Thanks for this idea goes to Laura and Linda Kemshall and is from their video how-to website, DMTV. They called it plant printing. I had a go using all sorts of plant material from the garden.
My results were mixed and I wonder if it's because I didn't have the requisite lump of rusty iron and used a natural iron oxide (artists' pigment dry ground - raw sienna) instead. It worked but I only used 1/2 teaspoon and may have needed more to deepen the colour. Iron Oxide pigments do come in other colours too including a chocolatey dark umber, so a bit of experimenting is called for I think, if only to justify buying an £8 wok for the boiling process from Wilko. (I didn't want to use my food prep pans, and none of the charity shops had any saucepans)
It was a fun way to spend an afternoon. When I get something better to show you, I'll post again. Below is heuchera and abutilon.