Propagating Nigella: Saving Seed and Handling Self-Sown Plants

Seed is the only route, and there is no second one

Nigella is a hardy annual. It germinates, flowers, sets seed and dies inside a single season, and there is no perennial part of it to divide, no woody stem to take a cutting from, and no crown to lift. Attempts at cuttings fail because the stems are hollow and soft and rot within days. There is nothing to layer. This is not a limitation in practice, because the plant produces seed in absurd quantity and germinates readily.

Because of the taproot, the same rule applies to seedlings as to mature plants: they will not move. This matters most with volunteers, which come up exactly where the pods fell rather than where you wanted them. You can shuffle a self-sown seedling a few inches with a trowel and a large undisturbed plug of soil while it still has only its seed leaves, and perhaps half will take. Beyond two true leaves, the answer is no. Sow fresh seed where you want plants and hoe out the volunteers you do not.

Seed comes true enough for garden purposes if you grow one strain in isolation, and the plant is insect pollinated, so a bed containing several colors will give you a mixture in the next generation. That mixture is usually pleasant rather than a problem, but if you care about keeping a color separate, grow it well away from the others or save from an isolated plant.

Saving seed from your own pods

Choose your seed plants early, before you start deadheading, and pick strong well-branched plants in the color you want to keep rather than whatever happens to be left at the end. Leave them entirely alone from then on: no cutting, no deadheading, no watering once the pods have formed. The pods swell, color, then fade to straw and go papery, and the seed inside turns from pale green to matt black.

Cut the stems when the pods are dry and rattling but before the crown at the top of the pod opens, which is a window of only a few days in warm weather. Hang the stems upside down inside a paper bag for a fortnight in a dry airy place; anything that splits early is caught by the bag. Then crush the pods over a bowl, tip out the black angular seed, and pick out the papery debris by tipping the bowl gently and letting the lighter chaff run off the edge.

Store the seed dry and cool in a paper envelope, labeled with the color and the year. It stays viable for three to four years, losing vigor slowly, and a single plant yields hundreds of seeds, so one good plant supplies a garden. Sow the surplus thickly somewhere rough rather than hoarding it; older seed germinates less evenly and there is always more next year.

1. Mark the seed plants before flowering finishes Pick well-branched plants of the color you want, tie a loop of string round the base, and do not cut anything from them for the rest of the season.
2. Stop watering once the pods inflate Pods ripening in dry conditions dry hard and clean. Wet pods go moldy and the seed inside is spoiled or germinates in the pod.
3. Cut when the pods rattle, before the top opens Test by shaking a stem. Rattling means ripe seed; a pod whose crown has already parted has probably shed half its contents.
4. Dry upside down inside a paper bag Two weeks somewhere dry and airy. The bag catches anything that splits, and a paper bag breathes where a plastic one sweats.
5. Crush, clean and label Break the pods over a bowl, tip the chaff off the edge, and store the black seed in a labeled paper envelope somewhere cool and dry.

Save only what you will sow within three years. Fresh seed germinates faster and more evenly than old, and a plant that sets hundreds of seeds a year does not need hoarding.

Self-sowing, which is propagation you did not ask for

Left to itself, a nigella pod splits at the crown and shakes its seed out over a foot or two (30 to 60 cm) of ground, and the following season those seeds germinate in a dense mat exactly where the parent stood. Once nigella is in a garden it is effectively permanent, and gardeners who like it stop sowing it entirely after the second year and simply manage what appears.

That is either the best thing about the plant or a nuisance, depending on the bed. In a gravel garden or an informal border it is what you want. In a tidy scheme or a vegetable bed the seedlings come up thick enough to smother small plants and to make the ground look unweeded all spring. The control is entirely in your hands, because every seedling comes from a pod you chose to leave.

To keep it and limit it, leave two or three plants for pods and deadhead the rest, then thin the following spring's volunteers ruthlessly to the same six to nine inches (15 to 23 cm) you would use for a sowing. To stop it, deadhead everything and pull the plants out at the end of the season before any pods dry. Seed already in the soil will keep germinating for two or three years afterwards, so it takes more than one season of discipline.

Keep a note of where the volunteers came up

Sketch or photograph where the self-sown seedlings appear each spring and note how thick they were. Volunteers show you where the conditions actually suit the plant, which is often not where you first sowed it, and the strongest self-sown drifts are worth building the following year's deliberate sowings around.

Record the year on every seed envelope and note how the germination went when you sow it. Nigella seed loses vigor gradually rather than suddenly, and having your own record of how three-year-old seed performed saves you either throwing away good seed or wasting a sowing slot on seed that was past it.