Nasturtiums are annuals that grow as either climbing plants or compact, bushy clumps of leaves and flowers. They don't tolerate transplanting well and therefore are usually planted as seeds. They do tolerate most soil conditions and are ideally planted in the spring after the last frost. If you want to collect the seeds from this year's plants and save a buck or two in the process, wait until fall when the flowers begin to fade to collect the seeds.
Look through your healthiest nasturtium plants with your hands to find green seed pods, which grow to about 1/3 inch in diameter. Nasturtium seed pods often grow in groups of three or four. Pull or snip off the plumpest pods with hand clippers.
Spread the seed pods on newspaper or a cookie tray. Set them in a dry, well-ventilated area of your home for one or two weeks until they dry completely and turn brown.
Separate the seeds from the pods. The pods should open up during the drying process. Take the seeds out with your fingers.
Throw out other debris and the pods and store the seeds in a paper bag or airtight containers like old jelly or olive jars. Place them in a cool and dark location like your garage or in the bottom drawer of your refrigerator.