from what i've read, polyploidy can be induced in an established plant by physical means (i.e. defoliation technique or possibly others, my knowledge is quite limited). It can certainly be induced to polyploidy by by chemical means using colchicine applied to the apical tip which is the main growth tip (i.e. the top of the plant). The preffered way, it seems, is to attempt to induce the seeds of the plant to polyploidy either by treating the mature seeds or the fertilized pistillate (female) flowers with colchicine. This method is preffered because the cultivator will not waste their time on plants that do not respond well to the treatment, the bad seeds simply won't grow and the good ones will. However, colchicine is toxic to humans and other animals, so the method of prolonged defoliation is safer to the cultivator. Please note that I have not had the opportunity to experiment with induced polyploidy personally, I'm only relaying information that I believe to be credible through amateur research. All that is certain is that induced polyploidy has had significant benefits in other plants, but generally causes the first generation to be sterile. Seedless watermelon is a perfect example of this. Happy growing, peace & love.
-Grammaticus Funk