I've been experimenting with sealing lettuce and herbs in ball jars. Here's what I know so far -
Lettuce keeps really well! I ended up using the jars in my experiment all within 5 days but it was so fresh and crispy that I know it would have lasted a good bit longer.
I used romaine heads. I tore it into bits but did not wash it. If you do wash it before sealing, you need to get it really dry. The people who seemed to know what they were doing washed it just prior to eating.
In one of the jars, I combined lettuce, carrots, tomatoes, parsley and cilantro. The tomatoes and parsley did fine. The carrots were a bit old but did OK. The cilantro was old to begin with and it was rather limp. I wasn't all that hopeful for it. Cilantro is pretty delicate.
My most recent sealing has been lettuce in jars - JUST greens, lettuce, sometimes with a little parsley or kale. My goal is less about convenience and more about keeping the lettuce fresh as long as possible. I read a number of posts with people saying they preferred doing it that way so they would have more flexibility with additions when they were ready to eat.
I'm very happy I finally got on the vacuum sealer bandwagon. I'm finding more uses for it every day that make sense/cents.