I know it's a Tamarisk, but it's so different from the variety that I've got, which flowers in the summer. How old is it?

to be honest Spritz don't know the variety, i bought in Woolworths year before last, so proberly one of the most common ones, it was only a tiny sprig, i grew it on in a pot for a year and planted it out last spring and it now stands about 5ft high, this is the first year it has flowered and it has only flowered on a side shoot, nothing on the main stem at all, do you know why this might be? and also how and when is best to prune them?