Sculpted by: Paul Harding
Release: 2016
Edition: Limited
Used material(s): Cast in yellow- and rose gold-plated white metal
One of: 675

Signed: Paul Harding

First appearance: Keys to the Kingdom #3
 Allows the control of roses

More information:
(possible spoilers)

Dodge used the Thorn Key in his series of attacks on the Locke family in February. However, the Lockes were able to fend him off through unknown means, although injured.

Note: The key only had one appearance, one page only, so what follows now is pure speculation based on observation.

The key can be used in the green house. Once it “unlocks” an artifact in the green house, vines grow out of it rapidly. There are flowers on the vines, resembling the face of the user. Apparently the vines are sentient and follow the will of the one who activated them.

Source: Locke & Key Wikia