Wattola. Canna recommends when plant develops "small fruit."
Some discussions, I read yesterday, on other forums suggest many use it as they, please, after flowers appear.
I think following Canna's suggestion, for a week, at second or third week of flower development--is safe. I used a small dose at 1/4 to 1/2 of recommended.
Also, add the Canna Boost if you have it. It is expensive.
In the future, I will be giving at least 10 days of plain water/cleaning solution. Canna suggests using the Boost all the way thru without mentioning a flush. I do think it is a good idea to flush and get rid of residue and push plants to start using mobile nutes. I mention this because I did not flush throughly on my last grow.