There is currently no known use for a 
Thorn Seed. I did some research and wasn't able to find any sources of others finding a use.
This isnt the only mysterious item that has an unknown usable feature. For example, I know the
Blood Gobet and 
Shrunken Head Necklace have the option to "use with" or "crosshairs" on something but we don't know what. I know the Shrunken Head Necklace also gives a speed bonus, but still, it has "use with" which has no use at the moment.
Well why are some items like this? Maybe we dont know the use. Maybe we are waiting on some updates for CipSoft to find a use for the item? Maybe it was meant to be for something but there was a change in direction? Here's a great question answered in a Tibiawiki interview with Delany:
After some major updates, we can find items that are apparently “ready to use”, however there is no way to obtain them in the game, yet. Can you tell us a little about some of the reasons that could explain why these “ready-to-use” items are sometimes implemented with no actual way to obtain them, but are made obtainable a few major updates later on (or are never made obtainable at all)? Does the dev team see any problem with such situations?
In fact this is something that has been a part of Tibia since the very beginning (at least in my opinion). Sometimes, it is a long-term plan of the developers to introduce items they want to use later to expand a story. Other times, circumstances might have changed that made it necessary to alter a previous plan and “ready-to-use” items are a remnant of that. In few cases it also may be a simple mistake.
We currently do not see any of these things as problems.
