Can I use 5 15 5 pro mix root booster for flower please?

What base nutrients are you using? (How much of each product?) You could use it, and get a reasonable P-boosted ratio (out of veg nutrients). It probably has growth hormones. No telling what that might do. In late flower it might act as a bud denser. Earlier in flower you might get weird stretch.

Why do you want to use this instead of a more typical "bloom formula?"
Spaghnum, perlite, sheep manure, compost, molasses, fish emulsion (very stinky), and Epsom salts. Compost tea throughout summer. Thank you for your kind reply a 2000. I decided to go bome meal, coffee grounds, banana tea, and or brewers yeast to keep on the natural regimen. Thank you very kindly for your great thoughts. Pink Kush 80 inches high right now, and bushy as hell.