Many hardy plants will selfset seed which is evident from our carpets of annual weeds. Most if not all of us have some plants from which we would like to collect the seeds. Wait until the seeds have fully developed, until just before they spring open, scattering the seeds far and wide. Put large pods to dry in shallow dishes, not in the full sun, at as near to a constant temperature of 15C as is possible; fluctuations of temperature can adversely affect viability. Heads of the same variety of tiny seeds can be put to dry in a paper bag hung from the roof of a shed or garage until the tiny seeds have fallen to the bottom of the bag. Dried seeds can be stored in paper envelopes until sowing time. Plastic bags should never be used as they trap moisture which causes seeds to loose viability. You can store a number of these packets together in a box or drawer but it pays to place a packet of silica gel in each box or drawer as insurance against moisture uptake.
Large seeds, such as tomato, that are coated with a sticky gel can either be washed and dried before storing or can be placed on an expanded polystyrene tray, allowing the gel to dry and glue the seeds to the tray. The seeds can then be lifted off with the point of a knife blade when required. Very fine seed is best mixed with a little dry sand to ensure a better spread of seed when sowing.
(Reproduced with the permission of B O G Birmingham Organic Gardeners)