Janis did this by bleach stamping. Her found stamp was the fern itself. Here's how she described it in a post to the Carvers Consortium mailing list:
Yes, I had a blast doing this one - didn't want to stop, but the bleach fumes finally got to me! I tried other things first that didn't work as well; I used an eggplant and tried bleaching black paper, but it didn't bleach much. Tissue paper was more successful. For the final one that I sent in, it was really very easy and straightforward. In fact, it's the same technique I use to do nature prints with printer's ink.I used sheets of oak tag, this time it was the kind you can buy at Staples, comes in a 9 x 12" pad. I brayered some inks on, all over. Then put the fern on a sheet of waxed paper, or maybe freezer paper, and painted the bleach on with a cheap disposable varnish brush. Grabbed the fern with tweezers so I could easily place it where I wanted. Then cover, momentarily with a scrap paper and pressed all over it with my hand/fingers, and voila!, lift off the paper and fern and you have a beautiful print! I varied the size fern so the end result would be interesting, but I reused the same ferns quite a few times before I felt they had taken enough abuse. Lots of fun!