Barberry Bark (Berberis vulgaris) is perfect for natural dyeing and comes in cut pieces which can easily be added and removed from the dye bath - you could also infuse first then use the dye liquor for your natural dyeing.  Whole dyestuff is perfect also for yarn dyeing where you wish to avoid any particles remaining in the batch. 
 
Barberry bark yields a clear yellow depending on quantity used, the fibres used, and the pH of your dye bath too (i.e. whether you use any dye modifiers such as acids or alkaline).  With copper and iron modifiers you can achieve green colour shades.  Mordants are recommended so use Alum mordant for when dyeing animal (protein) fibres, and a tannin plus ALUM or Aluminium acetate when dyeing plant (cellulosic) fibres.
 
Smallest pack size supplied is 100g
 
Recommended use is at 100% weight of fibre/fabric (WOF)