Delphinium Care: Thinning the Stems, Staking Every Spike and Feeding for Height

The spring shoots are the whole plant, and slugs eat them first

Nothing kills more delphiniums than slugs and snails working on the emerging shoots. The crown wakes early, while the soil is cold and wet and the plant has no leaves to spare, and it pushes up a small number of fat, soft, sugary shoots. A slug can take a whole shoot to ground level in one night. Take three or four shoots off a young clump and there is nothing left to grow from; the plant sits there, pushes weakly, and is gone by midsummer.

So the first care job of the year happens before there is anything to admire. As soon as the crown shows movement, clear every scrap of dead leaf, old stem and mulch away from the immediate crown so there is nowhere within a few inches for a slug to spend the day. Then put a physical barrier around it: a ring of sharp grit two inches (5 cm) wide and half an inch (1 cm) deep, or a copper collar, renewed after heavy rain.

Check by torchlight on the first mild damp evenings, which is when the damage actually happens. Ten minutes with a torch two or three nights running at the start of the shoot flush is worth more than any amount of daytime effort, because you catch the animals actually on the plant.

Keep this up until the shoots are eight to ten inches (20-25 cm) tall. Above that height the stems firm up, the slugs lose interest, and the danger period is over for the year. The full detail on baits, barriers and what else attacks a delphinium is on the pests and diseases page.

Every part of a delphinium is poisonous if eaten, and the sap irritates skin on some people. Wear gloves when you thin, cut back or divide, keep the trimmings away from anywhere livestock or pets graze, and wash your hands before eating.

Thin to five or six stems and you get spikes worth having

An established clump will push up a dozen or more shoots. If you leave them all, every one of them is thin, every spike is short and sparsely flowered, and the whole clump goes over in the first squall because no individual stem is thick enough to hold itself up. The plant has a fixed amount of root behind it, and spreading that across twelve stems buys you twelve disappointing ones.

When the shoots are four to six inches (10-15 cm) tall and you can see which are fattest, snap out the weakest at the base until five or six of the stoutest remain, spread evenly around the crown. Break them off cleanly rather than cutting halfway up, so no stub is left to resprout. On a first-year plant leave two or three; on a second-year plant, four.

The difference is not subtle. Those five stems get everything the twelve would have shared, so they come up thicker, stand better, and carry spikes that are longer and packed with far more florets. You also get daylight between the stems, which dries the foliage after rain and keeps powdery mildew out of the center of the clump.

Do not throw the thinnings away in spring. Shoots pulled with a sliver of the crown attached root readily as basal cuttings, which is the standard way of increasing a delphinium; that is covered on the propagation page.

One cane per spike, tied in early

A delphinium spike is a tall, top-heavy column of wet petals on a hollow stem. One storm at full flower snaps it, and a snapped spike does not recover: that stem is finished for the year. Grow-through ring supports and a loop of string around the clump do not work here, because the stems are too tall and the load sits far above the ring. Each spike needs its own cane.

Put the canes in when the stems are about a foot (30 cm) tall. Going in early matters for two reasons: you can push a cane down beside a shoot without spearing the fleshy crown roots, which you cannot do reliably later, and the stem grows up against the support instead of being bent back to it. Use a cane that reaches to roughly three-quarters of the expected final height and set it on the windward side of the stem.

Tie loosely with soft twine in a figure of eight, so the tie sits between cane and stem and cannot chafe. The first tie goes on at about eighteen inches (45 cm), then add another every twelve inches (30 cm) as the stem extends, with the last one just below where the flowers begin. Three or four ties per spike is normal.

Check the ties once a week while the spikes are extending. A tie that was loose in late spring can be cutting into the stem a month later, and a stem strangled at one point snaps there in the first gust.

1. Set the cane at a foot (30 cm) of growth Push a cane in four to six inches (10-15 cm) from the stem base, angled slightly away from the crown, until it is firm. Going in this early means you feel resistance and can move the cane rather than driving it through a root.
2. First tie at eighteen inches (45 cm) Loop soft twine around the cane, cross it, and loop it around the stem so there is a finger's width of slack. Never pull the stem to the cane; bring the tie to where the stem already is.
3. Add a tie every twelve inches (30 cm) As the spike extends, add ties at roughly one-foot (30 cm) intervals. The top tie should sit just below the lowest floret, which is where the leverage is greatest.
4. Check weekly and slacken as needed Run a finger under each tie. If it does not move freely, cut it and retie. Stems thicken fast during the last few weeks before flowering and outgrow a tie in days.

Water and feed like a plant building a five-foot stem

A delphinium puts up several feet of stem and a solid column of flower in a single season, and it does that on a shallow, fleshy root system. It has no drought tolerance to speak of. A clump that dries out while the spike is extending produces a short spike with gaps in it, and one that dries out at bud stage drops the lower florets before they open.

Water deeply and infrequently rather than little and often: enough to wet the top eight inches (20 cm), roughly once or twice a week through the extension and flowering period in normal weather, more in a hot dry spell. Push a finger into the soil at the edge of the clump; if it is dry to the second knuckle, water. Aim the can at the soil, not over the foliage, and water in the morning so the leaves are dry by evening.

For feeding, work an inch (2.5 cm) of garden compost around the crown as growth starts, then give a balanced general fertilizer once the shoots are six inches (15 cm) tall. A second, lighter feed as the buds color supports the spike. Stop feeding once flowering begins.

Do not push it with high-nitrogen feed in the hope of more height. Nitrogen-driven growth is soft and thin-walled: it flops, it snaps, and it is the growth that powdery mildew colonises first. Two inches (5 cm) of mulch over the root area, pulled back an inch or two from the crown itself, does more for height than extra feed by keeping the shallow roots cool and evenly moist.

Keep a note each spring of how many stems you left on each clump and how tall those spikes finished. Add the date the shoots first broke ground and the date you first saw slug damage. After two seasons you will know your own garden's thinning number and exactly when the barrier has to be in place, which no general figure can tell you.

Cut the spent spike to a low leaf for a second flowering

As the last florets at the top of a spike fade, cut that stem down. Not to the ground, and not just the dead tip: take it back to a healthy leaf low on the stem, roughly six to nine inches (15-23 cm) above soil level. Cut just above the leaf, and leave the leaf on.

That cut does two things. It stops the plant setting seed, which otherwise absorbs most of what it has left, and it wakes the dormant buds sitting at the base of the crown. Within a couple of weeks new shoots appear, and on a well-fed, well-watered plant those shoots flower again in autumn. The second spikes are shorter and slimmer than the first, but they arrive when very little else in the border is doing anything.

Water and give a light balanced feed straight after the cut-back to fuel that regrowth, then keep the plant watered through late summer. Treat the new shoots exactly like the spring ones: thin them if there are too many, and cane the ones you keep, because they blow over just as readily.

Once the autumn flowering is over and the foliage yellows, cut the whole plant to two or three inches (5-8 cm) and clear every leaf and stem fragment away from the crown. Leaving that litter in place gives slugs a perfect place to overwinter right where the new shoots will emerge, and hands mildew spores a way through to next year.