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I think the season varies between the hemispheres and according to distance from the equator.
All in the western hemisphere.http://www.microsofttranslator.com/bv.aspx?from=&to=en&a=http%3A%2F%2Fwww.scielo.br%2Fscielo.php%3Fscript%3Dsci_arttext%26pid%3DS0100-29452007000300044%26lng%3Den%26nrm%3Diso%26tlng%3Dpt%23fig02The equator traverses south America and they are found on both sides from Colombia to central Brazil and I understand some Brazilians grow them even further south than their natural distribution.
Seems like they are quite widespread so should be pretty easy to get.
Oscar that was not really a serious comment as I have found them hard to get. Weeds usually have masses of small seeds but it appears they can be dominant in secondary regrowth forest,especially near ecotones.