The thistle can represent our spiritual defiance in the face of physical adversity and difficult conditions. This is one of the reasons why it became the emblem of Scotland. Magically it is a fire herb and, therefore, represents courage.
Psychological / Emotional Perspective:
The thistle has a meaning of rebelliousness and spite, but also of affording protection. When dreaming of a thistle we may be being made aware of those qualities, either in people around us or in ourselves. The colour of the thistles may also be important.
Everyday Material Aspects:
To be conscious of thistles in a dream is to be aware of some discomfort in waking life. A field of thistles would suggest a difficult road ahead. A single thistle would indicate minor difficulties.
You might also like to consult the entries for Colour, Plants and Weeds.