is cactus already plural

Answer 1

No.

More than one cactus is either cacti or cactuses.

http://www.google.com/webhp?source=search_app#hl=en&q=cactus&tbs=dfn:1&tbo=u&sa=X&ei=HcM2UJLXMqb_ygHgioCYCA&sqi=2&ved=0CB0QkQ4&bav=on.2,or.r_gc.r_pw.r_qf.&fp=8195328a06f9244b&biw=693&bih=425

Answer 2

No, cacti.

Answer 3

no,cactus is a singular