Earlier quoted context omitted.
Most sources I can find say that the vast majority of heroin in the US comes from Mexico and South America.
The poppies used for heroin production can be grown in South America but most of it comes from Asia.
The vast majority of worldwide poppy for opiates is Asia, but the US is unique in that most of the poppy source for their heroin is coming from Columbia and Mexico. It is odd. Canada, for example, gets most of it's heroin via Afghanistan.
The size of the lines in this map from the above mentioned resource is relative to the actual production source...where the poppies are, and where they end up as heroin. Note the fat lines from the south, and the thin one from the east.