How to Sow Delphinium Seed: Freshness, Temperature, Depth and Growing On

Freshness and cold are what decide germination

Delphinium seed is short-lived. Left in a warm drawer it loses viability within months, and that single fact explains most failed sowings. Seed collected in the garden and sown as soon as it is ripe germinates thickly and quickly; the same seed sown a year later from a warm shelf comes up thin, slow and uneven, and it is easy to blame the compost or the temperature instead.

If you are not sowing straight away, dry the seed, put it in a sealed container and keep it in the refrigerator until you are. Cold, dry storage holds viability for a couple of years where warm storage does not manage one. Label the container with the date it was collected, because you cannot tell old delphinium seed from fresh by looking at it.

A short cold spell before sowing also helps stored seed germinate more evenly. Two to three weeks in the refrigerator in slightly damp vermiculite, in a sealed bag, before you sow will pull a straggling germination together. Fresh seed does not need it.

The seed is among the most poisonous parts of an already poisonous plant. Keep packets labeled and out of reach of children, and wash your hands after handling it and after sowing.

Sowing in trays

Sow in trays or pots rather than direct into the border. Delphinium seedlings are small and slow at first, and in open ground they are lost to slugs, to weeds and to being hoed off before you have identified them. In a tray you can control the temperature, keep them off the ground away from slugs, and prick them out at the right moment.

Use a fine, free-draining seed-starting mix, firmed lightly and watered before sowing rather than after, so the seed is not washed about. Sow thinly on the surface and cover with just an eighth of an inch (3 mm) of sieved compost or fine vermiculite. Vermiculite is the better cover here because it holds moisture without capping into a crust.

The temperature target is cool: 55-60F (13-15C) is right, and germination falls off badly above about 70F (21C). This is the opposite of most bedding seed, and a heated propagator is more likely to hurt than help. A bright unheated windowsill, a cold frame or an unheated greenhouse is usually a better place.

Expect emergence in two to three weeks. Keep the surface evenly damp throughout, watering from below where you can, and give plenty of ventilation. Once seedlings are up, thin any that are touching so each has air around it, because damping off moves fast through a crowded tray of delphinium seedlings.

1. Fill and pre-water Fill a tray with fine free-draining seed-starting mix, firm it lightly and level it, then water it thoroughly and let it drain before you sow. Sowing onto pre-watered compost keeps the seed where you put it.
2. Sow thinly on the surface Scatter the seed so the grains are around half an inch (1 cm) apart. Crowded sowings are the main cause of losses to damping off later, and thinning a dense tray of tiny seedlings is fiddly work.
3. Cover with an eighth of an inch (3 mm) Sieve fine compost or vermiculite over until the seed is just hidden. Delphinium seed needs covering, but only barely; deeper than a quarter of an inch (6 mm) and germination falls away.
4. Keep at 55-60F (13-15C), bright Stand the tray somewhere bright but out of direct midday sun, at a cool steady temperature. Avoid a heated propagator. Warmth is what stops delphinium seed, not cold.
5. Thin as soon as the first true leaf shows Remove any seedling touching its neighbor so air moves between them. Do it early, while the roots are small enough that pulling one does not disturb the rest.

Transplanting seedlings and growing on

Transplant when each seedling has one or two true leaves beyond the seed leaves, into individual small pots or cells of ordinary potting mix. Handle them by a leaf, never by the stem, and set them at the same depth they were growing. Delphinium seedlings resent being disturbed once the root has run, so it is better to move them slightly too early than too late.

Grow them on cool and bright. Warm, dim conditions produce drawn, floppy seedlings that never make good plants. Once the risk of hard frost has gone, move them outside to a sheltered spot and let them grow through the summer, potting up as roots fill each container. Keep them just damp; a fleshy delphinium root in cold wet compost rots.

Stand the pots up off the ground on a bench or table, and keep the area around them clear. Slugs treat a potted delphinium seedling the way they treat spring shoots in the border, and losing a summer's worth of growth in one night is a common and avoidable way to end the exercise.

Plant out into their permanent positions in autumn, spacing them eighteen inches to two feet (45-60 cm) apart on ground that does not lie wet in winter. Set the crown at soil level rather than in a dip, and mulch the root area while keeping the mulch itself clear of the crown.

The first year, and what seedlings will and will not give you

A seedling sown in its first spring may throw up a small spike late in its first summer. Pinch that bud out. The plant cannot build a crown, a root system and a flower spike at once, and letting it flower in year one is the usual reason a seed-raised delphinium is still feeble in year three. Sacrifice the first spike and you get a proper plant the following season.

Protect the first-year plants from slugs exactly as you would an established clump, and more carefully, because a young plant has only two or three shoots and no reserve at all. Thin a second-year plant to no more than four stems, and only go to five or six once the crown is genuinely established.

Understand what seed gives you. Delphinium seedlings do not come true: height, spike density and above all color vary across a batch raised from one parent, and some of them will be plainly worse than the plant you collected from. That is the trade. Seed is the cheap way to a lot of plants; basal cuttings taken in spring are the way to more of one particular plant.

The productive way to use both is to raise a batch from seed, let them flower in their second year, then take basal cuttings in the following spring from the one or two you genuinely like. That converts a random batch into a stock of a plant you chose, and it is how most good garden delphiniums get multiplied.

Write down the sowing date, the seed's age and storage, and the germination you actually got, then add each plant's position in the border and what it looked like when it first flowered. Seedlings are indistinguishable for their first year and by the time they differ your memory will not be reliable. That record is what lets you go back to the right clump for cuttings.