Preserving your own seeds is fun and easy to do. The hardest part is waiting for the seeds to mature, which means letting the fruit ripen until it begins to rot, or the seed heads to dry on your flowers. This means that you might have to redefine what you consider to be a messy garden. Obviously you cannot deadhead your flowers if you want to save some seed. Most annual seeds are easy to save. I like to save zinnia, marigold, four-o-clocks, and cosmos seed. I often plant large beds of these and this way I can have a bunch. Buying a lot of flower seed can be expensive at $1 - $3 per pack, and each pack has such a small amount in them. I like to direct sow seed because I hate transplanting lots of tiny seedlings into tiny pots, and then worry every day about them drying up. I'm much more a large motor movement person, and planting all those tiny seedlings makes me impatient, so broadcasting annual seeds works great for me. I'd rather broadcast more than can possibly fit, and then pull up the excess plants, rather than transplanting flowers individually, and then you can save only the strongest and biggest ones.
When the seed heads dry you'll have to catch some species of flowers quickly such as four-o-clocks because the pods pop open and spray out the seeds. Collect the drying pods, or snap off the dried up heads, and put them into individual boxes or paper bags to finish drying. Be sure to label what you picked. Mold is the no. 1 enemy of your seeds, so they must be absolutely dry before you put them into individual zip-lock baggies. The other enemies of seeds are moisture, heat, and light, so the best place to keep them for up to three years is in the freezer. You can also keep them in a frost free refrigerator for up to a year. If you only want to save them until the next planting season, I have successfully left them in a cool, dry, dark place for a winter such as in a closet or a drawer. If you leave them longer than that in a closet, insects are likely to infest them, so it's a good idea to store the dry seeds in small jars. Be sure you carefully date and label your seeds. I made the mistake of not dating some marigold seed, and ended up replanting them three times this summer because my old seeds had such poor germination rate. I was mad at myself for not dating and keeping the seed in the extra shop refrigerator, instead of being lazy and leaving them in an old bucket all summer. Boy, I was lucky to have found some good ones in the refrigerator where I had saved a lot of seeds last year, or else I would have missed out on my annual fall beds of marigolds which look so pretty as fall flowers. Last year they were beautiful past Thanksgiving. They get spider mites badly during the hot dry summer, so I don't plant them until late July or the 1st of August.
The daily watering of the seedlings keeps the spider mites at bay until they get bigger. By the time they get big, it cools off and they love the cooler weather by blooming beautifully up until frost. My problem is that the seeds are only in the stores in the spring, and so they are harder to find when I need them, unless I buy them from more expensive seed catalogs.
Zinnias make wonderful butterfly gardens and I like to sow them by the hundreds, which would be very expensive to buy by the packet. I need them by the bucketful, so you can see the practicality of me saving my own annual seed. You can do the same with parsley and sow rows of it in the garden as a host for the Black Swallow Tailed butterfly. Perennials such as purple coneflowers and Black-eyed Susan, also have easy seed heads to to save. But these will not sprout unless the temperature is between 45-52 F, and I like to start those in the late winter outside in cold frames. But germinating and saving are two different subjects. Poppies, and other spring flowers such as lupines (bluebonnets) and larkspur have seeds as tiny as dust. These spring bloomers must have periods of freezing and thawing before they will sprout so the best way to plant these is to sow the seeds in late fall on a prepared bed. These are the easiest of all to have large beds of flowers, once you get them started, but you will have to be diligent to save the dried seedpods, because these also pop open and spray out their minuscule seeds.
To save vegetable seeds such as cantaloupe that have a wet pulp, I let the fruit ripen to the point of nearly rotting, and then I scrape the seeds out of the pulp onto a paper plate and let them dry there. The seeds are easy to scrape off after that. Many vegetable seeds such as tomatoes have to actually ferment before you get a really good germination rate because they have a gelatinous seed coating to prevent them from germinating too quickly. Mash the extra ripe tomatoes, place it in a bowl or jar, and let the pulp stay for up to four days until the top forms a crust, then scrape out the seeds onto a paper plate and dry them as usual.
A quicker, less smelly way is to put the seeds in a cup. Fill it with water to measure one cup, add 1 tablespoon of Oxyclean soap powder and let it set for 30-45 min. Pour off the water through a strainer, and rinse the seeds until they are no longer slippery. Dry them on a paper plate as usual. If you have lots of seed, and don't care if not all the seed germinates, you can skip all the mess and just save the seed like you would the cantaloupe. When saving seeds you must remember that many hybrids do not have viable seed, and if they do, they will not come back true to the mother plant. Think of it like kids. The seeds carry the genes of their ancestors, and may look more like father, grandma, and Great Great Uncle Gene, instead of mother.
Many plants cross-pollinate easily. Squash is notorious for this, but fun, too. You never know what you'll get if you grow lots of squash near each other. Many plants like corn may be pollinated by the wind. These types of species may be pollinated by hand, and then the flower is kept covered by a paper bag stapled on so nothing else can pollinate it, such as insects. Crossing seeds is another whole other subject, but you need to be aware of how it works before you save seeds of plants with many varieties in close proximity. Saving seed is cheap, easy, and rewarding. It is also a great project for kids. Have fun saving your seeds!