A few of my favourite botanical dyes:
is a very sensitive dye. It's extremely versatile and will change colour depending on your pH, temperature and mordant. That means your method has to be very precise to produce the colour you want. The dye is found in the roots, which have to have been growing for 5 years to yield the best results, and should be harvested in winter when the nutrients retreat from the cold to hibernate.
is a small, parasitic beetle found in cacti in the americas. This is probably my favourite dye to work with. You can't tell just by looking at the dye bath what colour the yarn's going to come out. It's very illusory with many fugitive colours. As an extract it's brilliant orange, when in cold water it can turn black, but heat it up and it turns a rich magenta. Best of all, it's extremely intense so you don't need much.
is a real work-horse of a dye. Its high tannic acid content makes it suitable for use on cotton and was widely used in the First World War to dye the khaki of military uniforms. Derived from the trunk of the dyer's mulberry tree, it's still widely used as a commercial dye in the tanning industry.