Stake 3 Delphinium Spikes with Link-a-Grow Supports in a 35-Centimetre Terracotta Pot

A 35cm terracotta pot holds roughly 20 litres once you allow for the crocks and the shoulder you leave below the rim. That volume is the whole reason this works and also the reason it goes wrong. Three delphinium crowns crammed into 20 litres will fill the rootball fast, drink hard by June, and throw spikes that reach 1.2m or more on the taller Pacific Giant strains. Terracotta wicks moisture out through the wall, so the compost dries from the sides inward, and a pot that heavy top and light at the base is exactly what the wind wants to tip.

Get the cage in before the plants need it. That is the single thing most people leave too late.

Building the Link-a-Grow cage in stages

Link-a-Grow stakes come as straight uprights with clip-on horizontal links, and the point of the system is that you extend it as the plant grows instead of committing to a fixed height on day one. For three spikes in one pot, push three uprights down the inside edge of the rootball, spaced evenly, about 10cm in from the rim. Do this when the new growth is still around 20cm. Go deeper than feels necessary. In 20 litres of compost you only have maybe 25cm of usable depth, so drive each upright to the base and let it rest near the crocks for anchorage.

Clip the first ring of horizontal links at roughly 30cm to catch the spikes as they lift. As each stem passes that ring, add a second tier at 60cm, then a third near 90cm if you are growing a tall cultivar. The spacing matters more than the height. A single ring high up acts as a pivot and the stem folds over it in wind. Two or three tiers spread the load down the whole stem.

The edge case nobody warns you about: delphinium stems thicken as they flower, and a link clipped tight in May will bite into the stem by July and bruise it. Leave a finger’s width of slack at each contact point. The stem should sit inside the cage, not be strapped to it.

Why terracotta and not plastic for this job

The weight is the argument. A dry 35cm terracotta pot runs 4 to 6kg empty, and full of damp compost with three staked spikes on top you are near 25kg at the base. That ballast is doing real work against the sail area of three 1m flower stems. A plastic pot of the same size weighs under a kilo and will go over on a balcony in a stiff breeze no matter how you stake it.

The wicking cuts both ways. Terracotta lets the compost breathe and drains the excess that delphiniums hate around the crown, so crown rot is less likely than in a sealed plastic pot standing in its own runoff. The cost is water. In a July dry spell a 20-litre terracotta pot in full sun can lose enough to need a soak every day, sometimes twice. This is where a Hozelock drip kit earns its place. A single 4-litre-per-hour dripper on a timer, run for ten minutes morning and evening, keeps the rootball evenly damp without splashing the foliage and inviting mildew, which delphiniums get at the drop of a hat once air movement is poor.

One detail on the pot itself: soak a new terracotta pot in water for an hour before you plant. Bone-dry terracotta pulls moisture straight out of fresh compost and the plants sulk for a week while the clay saturates.

The compost mix that holds three crowns

Delphiniums are hungry and they want a compost with body, which is why a peat-free multipurpose on its own goes soft and structureless by midsummer and the rootball shrinks away from the pot wall. A loam-based mix holds its shape and its nutrients far longer in a container that stays put for a whole season.

John Innes No 3 is the one for established perennials in pots. The John Innes numbers are not brands, they are formulas: No 1 has the least added fertiliser and suits seedlings, No 2 is a general potting-on mix, and No 3 carries the most base feed and is meant for gross feeders and long-term plantings. Three delphinium crowns going in for the season sit squarely in No 3 territory. The Melcourt and Westland loam-based ranges both follow the classic John Innes formula, though genuine loam is getting scarcer, so read the bag for actual loam content rather than the number alone.

Blend it roughly two parts John Innes No 3 to one part peat-free multipurpose. The loam gives weight and structure, the multipurpose keeps it open and lightens the drink. Put a 3cm layer of crocks or coarse gravel over the drainage holes first. In a 35cm pot with three plants you cannot afford waterlogging at the base, because the lowest 5cm of compost holds water longest and that is exactly where the crowns will rot if it stays sodden.

Mix a handful of controlled-release feed through the top third at planting. The base feed in John Innes No 3 runs out around six weeks in, well before the spikes are at full stretch.

Dividing the crowns before they go in

Most container delphiniums start as a nursery pot with a single fat crown, and you get better spacing and better air movement by splitting a large three-year-old crown than by buying three separate young plants that flower unevenly. Lift the parent in early spring as the buds break, when you can see where the growth points sit. Wash the soil off the crown so you can read it.

Delphinium crowns split with a sharp knife through the woody base. Each division needs at least two or three visible shoots and a fist of healthy white root. Discard the old woody centre, it flowers poorly. Dust the cut faces with nothing and let them dry for twenty minutes before potting, because an open wet cut sitting straight into damp compost is the classic slug entry point, and slugs will strip a new delphinium shoot overnight.

Space the three divisions in a triangle inside the pot, each about 8cm from the wall and 12cm from each other. That gap is what lets air move between the spikes later and keeps powdery mildew off the lower leaves.

Overwintering the pot outdoors

Terracotta cracks in hard frost when saturated compost freezes and expands against the fired clay. Raise the pot on three pot feet so meltwater drains and the base is not sitting in a frozen puddle, and move it against a house wall on the sheltered side for the worst spells. The delphinium crowns themselves are hardy well below freezing and want the cold to trigger next year’s growth.

Reading the plant instead of the calendar

The timing that matters is not a date, it is the height of the new growth. Uprights go in at 20cm, first links at 30cm, and you add tiers as the spikes pass them. Miss that window and you are trying to thread a floppy 80cm stem through a rigid cage without snapping it, which is a job for two pairs of hands and a lot of swearing.

Watch the base of the pot too. When roots start showing through the drainage holes in the second summer, the three crowns have outgrown 20 litres and the drink becomes a losing battle no dripper fully wins. That is the point where the pot stops being the right home. Whether you split them again into two pots or move the lot into the ground is the question the container never answers for you.