A lot of people ask me about seed saving. Most likely because I started a seed bank in Olympia Washington, but also because I talk about seeds quite a bit- even when unprompted. I can't help it, it's what is on my mind.
Often when finding out for the first time that I have some knowledge in this area, people will begin with the following scenario: "My kale started flowering this spring, and I thought, what the heck I'm going to save the seed."
Usually my first response is to ask how many kale plants were flowering. Inevitably, it is only a few. This is where I drop the bomb. You can't save seed from just a few kale plants.
Just like isolated populations of animals, when you only have a few organisms (in this case kale plants) mating with each other, you will have in-breeding depression over time. This is not good. Problems like reduced vigor arise, or worse- inviable seeds.
There are admittedly some plants that can handle small population size, but they are few. The best advice I have without giving you a two hour lecture is this: save seeds from the best plants in a large population of the same variety, making sure nothing else that it can cross with (same genus and species) is flowering at the same time. Don't know the genus and species of your crop? As my parents used to always say when I asked what a word meant, "Look it up!"
Good luck and happy seed saving!