed immediately afterwards to fuel the regrowth.

Late summer and autumn: the second flush, then hygiene

Within a couple of weeks of the cut-back new shoots appear from the crown, and on a plant that is kept watered and was fed at the right moment they flower again before the season ends. The autumn spikes are shorter and slimmer than the first set, but they come when the border has little else standing.

Treat the regrowth as a second spring. Thin the new shoots to four or five, cane the ones you keep, and go back on slug patrol, because late summer slug numbers are often higher than spring ones and the new shoots are just as soft. Keep watering through any dry spell; a plant that has just rebuilt itself has no reserve to draw on.

Once the second flush is over and the foliage yellows, cut the whole plant down to two or three inches (5-8 cm) and clear away every leaf and stem fragment from around the crown. Do not leave the debris in place as a mulch. This is one perennial where autumn tidiness genuinely earns its keep: that litter is where slugs overwinter and where mildew spores survive to reinfect next year's shoots. Bin, rather than compost, anything off a clump that had mildew or rot.

Finally, on wet ground, make sure the crown is not sitting in a hollow that collects water over winter, and keep mulch pulled back from it. Winter wet on the crown is the main cause of a clump that simply never comes up again in spring.

Keep a short log with four dates for each clump: first shoots through, first slug damage, first spike open, and cut-back. Two seasons of that tells you when your barrier must be down, how long your garden needs between cut-back and the second flush, and therefore the last date in the season that cutting back is still worth doing.

A standing rule for the whole year

Every part of a delphinium is poisonous if eaten, and the sap irritates some people's skin, so gloves belong with every job on this page: thinning, cutting, staking, dividing and clearing up. Keep the cut material away from children, pets and any ground that livestock graze, and wash your hands afterwards.

The other rule worth holding in your head is that delphinium work is almost all preventive. Slug protection has to be in place before there is damage, canes have to be in before the stem needs them, thinning has to happen before the plant has spent itself on twelve stems, and the cut-back has to happen before seed sets. In each case, acting after you see the problem is too late.

That is why the trigger column in the table above is written in terms of what the plant is doing rather than what month it is. Read the plant, not the calendar, and the same set of jobs works wherever you garden.