You can go buy some seed in a packet and go ahead and toss your spent plants in the compost pile, but if you have the room and the patience to let your plants mature, you can save your own seed. Will it come true- will the new plants be like the old ones? Depends if you used a hybrid seed, if there are other varieties it could cross pollinate with, and if it is likely that it did.
I have a speckled romaine that comes from romaine and some red lettuce I grew that crossed, probably Red Sails. I like it. My own quasi Forellenschuss. I have just cleaned seed from a Brune D’Hiver French heirloom lettuce plant that was all by itself in the greenhouse, so probably it didn’t cross. I plan to save seed from the cool Baker Creek Rocky Top mix and see what happens. Life is too short to just grow one lettuce a year so you can save pure seed. Luckily I only have one kind of arugula. It volunteers in my garden all year long, but it is fun and easy to save.
Let your plants bloom and go to seed. This is actually good for your garden because you attract beneficial insects. I also find that the dreaded Harlequin beetles
will stay on flowering brassica crops and leave leafy crops alone, if pressure is not that high, It’s also pretty. Endives are in the chicory family so they have pretty blue flowers. Once the seedheads are dry enough for the seed to “shatter;” fall into your hand when you crush the seed head, you’re ready. Get a big clean bucket and cut the tops of the plants into it. Sometimes I just pick the tiny dandelion looking heads of the lettuce into a little bowl. If you pull up the plants it is easy to get crumbs of soil in with your seeds. This is fixable but extra work. Set the bucket somewhere to dry if there is any flexibility in the plant material. When you are ready to clean the seed out, crush the seed heads so the seeds fall into the bucket. Discard the stems. At this point I generally transfer the seeds and chaff to a light mixing bowl.
Arugula and mustards drop a lot of seedcases. The seed settles so you can actually pick most of that off the top and compost it. You should do this outdoors. Put a wide bowl on top of a sheet in a place where there is a light breeze. Holding the other bowl about 2 feet in the air, pour it into the bowl on he sheet. You will see that the chaff falls at a different angle from the seed, and even that different colored or sized seed falls differently. You will soon get the knack of winnowing- humans have been doing this for thousands of years- and won’t need a sheet any more. Look through your seed, blow on it to remove last bits of crud, check it for dirt and insects. Light seed will tend to blow away, leaving you with the best seed. Crush a seed with your fingernail to make sure it is dry enough to store. If you put moist seeds in plastic they will mold. Let is sit out in an open bowl until you are confident that it is dry enough to store. It’s less critical if you are using a paper envelope. Put it in an envelope, label and date it, and put it in your seed file.
With lettuce I tend to sit with my morning coffee and pick off seed heads, then crumble them into a small bowl. The fluff comes off as you rub it between your fingers. You can then go outdoors to winnow out the fluff. I just pour it into my palm, pour it into the bowl, blow on it, and play with it until I have a few teaspoons of clean seed to put away; enough to seed several hundred lettuces. Some people knit.
The next seed I will be saving is cilantro. It makes a lot of seed, and guess what- that is coriander seed! You should never have to buy that from the store. It’s too easy! I use a lot of that for Indian cooking, like this fabulicious venison curry, inspired by lamb vindaloo but not,
and it is so good for you! This keeps me from being too sad when the cilantro bolts. Speaking of which, it is now cool enough for me to go back out and seed some more cilantro. Later.
OK. I forgot to take pictures. I cut off the dead cilantro plants, carried to the shed, and let them sit on a rack out of the rain for a few days. Then I put them on a sheet and crumbled the seed heads to make the little round seeds come off. This left me with a lot of broken up dead plant material which went right in the compost. It rolled into a bunch on the sheet and I picked it up together. What was left I poured off the sheet into the big mixing bowl. I took a smaller bowl- no particular reason, – and went outside to winnow. I crumbled the seeds against each other to break off little stems, and rubbed them to get tiny chunks of dirt to become dust which is easy to winnow out. I poured the chaffy seeds from one bowl to another, holding the bowls up in the slight breeze. The chaff poured off to the side while the heavier seeds poured straight into the bowl. Eventually I had to resort to blowing, swirling, etc., and the dust was clinging, I wonder how hard it would be to get them dry if you washed coriander seeds. I poured them into a couple of small jam jars and labeled them.