The most frequently asked question regarding seed collecting is How do I know if seed is ripe?” Here’s a very general answer, for seeds of different plants ripen at different times, and their appearance, when ripe, can be as different as the individual plants that produced them. You need to keep a watchful eye. Observe the green immature pods over time, and suddenly you’ll notice brown seed capsules ready to split and burst. If you are not vigilant, you may discover empty seed pods days later, and your opportunity will have passed. (You can also try placing a paper bag over the immature seed capsule and securing it with string. When the seed ripens it will be contained in the bag.)
Collecting Tips: Mature seeds are usually dark in color, firm, and dry. Seeds that are green and moist are usually immature and generally will not germinate or will produce unhealthy seedlings. The flesh of pulpy fruits often becomes soft and changes from green/yellow, to red or blue-purple when ripe.
Cleaning: In a cool dry space, place dry seed capsules in a paper bag secured with string and hang upside down. Clustered seeds of composite plants such as Asters and Marigolds might benefit from being laid out on newspaper layers and allowed to dry more completely. Remove the chaff and other vegetable matter which may harbor fungal spores which will spread and infect the seeds. Moist seed from fleshy fruit such as tomatoes, or from ornamentals such as Arisaema, should have the mucilage (the wet medium surrounding seed) rinsed off. Place the ripened seed in a sieve and rinse off thoroughly. Spread rinsed seed on layers of newspaper and allow to air dry.
Storage: Only store cleaned and dry seed. The combination of moisture and warmth will cause spoilage. Store seed in paper envelopes or bags to allow them to breathe. Don’t use plastic baggies, which may trap moisture. Keep your seeds in a cool dark dry space. Your refrigerator would work, if you have room, or perhaps a closet or cabinet in a cool room. Clearly label all packets with all pertinent information.
Important: Remember the lower the humidity and temperature in storage, the longer the viability of the stored seed
For more info online: http://theseedsite.co.uk/harvesting.html If you really want the specifics, you need the seed germinating bible. Norman Deno’s book Seed Germination, Theory and Practice can be purchased through the North American Rock Garden Society’s Bookstore.