The sex is determined at polination, but sex expression is not.. China is correct, I have read more than a few general botany abstracts that support the notion that acetylsalicylic acid disrupts ethylene pathways in a similar fashion to GA or ionic silver solutions, and that is understood to be the mechanism behind inducing typically diocious plants to go monocious.. Different genetics will have different resilience to sex adaptation both early, and late season.. Its commonly believed that great environmental conditions early in veg can tilt the balance toward XX expression, while less desirable conditions later in life can tilt those expressing XX to grow staminate flowers.. It actually makes sense from a propogation of species POV.. Ignoring the fact that this might sound like plants have intelligence, an XX expressing plant that is stressed can fear that available breeding partners will not be able to polinate it, therefore it does what it can to produce its own pollen since male flower growth does not require a ton of effort to achieve.. Similarily, positive conditions early tell XY polinated seedlings that its likely they will be able to take on the bigger job of seed production themselves..
I would personally avoid inducing XX expression via these means though, because those particular genetics are not tightly bound to it, and my hypothesis is that these will be more likely to also grow male flowers.. Merely an opposite form of herming..
Lastly, although it might generally work out for simple classification, the XX/XY system is BS in plants.. Its not even really 100% effective for animals either (a Y chromosome is merely an X chromosome with a minor mutation apparently caused by testosterone iirc), but we tend to diverge from it much less than plants..
http://en.wikipedia.org/wiki/Cannabis#Mechanisms_of_sex_determination