There is no one particular sign for thorn. If it is important for that specific word to be expressed you can fingerspell it.
Often the context in which the signing occurs makes it obvious that you are talking about thorns or a thorn and so you wouldn't need to spell it out but rather you would just show what the thorn looks like.
For example, suppose you wanted to describe flower with lots of thorns along the stem you could use your base hand in a "1" handshape to represent the stem and slide a dominant "4" hand up the stem while using a mean facial expression:
Note: For the version above, you could either "slide" the four up the 1, or you could place it on the 1 a couple times at higher intervals.
Another way to show a thorn would be to use the dominant "1" hand to
show how the thorn extends up and away from the main stem. The
dominant "1" finger "grows" out of the stem tracing the shape of a
thorn.
Or you could use an "x" handshape (hooked finger) to represent the
shape of the thorns:
Occasionally people ask
me how they would show a "crown of thorns" (as in theology).
For that you again use strong facial expression and use your hands
to show the general shape and nature of the crown of thorns:
Notes: