The top layer of your skin is covered in dead cells which shed during everyday wear and tear. The second layer of cells are new and replace the dead layer. New cells also grow in the third layer. With psoriasis sufferers, these new cells grow a lot faster than the dead ones can shed, which causes the skin to have a thickened and scaley appearance.  

Avoid the essential oils of Lavender, Palmarosa, Patchouli and Rosewood as these promote the growth of new skin cells.

There are a lot of aromatherapy/essential oil books that have Lavender as a recommended essential oil in the treatment of psoriasis. I cannot and would not recommend this, along with the other essential oils mentioned above, purely because of the fact that it promotes the growth of new skin cells. Which should not be encouraged, seeing as psoriasis is an acceleration of skin cell growth. I found a very informative website on psoriasis, www.psoriasis.org, which is full of information and well worth a look.