Kevin Miller, aged 67, had Delta-9 THC in his system - the main psychoactive part of cannabis - when he was caught driving in Fford Eglys, Newtown.
A drug test found that Miller, who was caught on August 30 last year, had 2.5 microgrammes of the substance in a litre of blood. The legal limit, taking into consideration the possibility of accidental exposure, is 2mcg.
Miller, of Fford Eglys, Newtown, pleaded guilty at Welshpool Magistrates Court to one count of driving with a proportion of a controlled drug above the specified limit.
As well as the road ban, magistrates also fined him £80 and ordered him to pay £117 in other court costs.