NEVER. That's my opinion at least. It's not neccessary and doesn't really help, it's just an old tale. Plants don't need granny's sugar syrup added to their water, that's ridiculous, they're plants. If some ppl think it helps, that's fine, but I think it's kind of like a placebo affect of sorts. There's also "the pill", which apparently works wonders, too. But I might actually believe that one, never know with hormones. Kind of curious to try it. Also, aspirin will turn a plant male in a hurry, though that's not usually helpful.