1. To save money
Saving seeds is essentially free, all it takes is a little organisation and patience. Swapping seeds with other seed savers can be a great way to get hold of varieties you don't currently grow.
2. To propagate an unusual or particularly fine specimen.
If you have managed to grow a particularly fine crop of tomatoes, or a giant pumpkin, by saving their seeds you will (hopefully) capture those fantastic genetics for next year's crop too. Please note that plants that have cross pollinated will also have characteristics from the pollinating plant, which may give you different characteristics to the ones you expect.
3. To preserve wild plants in your area
Some of the photos in this post are of wild plants. We are in the really fortunate position to have lots growing in our garden, as well as in the fields surrounding us - where we have permission to forage. You can read more of our Wild Food posts here. Collecting seed is governed by the same laws that protect all our wild plants, and permission from the land owner should also be sought before any collection. The Wildflower Society states that "collecting wild flower seed for private gardening must also be done sparingly and only common species should be gathered." Provided you stay within the law, and follow guidelines, collecting seed from wild plants can be a great way to introduce natural species to your garden.
4. For educational purposes
Collecting seed is a great way to demonstrate the full life-cycle of a plant. Kids have usually experienced planting seeds and seen them grow into a plant. Watching the fruits ripen, and collecting those seeds completes the cycle, and is fascinating to watch.
5. To give as a present
A home-made packet of seeds makes a lovely, unusual gift for someone. Why not make a little. home-made envelope and decorate it, before filling with seeds.
6. For food
Last but not least, many seeds can be consumed as food. You may want to harvest your beans and dry them as pulses, or even collect poppy seeds to use in baking. Our ancestors would have gathered, and included many seeds in their diets. Here in the UK, plantain and nettle are two very common wild plants with highly nutritious seeds, as well as the wonderful nuts we collect in Autumn.
It is usually obvious when seeds are ready to be collected from a plant, after all, nature has to disperse those seeds when they are ripe. If you keep your eyes open you'll see seeds falling from trees, pods darkening and splitting, and tufts of seeds floating on the wind. One great method for collecting smaller seeds in the garden is to tie a paper bag around the seed head. You can do this after harvesting the plant or even in situ.
Once you've gathered your seeds, spread them out on newspaper or kitchen towel and let them air dry for about a week. It is important that seeds are fully dry before storage - they should be so dry that they shatter if hit with something heavy! Once dry, pack your seeds in small paper packets, and label with their name and date you collected them.