I can't short a non-marginable (and, PHOT is a non-marginable stock) via my TDAmeritrade platform)....
and......
here's what Scottrade says (the link follows; I added the red):
Canadian stocks, options and any stock below $3.00 per share is not marginable and therefore cannot be shorted. Structured fixed income products, which include bonds tied to equities or indexes, are not marginable at Scottrade.