Thank you all!
Salmonsweet - Thank you for the book tip! I'll see if I can get Jenny Deans book somewhere. The book about fungus dyeing arrived yeaterday and it's fabulous! I'm out for a "fungus hunt" tonight!
Romanojg - I'll also check out what Georgia Bonsteels got.
DebraK - please let me know if you find the instructions of dyeing with marigold.
Fabric_fancy and Janallyn - from what I found out so far, silk and wool could be treated the same way, but -as salmonsweet said- plant fibre requires another kind of mordants.
/Karin :)