I found these beautiful roses online -- and they were made with napa cabbages. We don't eat napa cabbage, but I've been able to make similar imprints with romaine lettuce.
First, cut off the stem part and fix your salad. Save the stem/base. Make sure the cut is as flat as you can make it. Put ink/paint/mist on your stem and stamp. You don't want it too wet -- but wet enough for all of it to print.
I used ink with the smaller heads,
but this big one was more than I wanted to ink up.
I thought I had paint in the right color, but didn't. So I improvised with spray mist.
You can see in the top pic where I cut one out to use. You could print right to your background paper, but if you're like me, I need to just print when I have them and use them later. So I just print on white cs and cut and use as needed.
One of my friends who saw one on a layout made the comment that it looks so organic. It's not perfect, it's much more natural. I like that.