Pruning Delphinium
When and how — Delphinium elatum
Prune your delphinium in March and November — the optimal month is usually November.
The next pruning window is November.
When to prune?
The perennial delphinium is pruned in March and November.
With perennials, pruning is really seasonal management.
You don't prune perennials the way you prune shrubs. The work happens at three moments: (1) deadheading spent flower stems during the season to encourage repeat bloom, (2) optionally cutting back to about 10–15 cm above ground in late autumn, and (3) clearing all the old foliage in March before the new shoots emerge. Many gardeners now deliberately leave the old growth standing through winter — it protects the crown and shelters overwintering insects. Which approach to choose depends on taste and species: evergreen perennials (hellebore, bergenia) look better left alone, while wet-rotting species (hosta) need to come down after the first frost.
How to prune delphinium
Delphiniums benefit from two distinct pruning sessions each year: one in early spring and one after flowering. In March, as new growth begins, cut back any old, dead stems left from the previous year to ground level. This tidies the plant and reduces the risk of slug damage to emerging shoots. At the same time, thin out the young shoots when they reach about 10–15 cm tall, leaving only four to six of the strongest per clump. This may seem drastic, but it channels the plant's energy into fewer, sturdier stems that produce larger, more impressive flower spikes and are less prone to wind damage. After the main flush of flowers fades in mid to late summer, cut the spent flower spikes back to a healthy side shoot or to just above a set of leaves lower down the stem. This often encourages a second, smaller flush of blooms in late summer or early autumn. Once this secondary flowering finishes, or by November at the latest, cut all stems down to ground level. Remove and compost or bin the foliage to minimise overwintering of pests and diseases, particularly slugs and mildew spores. Use clean, sharp secateurs or loppers for all cuts. Delphinium stems are hollow and can collect water if cut at an angle, so a straight cut just above the crown is best. Wear gloves when handling delphiniums, as all parts of the plant are toxic if ingested and the sap can irritate skin.
Common mistakes
✗ Cutting back too early in spring
Late frost can still strike and the old foliage protects the crown. Wait until the first new shoots are visible (usually mid-March) — then you know the season has actually started.
✗ Skipping deadheading
Hardy geranium, salvia, lupin and delphinium will give a second flush if you cut spent stems back to just above a pair of healthy leaves as soon as the first flowers fade.
✗ Cutting ornamental grasses down in autumn
The dry stems are the whole point of winter interest, AND they protect the crown from frost and waterlogging. Cut down to a fist's height only in late February.
Combine with feeding
In March you can combine pruning with feeding — efficient, and you only disturb the plant once. Read the full care guide for delphinium →
Hold off on pruning
Better to wait than prune at the wrong moment. The next optimal window is November. Until then: leave the plant alone — only remove dead or diseased wood (which you can do year-round).
