Seeds can be harvested/collected from the flower heads and fruits of virtually any type of plant, tree, shrub, grass, perennial, annual etc. Once the flowers have faded and the seed is set and ripened, you can begin to harvest the seeds for sowing at a future date. Not so many years ago it was customary for gardeners to collect their own seeds for the next sowing, particularly for vegetables, and many still do. However there can be many problems collecting vegetable seeds yourself because transferable diseases can be passed on though each generation. These days it is advantageous to buy vegetable seeds commercially as they are specially breed to be either disease free or resistant.
Seed collected from native and named species will provide better results than seed collected from F1 or F2 hybrids or special strains and other cultivars such as grafted plants. Because F1 hybrids and other selectively breed plants are cross-pollinated they will not breed true to type unless the same parent plants are used in the pollination processes.
Obviously, if ripe seed heads are left on plants they will eventually drop their own seeds or disperse them in the wind, which is particularly true of wild flowers and weeds, as we all discover to our dismay at weeding time. So why bother collecting the seeds yourself? Why not just let them fall and germinate in the ground? Of course this can be done and is very successful for certain varieties that seed easily. This is often a source of delight for gardeners who come across a new plant growing in the garden that they did not plant themselves.
However, if you want to control the germination process and give the young plants the best chances of survival, then collecting the seed and sowing them under controlled conditions will give you far better results, and you can keep an eye on the plants at all stages. You can then transplant those plants in a different part of the garden, or to give them to a friend or neighbour.
The ideal time to collect seed is on a still, dry day, when the seed pods are just about to open. If you can, collect the seed over a few weeks. That way, if they were not fully ripe on one occasion, then the chances are one of the collections will be. The best way to trap all the seeds is to snip off the seed head and pop it in a paper bag or large envelope and shake the head, upside-down, to get all the seeds out. Some species (Impatiens for example) actually expel the seeds if the ripe pods are touched, so be prepared.
All collected seed must be completely dry before storage. If there is any hint of dampness, spread the seeds out on tissue paper on a sunny windowsill for a few days. Once dry, store them in envelopes and clearly label and date them. The envelopes can then be stored in plastic boxes, jars or tins with tight-fitting lids. Keep the seeds in a cool, dry place, at around 5°C (41°F).
The length of time seeds can be kept depends on the variety and how well you store them. Certain seed will only germinate if freshly ripe, such as cyclamen.
The method of sowing seeds that you have collected yourself is the same as it is for commercial seeds. However, as nature does not provide you with a seed packet with sowing instructions on it, you will need to look-up the best method depending on the species.