Although the simplest method of propagating herbaceous perennials is by lifting and dividing, growing them by seed is very useful when you need a large number of new plants. However, this technique should only be used for propagating species rather than cultivars, as the majority of cultivars do not come true from seed. For full details on how to sow seed, take a look the handy guide in the Propagation section.

Collecting Seed

When collecting seed from your own garden, always make sure that you use plants with the best flower and growth characteristics. Seed from early-flowering perennials can usually be collected in early to mid-summer; if this is sown as soon as it is ripe, it will germinate quickly, producing plants that can be successfully overwintered in open ground or in a cold-frame.

Collect ripened seed pods when they have turned brown. Shake the seeds from the pods onto a piece of paper or hang the seed-heads upside-down in paper bags so that the seeds fall into the bags. When the seeds are completely dry, store them in clearly labelled envelopes of paper bags in a cool, dry place until ready to sow.

All seeds gathered from later-flowering perennials should be sown in autumn (as soon as they have been gathered), so that they will germinate the following spring. Alternatively, store the seeds in a cool, dry place over winter and sow in the spring.

Dormancy

Some herbaceous plant seeds such as monkshood (Aconitum) and peonies (Paeonia) remain dormant unless they are subjected to particular conditions to break their dormancy and induce germination. This may include removal of their hard seed coats (e.g. Baptisia), chilling (e.g. Primula) or exposure to light (e.g. Gentiana).

Scarifying

Remove the hard seed coats from perennials such as Leguminosae by rubbing them with sandpaper or chipping them slightly with a sharp blade. Carrying out this treatment just before sowing will allow the seed to take up water and germinate.

Chilling

Sow in autumn, covering the pots with a thin layer of fine compost and a grit top-dressing. Then plunge the pots in an open sand bed outside to encourage the seeds to germinate. Alternatively, chill the seeds in a refrigerator for a few weeks before sowing in spring. Use these methods for genera such as Aconitum, Adonis, Campanula and Primula.

Soaking

Other perennials such as Arum, Baptisia and Euphorbia may have their seed coats softened by soaking them in hot (not boiling) water for 12-24 hours before sowing. Again, this will allow the seed to absorb water and germination to take place.

Double Dormancy

Seeds from genera such as Paeonia and Trillium need to have two periods of cold to trigger germination of both roots and shoots. The first season's cold triggers root development, whilst the second season (usually during the following winter) will trigger the shoots.