Saving Nasturtium Seed and Keeping Self-Seeding Under Control

Ripe seed falls, so collect it off the ground

Nasturtium does not hold its seed. The green pod swells, turns pale and corky, and then drops off at a touch, which means that by the time you notice a plant has finished flowering, most of the seed is already lying under it. So the collecting is done on hands and knees, lifting the runners and picking the pale wrinkled seeds off the soil surface.

The seed you want is fully swollen, pale beige to light brown, hard, and deeply ridged. Green seed that you pick early will shrivel to nothing in storage, and seed that has been lying on wet ground for a fortnight is often already rotting or sprouting inside. Late summer to early autumn, on a dry day, is the productive time to look.

1. Lift the runners and look underneath Most of the crop is on the ground under the leaf canopy, not on the plant. Roll the stems aside gently and gather what is lying there, then put them back.
2. Take some from the plant too Pods still attached but already pale and corky are fully ripe and cleaner than ground fallen ones. Pull them off with a light tug.
3. Dry them a fortnight indoors Spread them one layer deep on paper somewhere dry and airy, out of the sun, and leave them two weeks. Bagged straight from the garden they go moldy.
4. Store cool, dark and dry A paper envelope in a tin in an unheated room is ideal. Label with the year and the plant type: trailing or compact matters more than anything else next spring.

Nasturtium seed stays viable for four or five years stored dry, so one good collection covers several seasons. Test an old batch by soaking a few overnight and seeing which swell.

Saved seed rarely comes true, and it does not matter much

Nasturtium is pollinated by bees and long tongued insects and crosses freely between anything flowering nearby, so seed saved from a mixed planting gives a mixed batch: colors drift towards orange and red over a few generations, and creams and pale yellows are usually the first to disappear. Doubles and semi-doubles seed unreliably as well.

The one trait worth keeping separate is habit. A trailing plant crossed with a compact one gives seedlings of both sorts, and that matters because the spacing is completely different. If you want to keep a compact strain, save from plants grown well away from any trailing ones, and pull any seedling next spring that starts throwing long runners.

Volunteers: welcome them or weed them

Any nasturtium left to set seed will come up again the following spring, often in large numbers, and the seedlings appear later than you expect because they wait for the soil to warm. In a gravel path or the edge of a shrub border that is free planting. In a vegetable bed it is a weed, because a nasturtium mound will smother a row of young carrots within a month.

The seed is large and the seedlings are unmistakable: two fat oval seed leaves followed by round shield shaped leaves on long stalks. They pull out easily while young and the taproot comes with them. Leave them a month and the taproot breaks when pulled and the plant regrows.

1. Decide in late spring, not midsummer Go through the beds when the seedlings have two or three true leaves and pull everything you do not want then. At that size the whole root lifts cleanly.
2. Keep the strongest, spaced properly Where volunteers are welcome, thin them to the spacing you would have sown at. A dense stand of self-sown plants stretches and mildews.
3. Move a volunteer only when tiny A seedling at the two leaf stage can be lifted with a trowel and a good lump of soil and moved successfully. Beyond four leaves the taproot is too long and it will sulk.
4. Clear the plants before the seed drops To stop the cycle in a bed, pull the plants as soon as the first pods start turning pale, and rake the surface over to expose any seed already fallen.

Frost kills the parent plants outright but does nothing to the seed lying in the soil. Clearing the dead plants in autumn does not prevent next spring's volunteers.

Cuttings, for keeping a particular plant

Nasturtium roots readily from stem cuttings, which is the only way to reproduce a specific plant exactly, since its seed will not come true. It is worth doing if you have a plant with a color or a variegation you want to keep, and it is the standard method for the perennial climbing relatives that set little viable seed in cool climates.

Take four inch (10 cm) tip cuttings in late summer while the growth is still soft, strip the lower leaves, and put them in water or in gritty potting mix. In water, roots show in about a week and the cutting can be potted at half an inch of root. Overwinter the young plants on a bright windowsill above 50F (10C), keep them nearly dry, and plant out after the last frost.

1. Choose a non-flowering shoot A tip with flowers or pods on it puts its energy there and roots slowly. Pick a plain growing shoot with three or four leaves.
2. Cut just below a leaf joint Roots come from the joint. Cut a quarter inch (5 mm) below it with a clean blade and remove the leaves from the lowest two joints.
3. Root in water or gritty potting mix A glass of water on a windowsill is simplest and lets you see progress. In compost, use half grit and keep it just moist, because the hollow stems rot in wet compost.
4. Pot up early and keep it lean Pot as soon as there is half an inch of root, into ordinary compost with nothing added. Fed young plants make soft growth that will not overwinter.

The stems are hollow and bruise easily. A cutting crushed by blunt scissors rots at the base instead of rooting, so use something genuinely sharp.

Write down what you saved and from where

Because nasturtium crosses so freely, an unlabelled envelope of seed is close to useless a year later: you cannot tell trailing from compact, or a deliberate saving from a handful swept off a path. Write the year, the habit, the color of the parent, and roughly where in the garden it grew, and keep the envelopes together.

It is also worth noting where volunteers appeared and how many. That map tells you next year which beds need clearing early and which corners you can leave to look after themselves, and over a few seasons it shows how far the seed actually travels, which is usually less far than people assume.