Firstly, only collect seed during a dry spell. The least hint of damp and the seeds become mildewed and won’t remain viable.
You will need
Paper bags (not plastic) like the ones you get from the greengrocer.
For sorting and cleaning seed.
Sheets of white paper or large white envelopes cut open.
A willingness to squash insects with your finger.
For storing seed.
Small glassine envelopes such as those stamp collectors use, or bankers’ envelopes.
How do you know when seed is ripe? The seed heads will be brown, will rattle, and the seed falls willingly out without any encouragement. Plants like foxgloves produce thousands of tiny seeds which will shake easily from the stem. With some other plants you may need to cut off the head and rub it gently between your fingers to tempt the seed out. Cerinthe major ‘Purpurascens’ has big, black seeds which are ripe when they come away easily in your hand. I can never understand why some seed suppliers (Hello, Sarah Raven), charge so much for these seeds, which are abundantly produced and easy to harvest. Lathyrus varieties have pods with quite large, round seeds inside.
Round the garden you go, armed with your trusty Felcos and a paper bag. Cut off the top of the stem containing the seed heads and shake gently into the bag. Bear your spoils off to the potting bench if you have one, an outdoor table (if it’s not windy) or your kitchen if you can stand mess. Fold a sheet of paper in half, with a sharp crease, or open the envelope, which will have a natural crease. Tip out the contents of the bag. You will observe immediately that some of your seed appears to be moving. I have no idea what these minute creatures are, but you almost invariably find them Deal with them (see above).
Now the seed needs to be cleaned of chaff and any other unwanted debris which may cause the seed to rot. Do this by blowing very gently along the crease in the paper. You may think the seeds will disappear but fear not, the seed is heavier than the chaff and will remain on the page. Using the handy groove, tip the seed into the small envelope, label clearly, date it and seal. Store in the proverbial cool, dry place. That’s it.
Late summer and early autumn is a good time to sow the seeds of hardy perennials. Annuals can be saved for next year. Raising plants from your own seed will not suit the impatient gardener, but can be one of the most satisfying aspects of gardening. Who knows what variations you may find when the plants eventually flower?