I spent yesterday trying out the plant material using two different methods:
Experiment 1 - making bundles or folds of dry fabrics with plant material enclosed inside, which were fastened with thread or metal clips.
Experiment 2 - boiling plant material in a little water for 1 hour to release the colour, then removing from the heat source and placing damp fabric in the dye/stain.
Below are a few images that demonstrate the initial plant material and some of the resulting fabrics.
Brown onion skins
Bilberries (harvested on Stanton Moor)
Green walnuts (collected from the ground, Chatsworth House)
I am particularly happy with the results of the walnut and bilberry stains. I was fascinated by the results of experiment 1, where the plant material was enclosed in the fabric; the slices of walnut have created wonderfully subtle embossed prints on velvet.